Building a 22 GHz network analyzer for under $1000 - Page 1
Forum section: electronics / rf-microwave

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 07, 2024, 12:13:07 am
content: I am thinking about trying to build a DIY network analyzer based on the LiteVNA architecture.There are several issues (more like tradeoffs to make the device cheaper) with the LiteVNA design. They are as follows:1. Bad directivity with the bridge. The bridge starts to lose directivity at around 6 GHz, and the noise floor for S11 quickly increases to about -15dB or so at 8-9 GHz. To be fair, it was never designed to operate that high.2. Bad mixer. For cost reasons, the mixer used is the AD8342, which is only supposed to work up to 3.8GHz. This is why the noise floor increases quickly for S21 measurements after 4 GHz.3. Bad isolation with the RF switches. An RF switch is used to switch between the reference, reflected, and transmitted signals. The RF switch (MXD8641) is rated only to 3 GHz. That's probably another reason why the noise floor increases past 3 GHz, at least with S21.My idea is to use easily obtainable used RF modules which can be found at ham swap meets. I have a couple 7-18 GHz high directivity couplers from Narda, which have >25dB directivity. I also have several wideband mixers which cover the entire frequency range. My plan is to use two LMX2820 chips, which can be had for about $100, and a few RF amplifier ICs (like the MAAM-011100, which costs $12) to power the mixers and generate the signal for Port 1. The two chips will be set to have an offset of 10kHz, allowing a cheap audio ADC to be used to digitize the signals. I could also use mechanical RF relays to solve the isolation issue. Any issues with my idea?

author: joeqsmith
author_rank: Super Contributor
timestamp: June 07, 2024, 01:07:04 am
content: How low of a frequency do you plan to support? Are you planning to support more complex calibration methods? Did you try your 7-18GHz couplers are 22GHz and found good enough performance?Keep us posted.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: June 07, 2024, 02:04:12 am
content: Have you seen this here:<https://www.analog.com/en/products/adl5960.html> ?regards, Gerhard(who is just writing a driver for LMX2594/95 )In the end, directivity performance is the result of a lot of mathinvested in 12-term-error correction etc.

author: szoftveres
author_rank: Regular Contributor
timestamp: June 07, 2024, 02:16:57 am
content: If I could change only one thing with the existing LiteVNA, I would probably modify it for lower output power (e.g. attenuate the source by 20dB, and add 20dB LNAs before the mixers). It would enable measurement of active devices in their linear region without giving up of dynamic range of the VNA.Beyond that, an option to be able to switch between "normal" or "-20dB" output would be the ultimate usability improvement for me.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 29, 2024, 08:14:32 am
content: I bought a LMX2820 test board from Aliexpress for around $120 (Link:https://www.aliexpress.us/item/3256806758049656.html, it was also on sale when I bought it).The chip seems to work well, but amplitude falls off quickly after 20 GHz. I'm not sure if this is due to some random resonance in my setup or not (probably need to tighten down all the connectors), but the amplitude does drop sharply after around 20 GHz. I was able to get 0dBm out at 20 GHz but only around -5 to -6dBm out at 22 GHz. At lower frequencies the output amplitude is close to 7dBm.I think a lot of it is due to FR4 losses. In any case -6dBm + 12 dB from an amplifier is enough to drive a mixer (but there probably needs to be some kind of leveling to avoid damaging the mixer at lower frequencies).

author: ftg
author_rank: Frequent Contributor
timestamp: July 02, 2024, 02:42:18 pm
content: Asking FR-4 and SMA's to do 20GHz might be a bit much.I'd blame the lossy FR-4 and then the PCB->SMA transition might not be the best available.That said, the manufacturer does claim that the SMA's used are infact 3.5mm connectors made for 26.5GHz.The claimed output power is 0dBm on 20GHz, -0.8dBm on 21GHz, -3.8dBm on 22GHz and -3.5dBm on 22.5GHz.I also grabbed the images of the from that Aliexpress listing so that they can be seen more easily and will be available even when that listing is gone.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 03, 2024, 05:01:07 am
content: Here's my annotated version of the board.There are some dubious design choices (as with all cheap Chinese boards). The main ones are:1) Using the same loop filter components as the datasheet while using a different PFD frequency (though 140 MHz vs 200 MHz might be close enough that this doesn't make a difference, assuming they are multiplying the input frequency by 2 and then 7)2) Loading the output of a (presumably CMOS) oscillator with 25 ohms (2 x 50 ohms). I'll probably try removing them to see if that does anything.3) Not using C0G capacitors for the loop filterThey're also using LC filters for the power rails, and I doubt they've considered resonance.Anyways, it looks like I might be able to get the chips for $50 each off LCSC, which is much better than the $107 that Arrow charges (or $123 for Mouser). I've had good luck buying parts off of there, and they should be legit parts.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 03, 2024, 02:18:07 pm
content: What is the bandwidth of your loop filter?"About 10 degrees Celsius

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 04, 2024, 12:32:33 am
content: Removing both 50 ohm resistors seems to have decreased phase noise by a few dB (which makes sense, since loading a CMOS oscillator with 25 ohms is not a good idea).Other changes I tried (such as more decoupling and adding resistors to get rid of potential resonances) didn't do much at all.The power supply filtering is also not perfect, since some switching noise gets in. When I power it off a crappy USB power bank, you can see 500kHz noise from the switching.Also, it appears that the PFD frequency is only 10 MHz, which is different from the datasheet's 200 MHz. This might mean that the loop filter needs to be changed. I determined the PFD frequency by looking for reference spurs, and there seems to be one at 10 MHz no matter what frequency I choose.Edit: Dropping the PFD frequency by 20 times means the loop filter's bandwidth is now 20 times less - which is not very good.Also, here is the expected phase noise (with 200MHz PFD frequency) and here is the actual phase noise (with 10 MHz PFD). You can see that the bump around 30kHz lines up with my spectrum analyzer measurements

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 14, 2024, 03:54:09 am
content: I'm currently working on the PCB design for the new VCO.I found two couplers from (presumably) some HP VNAs in my rusty parts bin. The first one had 12dB directivity (and a bent APC-7 connector which I "fixed" using pliers and quite a bit of force), while the other had a bit over than 20dB (I measured from 0 - 9.3GHz, which is the best my LiteVNA could do). Before someone gets mad at me for "fixing" RF connectors, I connected a piece of coax to act as a "port saver", and then I marked all the bad connectors with Sharpie just to make sure I never use those connectors again with good partsHere's what's inside the 12dB directivity one. It is much simpler than I expected, and a lot wider in bandwidth (2GHz to >9.3GHz) than ones made with microstrip. There is a triangular piece of metal which is clamped with pressure. Moving the triangle with a screwdriver didn't do much in terms of return loss or directivity, as measured by my cheap LiteVNA. The directivity was always around 12dB at lower frequencies.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 28, 2024, 08:56:50 am
content: I have finished the schematics for the boards.The MCU board is going to be 100x100mm and 2 layers, while the two smaller 50x50mm oscillator boards will be done using 4 layers. The MCU board consists of the STM32F407, which should be able to handle I2S, as well as a TLV320AIC3204 ADC, which will sample the downconverted signals from the mixer. I am planning on using an IF frequency of 12kHz, which is exactly a quarter of the sampling rate of the ADC (which will allow me to do math very easily as sine/cosine is always 1, 0, or -1). The MCU board also contains the power supply for the RF relays, as well as some drivers for them. Finally, the MCU board also controls the gain of all the amplifiers, allowing some crude leveling to be done. I have some more directional couplers, so it might be possible to do ALC later with cheap diode detectors.The oscillator board contains the LMX2820, as well as protection circuitry for the RF amplifiers (MAAM-011100). The comparator should make sure that the transistor can't have 5V connected to it when the negative bias for the gate is not present.I've also ordered all the parts already. The MAAM-011100 was bought on Richardson RFPD (https://shop.richardsonrfpd.com/Products/Product/MAAM-011100) for $5.2 each, while the LMX2820 was bought on LCSC for $49.7 (https://www.lcsc.com/product-detail/RF-Misc-ICs-and-Modules_Texas-Instruments-LMX2820RTCT_C2871859.html, you will have to submit an RFQ). All the other parts cost basically nothing.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 04, 2024, 09:23:10 am
content: I've ordered the boards (after making some changes to my PCB schematic and layout). If anyone wants to mess around with the design files I also shared them (use JLC04161H-7628 Stackup for the 4 layer board).I also was finally able to get my hands on a transfer switch and an absorptive relay for $5 each at the ham swap meet. This will simplify my design a lot, as I no longer need 3 reflective relays to switch between the transmitted and reflected signal. They are both DC-18GHz (one of them has the markings rubbed off but the part number seems to end in 33S60). I guess I might have to change my driver circuitry since the relays take TTL inputs instead of 28V (the controller is inside the relay).

author: points2
author_rank: Regular Contributor
timestamp: August 16, 2024, 11:58:33 pm
content: hi EggertEnjoyer123,1st, thanks for sharing about such a high-profile project !far from what I could...2nd,do you have an idea about / can you rank : what matters in this "upgrade from a nanoVNA" so to speak...?To jump to 22Ghz, of course you can reply : all matter ! and you're right.but,what's the influence of the oscillator- vs the perf of a such device ? (nano-VNA & your project) ?- vs the other parts to improve : PCB design / SMA in/out / perf of the IC.. etc...my e-tweak level is currently about oscilllators

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 18, 2024, 07:53:37 am
content: Quote from: points2 on August 16, 2024, 11:58:33 pmThe VNA works by sending out a signal of a certain frequency and measuring the reflected/transmitted signal. The LiteVNA has a PLL IC (MAX2870) that generates the 6.3 GHz signal (I blelieve they are cheating and using the chip out of the rated frequency limit, but it works anyways). You can go past 6.3GHz, but then you're using the harmonics of the PLL IC to generate the signal (so at 7 GHz, the oscillator IC will actually send out 3.5GHz, and we measure the 7GHz harmonic). The downside of this approach is that with amplifiers, it could be the case that the lower harmonic saturates the amplifier, which would throw off your measurements). Of course, my design isn't much better (since the LMX2820 outputs a square wave at lower frequencies), and commercial VNAs have filter banks to deal with this issue (but that would cost too much for me). For a VNA, you actually need two oscillators (one to drive the mixer and one to drive the output of Port 1). The second oscillator is set to a slightly lower frequency than the first oscillator (for me, probably somewhere around 10-20kHz lower, since the output would then be in the audio range, and I can use a cheap audio ADC to digitize it). That way, after mixing you get a very low frequency which is much easier to analyze and get the magnitude/phase information from.The architecture of my design is going to be pretty much the exact same as the LiteVNA, but with higher quality parts. You'd actually be surprised by how much the LiteVNA creators got away with to save money. The mixer that is used in the LiteVNA is only rated to 3.8 GHz, which is why the S21 noise floor degrades after that frequency. The switches in the LiteVNA are actually only rated for 3 GHz, and leakage becomes a big issue at 6 GHz (another reason why the noise floor increases). The directional bridge in the LiteVNA is built using ferrite baluns, and those also lose directivity at high frequencies (causing your S11 noise floor to increase). In my design, I'll use mixer modules that are rated to 26.5 GHz, some 18GHz directional coupler modules (hopefully they still work at 22 GHz), and mechanical RF switches to solve all of those issues. The LiteVNA also is unable to switch the ports (you can get S11 and S21, but not S12 and S22), but my design will have a transfer switch.PCB design is actually a lot easier than you'd think. Since all the chips are matched, routing the PCB is just making sure all the traces are 50 ohms impedance. The SMA connector just has to have a center pin with the same thickness as the trace. The return loss of the entire system is dominated by the amplifier MMIC I'm using, which could have S22 as bad as -5dB. The MMIC return loss will definitely dominate over everything else in the system. To fix the return loss issue I'll probably just put an attenuator after everything. Also, I'm still using FR-4 because the Aliexpress board uses FR-4 and the output amplitude is still high enough to get to the 10dBm needed to drive the mixers (if I use two amplifiers in the middle). Switching to a good substrate like Rogers would cost way more than the $5 I spent on the extra amplifier. The only other real gotcha is making sure all the SMAs are torqued correctly. Unfortunately, the only torque wrench I have is from Aliexpress.I also got all the boards back, and I'm working on assembling everything. So far I found a mistake with the TL431 on one of my boards. Going to have to bodge that one...

author: points2
author_rank: Regular Contributor
timestamp: August 20, 2024, 09:59:18 pm
content: Thanks EggertEnjoyer123 for this precise feedback.To be honest, I had a look at a NanoVNA-H4 a while ago, noticed a few things to test in order to improve it, but I switched to another topic in the mean time.A full PCB design is out of my current skill (but it's on my road map :-) ).I just do some works looking at low level PCB behavior, I mean : PDN.Given the PDN influence on any transient state (clock signal is a "basic" transient state that occurs on a PCB at low level), a specific care upon the PDN of the main crystals/oscillators can improve the overall efficiency of the device (tested on a FMC, fiber-media-converter vs RT applications).So, why not in a such a device with transients all over the place ?Oups ! EggertEnjoyer123, here is just my humble input... and your feedback is appreciated (bad or bad :-) )

author: Bud
author_rank: Super Contributor
timestamp: August 20, 2024, 10:20:24 pm
content: Why do you need 22GHz?Do you have an idea how are you going to calibrate such device in-place?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 22, 2024, 05:27:15 am
content: Quote from: Bud on August 20, 2024, 10:20:24 pmMostly just for fun (though it would be nice for 10 GHz stuff if I ever get into doing that)I'd probably need to buy an actual calibration kit (which would cost several times as much as my entire project) if I ever wanted to get good results. But for now I could probably just see how far I can get with the LiteVNA cal kit.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 24, 2024, 08:40:18 am
content: I got the boards back and I assembled some of them. So far there's been a few minor problems (I forgot the ferrite bead for the crystal oscillator, and I messed up the pinout of one of the TL431s).Programming the firmware is quite annoying, since bugs will cause the chip to sometimes work and sometimes not work. In the first spectrum analyzer picture, we have the amplifier running at a lower control voltage (-1.5V vs 1V), and the amplitude is about 25dB less than the maximum output. I also had the attenuator in place (made of 0201 resistors). Each peak is spaced apart by 1GHz. There is definitely enough here to drive a double balanced mixer at full power. Unfortunately the firmware broke right after I took the picture, and it refuses to lock to 20GHz (but it locked to 22 GHz). The spurs were mostly gone after I put in a ferrite bead to kill any 100MHz noise from the crystals from coming back through the power rails. It looks like I might need more filtering in a later revision.I ended up removing the attenuator to get more power, since I found that there wasn't enough output power at 1V control voltage. At -1.5V control voltage, the amplitude increased by 4dB, but the amplitude only increased by 1.4 dB at 1V control voltage, which means my amplifiers are in saturation. I measured with a power meter and I got 8.6dBm, compared to the spectrum analyzer's 5.6dBm. Assuming that there's 4dB loss through all the connectors and through the FR4 (which is very lossy at 22 GHz), I think 8dBm would make sense since the datasheet has 12dBm for the P1dB at 20 GHz.I'll probably have to fix all the bugs this weekend and maybe try to get more power by increasing the gate voltage, which should allow more current to flow through the device.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 26, 2024, 10:25:25 pm
content: Here's a plot of the output power from 16 GHz to 22.6 GHz. For some reason, the board loses lock at high frequencies (probably a VCO calibration issue). I found the issue with my board, which was that one of my ferrite beads broke and there was no power supplied to the crystal oscillator. I also messed up my SPI setup in STM32CubeIDE.The power is over 10dBm up to 20 GHz, which is definitely enough to drive a mixer. I could probably just take a chance and hope that there's enough power to drive the mixer at 22 GHz (since generally mixers still work at lower LO powers, just not as efficiently).

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 11, 2024, 10:01:50 pm
content: I fixed all the issues with my code, and I soldered up and tested another oscillator board. I also did crude leveling by changing the gain of the amplifier using the control pin.It also seems like you can get >23GHz out of the chip even though it's only rated for 22.6. The amplitude sucks though.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 12, 2024, 06:56:47 am
content: Here's my results. The isolation was measured by disconnecting the coax for port 2 from the coupler, and terminating everything in 50 ohms. 50dB gain was applied to the signal to boost it to a level which can be measured. The isolation isn't great (60-70 dB) compared to commercial VNAs. I'm not sure whether this is from the RF signal leaking to the LO of the mixer and somehow making it into the other port, leakage through the power rails, leakage through the control port, or just radiative leakage (since I have no metal shielding on my RF boards)The open and short look good at all frequencies (they should be 180 degrees apart). The load looks good at all frequencies except for 22 GHz. The coupler is probably losing directivity at 22 GHz, or it could be that my load is not rated for that high of a frequency (but I don't think that's the case since I tried quite a few of them). Still, it might be possible to fix that with calibration (not ideal), and maybe get a decent result. My "cal kit" comes from the LiteVNA, so it probably sucks.My IF frequency is 12kHz because that is 1/4 the sampling frequency of 48kHz. I chose 12kHz since sine and cosine are all -1, 0, or 1, so no multiplication or math is needed to get the phase and magnitude.Edit: the Narda 5292 1-18 GHz coupler seems to work a lot better than the 3096 ones I'm using now. The coupler can be had for around 100 bucks on ebay

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 13, 2024, 07:38:27 am
content: Here is a sweep from 1 GHz to 22 GHz with open, short, and load, using a Narda 5292 1-18GHz directional coupler.I am still not sure why the amplitude dips by 6dB at 6.5 GHz when the output is shorted. Tightening the connectors more did nothing. There are weird spikes in the "load" trace after 18 GHz, but I think there's probably still enough difference between open, short, and load that the VNA should have no trouble differentiating between them.I connected a LiteVNA with the output shorted, but I don't see any dip (6dB should be very noticeable). The worst S11 that the amplifier should see is -12dB, since there is a 6dB attenuator in the signal path. I don't think this is enough to cause 6dB of mismatch loss.I also figured out that isolation can be fixed easily, since putting a piece of copper between the two PLL boards increased isolation by around 10dB. I'm also wondering if it's a good idea to print a conductive case out of ESD PLA or conductive PLA.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 15, 2024, 07:42:06 am
content: After doing some further experimentation, I concluded that the dip at 6.8 GHz was actually just due to me not waiting long enough for the output to settle. After increasing the time between frequency changes, the dip went away. However, it seems like there is a fundamental limit for APC-7 based couplers, in that they seem to have a resonance a bit over 19 GHz.In the first plot with the big 1-18GHz directional coupler, I am sweeping from 18 GHz to 22 GHz. Each "step" is a change of 100 MHz. You can see that at 18-19 GHz, the amplitude of the green trace (load) is significantly smaller than the amplitude of the open and short (yellow and pink respectively). This makes sense, because the load should reflect a lot less power back compared to the open and short. However, at around 19.6 GHz, the green trace grows in amplitude while the pink trace goes down (in fact the green trace actually has higher amplitude than the pink trace). This is probably because the APC-7 connector is only designed to work at 18GHz. I'm not sure exactly why the system resonates at that frequency (I have tried several different loads and this happens every time). I think it might be due to multiple modes existing inside the coupler/connectors at that frequency?Before someone asks, I am not using the Chinese APC-7 adapters, so that can't be the problem.In the second plot, I switched out the directional coupler for a 12.4 - 18GHz SMA one. As you can see, the green trace remains smaller than the pink and yellow trace even though I am using the coupler above the rated frequency, and there is no frequency where the green trace comes close to beating the pink or yellow trace. This is what the behavior should look like for a properly working directional coupler. I think I will probably need to buy a wideband directional coupler with SMA connectors instead if I want to go above 19 GHz. I know HP uses regular SMA connectors up to 26.5 GHz, so there shouldn't be any issues below that frequency.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 17, 2024, 03:19:42 pm
content: What mixers are you using?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 18, 2024, 01:53:34 am
content: I'm using two different mixers (I have multiple mixers but I can't find two that are the same).The P/Ns are:MMDC-00126 (1 - 26GHz)STDB-1001-AL (also 1 - 26 GHz)The datasheet for the STDB-1001-AL is attached (see page 32). The internals of the MMDC-00126 are also shown here:https://www.eevblog.com/forum/rf-microwave/inside-an-rf-mixer-and-rf-sampler/

author: arlo_g
author_rank: Contributor
timestamp: September 20, 2024, 10:31:18 pm
content: Looks like a fun project and you have some great parts to work with!Your photo of the inside of an HP directional coupler is quite interesting. It looks like yours is similar to the 779D couplers. They claimed in the HP journal articles about those that the “arrowhead” metal triangles was important for directivity.I think that most high performance VNA’s since the HP 8510 have used directional bridges for frequencies below mm wave. Directional bridges can be built with coaxial baluns to deliver 1000:1 frequency ranges, while getting even a 50:1 frequency range with good directivity and uniform coupling out of a directional coupler is pretty heroic. The Julius Botka “triaxial bridge” patents have a lot of good detail about the very high performance 8510 bridges. Henrik Forsten seemed to get good performance to 6GHz from his directional bridge build, but I think that he hadn’t fully enclosed them, and higher frequencies are likely a challenge on several fronts. Since you have nice directional couplers already, it makes perfect sense to use them.If you wanted to bootstrap your way to a calibration kit, then a coax LRL calibration would be a good place to start. A single “line” length (assuming you don’t count your 0 length through as a line) only gives a good calibration over an 8:1 frequency range though, so multiple lines, or hybrid schemes like LRM would be needed to cover your full range.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 20, 2024, 11:40:38 pm
content: My plan in the future is to use two different paths, with one going to a high frequency directional coupler and one going into the classic return loss bridge with balun. An RF switch could be used to switch between the two depending on the frequency, which would let you cover both frequencies in the low megahertz and mmwave frequencies.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 25, 2024, 07:57:14 pm
content: I finally got the S11 measurement to mostly work. (For some reason my code doesn't work when I try sweeping from 5 - 22 GHz, but it does when I try sweeping from 18 - 22GHz. It also breaks if I try doing more than 201 points).Calibration was done with the cheap calkit from the LiteVNA. I'm pretty sure it's not good at 22 GHz.The setup is mostly similar to what I did before, except I'm using some random splitter in my collection to get the reference signal. I'm pretty sure the performance of the splitter isn't critical since any errors will just be calibrated out. The antistatic bag helps increase isolation between the two oscillator boards (I found in my previous tests that adding the bag increased isolation by at least 10dB).I calibrated everything using the set, then I took off the calibration sets and remeasured them to determine repeatability. It looks like we're doing better than 0.5dB. I also measured a shorted transmission line, which seems correct (we should expect the line to go around the Smith Chart). The shorted transmission line is just two SMA barrels and the short from the cal kit.I also measured a 10dB attenuator, with the other end open and terminated.Finally, I verified that my setup was indeed sweeping from 18 - 22 GHz. The amplitude in the picture is a lot lower than expected since the signal had to go through a lot of coax to make it to the spectrum analyzer.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: September 25, 2024, 08:18:03 pm
content: I can't really contribute too much but maybe this could get you a big step forward:https://github.com/scott-guthridge/libvna13GHz would be a reasonable compromise for be.... That's right above where you can get troubles with 2.4G ISM radio.But I tend to look into TDR for this because CML Logic seems fast enough for the "heavy lifting".However, if you need some testing... I have a receiver that can do 26.5GHz and some anechoic room good for double digit GHz is coming soon.Regards

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 25, 2024, 09:56:56 pm
content: I fixed one of the software bugs and now I can sweep from 3GHz to 23GHz. Even though my coupler is rated for 12.4 - 18 GHz, there seems to be enough directivity left at lower and higher frequencies for my system to work correctly. The amount of signal coupled out definitely changes, but that doesn't matter and only the directivity matters.Here are the new plots from 3 - 23 GHz. I also measured the return loss of a 6.2 GHz lowpass filter. I think I need better calibration standards since the LiteVNA ones suck at high frequency.I also plotted the return loss of a 17.7 - 19.7 GHz isolator, and a WR18 waveguide adapter.(In case anyone is wondering, I basically programmed my board to act as a LiteVNA, and I am using the NanoVNASaver application to display my result).

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 27, 2024, 12:19:36 am
content: I now have S21 working. An RF switch is used to switch between measuring S11 and S21.The isolation sucks (maybe -30dB at some frequencies), but that is probably because I have no shielding. All my measurements have zero averaging so far. My C program takes 32 samples at 48 kHz (the IF is 12kHz) and just adds/subtracts every other sample to get the sine and cosine amplitude.I'm also not sure why the S21 measurement isn't accurate below 6 GHz. I think it might be some weird programming error, because the points seem to be in the same place between sweeps if a 10dB attenuator is added, but without the 10dB attenuator the points jump around. It could also be that I'm saturating my ADC or mixer.Here are my measurements for a 10dB attenuator, a 17.7 - 19.7 GHz isolator (both directions), a 6.2GHz lowpass filter, and a directional coupler which seems to be a 2-8 GHz coupler with -16dB coupling according to my LiteVNA (coupling and isolation were measured). The isolation is also shown. I wonder if it's possible to build a good RF shield using conductive filament.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 28, 2024, 08:16:21 am
content: I tried wrapping everything in the tinfoil bags that you get when you buy SMD components. That didn't really help much.The interesting thing is that the isolation goes up with averaging, so there is some random aspect. Averaging 10 times seems to improve the isolation by around 10dB (the two plots are from an uncalibrated setup, so you can't compare them with my previous results, and the actual isolation is around 5-10dB worse). You can see that the peak goes down, but at other frequencies the isolation is the same, which is what you would expect if the leakage path is deterministic. (If you have a 70dB attenuator connected between the ports, no matter how much averaging you do you will always get 70dB). I've ruled out the issue being the RF switch (which is only rated to 20 GHz) and the mixer leakage, because the problem remained even after disconnecting the reference signal and the reflected signal. It could be the power rails potentially, or maybe the control voltage for the amplifier. Touching the pins with my fingers had no effect though.The isolation also "increases" after 22 GHz, but that's only because I run out of power to drive the mixers, so the conversion loss increases by 20dB.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 29, 2024, 12:24:23 am
content: It seems like to get TDR to work, I need to have low frequency data, which I can't do right now without having to splice together S2P files from the LiteVNA.In any case the software I'm using (NanoVNASaver) does not calculate the TDR correctly. I have submitted a pull request to fix the issue:https://github.com/NanoVNA-Saver/nanovna-saver/pull/715The software takes the raw S11 data, windows it, and then does an inverse FFT to get the impulse respose from the S11 data. The resulting signal is complex, while the impulse response should be real, so the developer used the absolute value function to make it real. This is the wrong way to do it (for starters, a short and open would look exactly the same, since their S11 values are just negatives of each other, so if you take the absolute value you will get the same result). The correct way is to take the S11 data and append the conjugate of the measured data. Basically, for real signals, the FFT must be symmetric (in that the negative frequencies must be the complex conjugate of the positive frequencies). After adding terms to the end so that the negative frequencies and positive frequencies are conjugates of each other, if you take the inverse FFT, the signal is real, and you get the correct impulse response.I probably need to design a better VNA which has a copy of the LiteVNA circuit, and some RF switches to switch between the high band (>2 GHz, which I have now) and low band (50kHz - 2 GHz). That way I can do TDR.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 29, 2024, 04:01:44 am
content: I also have a major issue with thermals. At high frequencies I run out of power to drive the mixers, and as a result the conversion loss is highly dependent on the LO amplitude. As everything heats up it seems like the output amplitude goes down. The difference between hot and cold is quite a bit (like about 5dB), but it only seems to be an issue above 21 GHz.I calibrated my setup, and then power cycled everything a few times. As you can see, for all frequencies below 21 GHz, nothing bad seems to happen. However, since after power cycling the board is cooler, the amount of LO power going to the mixer is higher, which means the conversion loss is lower. If the board is completely cold the peak reaches about 6dB, and drops below 0dB after the board is warmed up more. All frequencies below 21 GHz remain the same though since there is enough LO power to make the conversion loss roughly constant.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 29, 2024, 03:17:52 pm
content: Quote from: EggertEnjoyer123 on September 29, 2024, 12:24:23 amFrom Brian and CMT, they use chirp-Z transform:https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5656049/#msg5656049

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 30, 2024, 02:53:33 am
content: The software I'm using doesn't seem to implement that. I read through the code for the TDR measurements and it just does the IFFT. Nothing fancy.Anyways I swapped the two mixers and now I seem to have >50dB isolation across the entire band. I'm not exactly sure why this happened. The blue line is what I get when I connect a cable, and the brown line is with nothing connected. The difference should be the isolation. There is a peak in the blue and brown lines near 22 GHz but that is just because the mixers are running out of LO power, and one of them seems to be significantly better at lower amplitude than the other (probably because they're using different diodes). In my next design I'll probably pay $30 more and buy higher power amplifiers.I'm not sure what's causing the remainder of the error. If I disconnect the ADC the noise floor is around -90 dB, so it's not the ADC. It's not the RF relay either.Edit: my IF bandwidth is 1.3 kHz. Here is the frequency response of my "filter" (which is just correlating the sequences 1, 0, -1, 0, 1, 0, -1, 0, ... and 0, 1, 0, -1, 0, 1, 0, -1, ... with the received signal at 48 ksps). The IF frequency is 12 kHz.

author: smaultre
author_rank: Regular Contributor
timestamp: September 30, 2024, 05:43:51 am
content: Why don't you try to buy some used HP? VNA 26-40GHz acquisition frontend?And add some modern, generators, ADC, & PC-based software?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 01, 2024, 03:45:48 am
content: Quote from: smaultre on September 30, 2024, 05:43:51 amGetting the S parameter test set for 22 GHz will probably set me back at least $500, and I'd still need to buy the PLL ICs which make up most of my cost right now. (Of course I get to save one IC because the HP test sets use sampling as opposed to mixing, but the cost difference is still quite a bit). The advantage of buying the HP test equipment is that I won't have to worry about shielding that much.Anyways I seem to have found the main source of error, which is the splitter. It turns out that the performance of the splitter does matter and can't be "calibrated out." The splitter is way too lossy at 20 GHz, and unfortunately I don't have any that would work at that frequency. My signal processing also has issues, because if I measure the isolation with an oscilloscope I get about a 9dB improvement.Here is the isolation with a lower IF bandwidth of 20 Hz. The filter frequency response for the IF is attached. The "peak" is at 20.93 GHz, where the isolation is only 50 dB.I swept from 20 - 20.01 GHz and I got an isolation of 70dB. Using an oscilloscope, I got 81 dB (the 12kHz peak had a power of -56.4 dBm with port 1 and 2 disconnected and with 50dB gain, and the 12kHz peak had a power of -24.9 dBm when port 1 and 2 were conneected).At 20.93 - 20.94 GHz, the isolation is under 50dB, but with the scope I got 58dB isolation. The power with port 1 and 2 disconnected was -46.4 dBm (with 50 dB gain), but the power with ports 1 and 2 connected dropped to -38.1 dBm. Because the power drops so significantly with both ports connected, and the isolation doesn't change that much (it was quite hard to measure because the amplitude of it is so small), I suspect the splitter to be the main culprit. Theoretically, the amount of leakage should be the same, as both the source and LO oscillator output the same amount of power at 20 and 20.94 GHz. However, the amount of received power with the two ports connected is significantly lower, so if we look at the difference, we will see a loss of isolation even though the leakage is the same.I think the reason why there's a ~10dB difference between the measurements from the ADC and from the scope are because of low frequencies. My digital IF filter does not filter out low frequencies that well, and looking at the oscilloscope readings there is a huge amount of low frequencies compared to the amount of 12kHz signal. (This is because the DC offset of the IF changes every time the frequency is changed, causing huge amounts of ringing). With a better digital filter, I should be able to eliminate this issue. (The oscilloscope capture shows the IF output with nothing connected at 20.93 GHz).

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: October 01, 2024, 11:31:08 am
content: Is there a reason why you ignore<https://www.analog.com/media/en/technical-documentation/data-sheets/adl5960.pdf>from post #3?bidirectional couplers, 6 GHz synth input with /2 ... *4 frequency gearbox, mixers,stepped if gain & if filters, interface to frequency offset osc, all controllable via SPI,sync-able to others for more than 2 ports, all on one chip??

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 02, 2024, 05:58:54 am
content: Quote from: Gerhard_dk4xp on October 01, 2024, 11:31:08 amI have a few reasons for not using the chip.1) The part costs $150. While it seems like I would be able to save money, this is not actually true. This is because the IC only does S11 measurements, and I need to do both S11 and S21 measurements. The IC does not have a source oscillator, so I still need to buy a LMX2820, and unless I buy another IC to do the S21 measurements (which brings my BOM cost to $300), I would have to buy another LMX2820 to generate an offset frequency to mix the received signal on the second port. Right now, my entire design cost me about 200 dollars in total (not counting extra parts I ordered). The two LMX2820s were $50 each, and I have 6 RF amplifiers at $5 each. Everything else on the board is <$5 and all the RF modules were bought for about $5 each from the ham radio swap meet.2) The part doesn't allow for much experimentation. I'd like to eventually see if I can do harmonic mixing, which is used by the original NanoVNA and the newer LiteVNA to extend the frequency range at the cost of decreased performance. If I can get it to work on my design, then I might be able to increase the range to 40 GHz (that's a big if though). With my current design I can just swap out the directional bridge to achieve that, but I can't do that if I use the chip, since it runs out of directivity fast after 20 GHz.3) The part does not support 12kHz IF frequency, so I'd probably have to choose a more expensive ADC instead of a cheap audio codec, and I'd need to do more data processing in the microcontroller, which means more programming (which I really don't want to do).There are several advantages though.1) With 2 ADL5960s, you can get all 4 S parameters without needing a transfer switch. A normal RF switch can be used to switch the source oscillator between the two ports. The downside is that you lose some dynamic range with S21 measurements. No matter if you use the ADL5960 or a mixer for getting the S21, you need to put an attenuator in front. Otherwise, the return loss of the mixer/chip would ruin your measurements because port 2 would not look like 50 ohms. For example, the return loss of an amplifier is highly dependent on the output impedance. The difference is that there is an additional loss due to the coupler inside. So if we look at the signal path before going to the mixer, in the current design there would be 10dB attenuation before the signal hits the mixer. In the ADL design there would be 10dB attenuation, but then the signal is not at the mixer yet. It has to go through the coupler, which adds another 10dB or so loss. Analog Devices does not share what the coupling loss is, but we can estimate because the insertion loss is -2dB. Assuming everything is perfect, the coupling has to be below 20 log (1 - 10^-0.2) which gets you -8.6dB coupling. However, this is assuming everything is ideal, and the actual number is probably 10.2) You can get measurements faster with 2 ADL5960s. Right now I only have 2 mixers, but I have three signals: reference, reflected, and transmitted. Having 2 ADL5960s would let you downconvert all three signals at the same time.3) You can do low frequency. My coupler runs out of coupling at megahertz frequencies, so I can't really get any data below 1-2 GHz.Anyways, if someone hired me to design a mass producible 20 GHz VNA, I would probably use the chip. But I'm just doing this at home for fun.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 02, 2024, 08:18:16 am
content: I did more research into the ADL5960, and I think my current design has superior performance when it comes to measuring S11 at least.I read the entire datasheet and the reference design datasheet. You'd think that a multi-billion dollar company would be able to provide plots showing them measuring a decent 50 ohm calibration load with the chip, but that doesn't seem to be anywhere in the datasheet at all.Anyways I'm basing my comparison on the following slideshow:https://www.analog.com/media/en/training-seminars/seminar-materials/01-20-ghz-small-form-factor-multiport-network-analyzer.pdfOn slide 26, we can see their measurements of (presumably) a 9.5 - 11.5 GHz filter. You can see that their S11 trace has a lot of noise at 20 GHz. Their graph is zoomed out a lot, but it appears to be around +/- 2dB. I'm getting pretty much zero noise to 20 GHz even with no averaging (see post 33). Of course, this might not be a fair comparison since I'm measuring a short that I calibrated the VNA with, while they're measuring a filter (which we don't know the actual S11 of). However, the fact that the points change so rapidly between close frequencies and the fact that it looks "random" as opposed to being smooth seems to point to this being a noise issue, and not an issue with the device they're measuring. If you look at post 30, where I measure a filter, the S11 curve seems smooth and while the S11 is obviously not correct (thanks to my bad calibration set), there seems to be no random noise.For S21 I don't think a fair comparison can be made. A picture of their setup is on page 16, and you can see that they have no shielding. So their S21 is not the best that it could be with proper design.I think my setup can also be improved more, but I can't find a good power divider at all in my collection. It would ideally be resistive and go up to a decent frequency (like 18 GHz)

author: tszaboo
author_rank: Super Contributor
timestamp: October 02, 2024, 11:15:20 am
content: Quote from: EggertEnjoyer123 on October 02, 2024, 05:58:54 amI know that the nano and the liteVNA does this, but you really shouldn't use an audio CODEC for these sort of measurements. The DC/gain parameters of them is not characterized, or guaranteed. Same with INL or DNL. An industrial ADC will have all these parameters characterized and it will have a reference voltage, either built in or external. The reference voltage is something that has the same (meaning 1=1) importance in an ADC as the input. The audio codecs will only have this internal, with questionable accuracy. ADCs that will do 24 bit and 50-100KSPS are below 5 EUR in cost.The ADL5960 is very interesting, but indeed you need one on every port of a VNA. I'm very tempted to attempt to build a VNA with it, but maybe after I'm done with the current projects that I'm doing.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 08, 2024, 02:30:13 am
content: I bought some SMA calibration loads which are similar to the ones from the LiteVNA and sent them to my friend, who has a 26.5GHz VNA in his lab. Here is the link:https://www.aliexpress.us/item/3256805982526182.htmlIt seems like the load is worthless (the seller claims 1.02 VSWR from 1M-6G which is laughable), and I should probably get my money back because the S11 is at -10dB at 6 GHz. The short and open seem to be good according to my friend's VNA, and there seems to be very little variation between two different opens and two different shorts, so you could probably use them for calibration up to 26.5 GHz.The offset delay for the short is approximately 0.077 ps. The open seems to be nonlinear, but I'm sure someone here can help me get the C0, C1, C2, C3, and offset delay from the data.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 10, 2024, 03:57:58 am
content: Here's my calculations for the coefficients for the open.I got the following numbers:C0 = 2.377218773138113e-15C1 = -3.77944625e-46C2 = 7.90828757e-36C3 = -4.21470232e-48Offset: 1.4244367406724633e-12 sMy Jupyter notebook is attached if anyone wants to check my work. I also attached plots of the regression vs actual data. My code sucks and all it does is try out all the offset values, and then it does a 3rd order regression, calculates MMSE, and chooses the offset which gets the lowest MMSE. I found out that low frequency data also has to be eliminated during the process.Apparently the cal standards for the Anritsu 22N50 are these values:Open: offset 17.83mm, C0=4e-15 F, C1=200e-27 F/Hz, C2=0 F/Hz^2, C3=1.1e-45 F/Hz^3Short: offset 17.83mm, L0=0, L1=0, L2=0, L3=0Looks like this agrees with what I found, namely that the short is perfect and the open has some fringing capacitance.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 10, 2024, 06:00:15 am
content: It looks like everything works now and I get good results. I had to experimentally determine the capacitance of my 50 ohm load though, by hooking up a short transmission line and manually adjusting it until the ripples were small (and were equal on both low and high frequency). After that, I tested the filter and I was surprised to see that the S11 line was completely flat, which means that my calibration worked. I think the program has a bug where it would randomly write "50" to C0, but it doesn't actually put in 50 (otherwise my results would be completely different)I used the same numbers from the previous post. Here are the short, open, and load after measuring with the calibration standards set. As you can see, they are all reasonable and match the results that my friend obtained before. The load is also somewhat reasonable (the VSWR is 1.32 at 18 GHz). This small difference is enough to throw the measurements off by 3dB though.I guess good open and short standards exist and are fairly easy to find. Getting a good load might be extremely tricky though, and not everyone has a friend who works in a microwave lab.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 10, 2024, 06:33:20 am
content: It looks like the software is buggy and randomly replaces my C0 value with 50 fF. Going to have to correct that one and then pull request it on Github...You can see the difference between the actually measured open and my "calibrated" open. I bet the 55fF of capacitance goes away when I correct for this.Edit: I changed it back to 2.377 fF and I deleted the 55 fF of capacitance from the load. As you can see everything works now, and I didn't even have to guess any values! The load I'm using is from SV Microwave, I found it in the dumpster at school and it has a blue dielectric instead of the standard white one. It seemed to have the lowest amount of reflections so I picked it to use for calibration.It looks like this:https://octopart.com/sf8018-6005-amphenol+sv+microwave-30479673?r=spbut brass instead of silver. Anyways, for the price of $20 you can buy a <1.05 VSWR load to 18 GHz. Quite impressive...Edit again: I missed the +0.01 * f part. Whoops. A 1.1 VSWR load is fairly inexpensive though:https://www.centricrf.com/terminations-loads/sma-terminations/sma-terminations-loads-6-27ghz-1-2-watts/c18s6-sma-miniature-male-termination-1watt-18ghz-vswr-1-10-s-steel/and it's probably good enough for government work.Also these are $15 each and judging by the typical performance data they should be much better than 1.1 VSWR:https://www.fairviewmicrowave.com/product/rf-terminations/coaxial-rf-terminations/sma-terminations/rf-load-1-watts-18-ghz-sma-male-st1827.html?srsltid=AfmBOooyBVU_AtmYB_hiXh7Weh9wFROYSK9GgAz4zkoQmdc0K1jBYl3j

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 11, 2024, 03:30:55 am
content: It looks like none of the Chinese loads I have in my possession can be used anywhere close to 18 GHz, when compared to my "good" load.All of them have return loss under 10 dB at 18 GHz. The return loss of the load that came with my LiteVNA looks like the attached picture.The only good way to get a decent calibration set to mmWave frequencies is probably by buying from a reputable company. I'll probably have to order a few of these to test soon:https://www.fairviewmicrowave.com/content/dam/infinite-electronics/product-assets/fairview-microwave/product-datasheets/ST1827.pdfAlso I've changed my mind on the ADL5960, and I think it's probably not as bad as the graphs in the presentation show. The design that Analog Devices chose to use in their slide deck (https://www.analog.com/media/en/training-seminars/seminar-materials/01-20-ghz-small-form-factor-multiport-network-analyzer.pdf) is laughably bad. The total cost of those parts is probably well into the thousands too when a decent VNA using their chips could be made for $500. (I guess they didn't want to use TI PLL chips, but they have their own PLL chips and they could have just selected two of them instead of using the current overcomplicated setup). It looks like the LO and the source oscillator are at the same frequency, since the output of the upconverter they're using is differential so both the LO path and the source path must have the same frequency. This is stupid design because any leakage from the LO to the source will mess up your measurements. You can try to calibrate it out, but it's going to be impossible to get any good dynamic range out of it. I'm also not sure how they're getting frequencies below 6 GHz if their upconverter is only rated for 5.9 GHz. Anyways, some Keysight VNAs use integrated circuits to replace the coupler/mixer/etc, so I think the chip has potential. I'm honestly not sure why their application engineers never thought about making an actual well-designed VNA using their chip if they're going to spend millions developing it.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: October 12, 2024, 04:20:11 am
content: Your synthesizer price seems quite optimistic when compared toan official distributor. When creating new things I can't proveanything when using parts from unknown heritage. 1 hour lossand the advantage is gone.They only need a 6 GHz synthesizer: A 0.5 divider and atimes 2 and times 4 multiplier are included, and equal mixers,and mixer LO drivers for the entire range.IIRC someone in a neighbor tread declared an ADF**** asking of the hill with regard to phase noise at 22 GHz, so theyprobably don't need TI.I must stop now. Ressurected from the Death after 3 days.Probably Covid. Still don't dare to go downstairs.Gerhard

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: October 14, 2024, 03:44:47 am
content: Quote from: Gerhard_dk4xp on October 12, 2024, 04:20:11 amThe LMX2820 is $54 on LCSC though, and I've already purchased three with no issues.QuoteThe ADL5960 still requires an external source oscillator to generate the 0.01 - 20 GHz. If you look at the block diagram of the chip, there is nothing connected to RFIN. You have to set up your own oscillator and feed the output into the chip.Also, an 8GHz LO frequency is still required, because the doubler works from 2-8 GHz and the quadrupler works from 4-8 GHz. In 4x mode the minimum frequency is 16 GHz, so if you want to set the LO to 14 GHz you must put in 7 GHz while having the LO circuit on x2. The LMX2594 is $21 on LCSC though and goes up to 15 GHz, so it's not really a problem.QuoteAll of the chips have roughly the same jitter at 6 GHz (around 35 - 40ps). Not sure what the jitter is like at 22 GHz but given that all the chips use multipliers to get there I don't think there is much of a difference.Anyways I'm now convinced that the ADL5960 isn't as bad as I first thought it was, and that the bad performance is probably due to their designers being bad. At least there isn't really anything fundamentally wrong with the chip based on the datasheet. I think I'll try using it in my next attempt since my current design would require the use of an RF switch to select between a low and high frequency path, and this chip doesn't. Without a low frequency path, it would be cumbersome to do TDR measurements.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: November 23, 2024, 03:38:00 am
content: I sent a bunch of 50 ohm loads to my friend, who measured them with his lab's fancy network analyzer. I bought 7 loads from some seller on eBay.It seems like load 2 (the blue line) is the best load, and the VSWR is <1.05 up to 26.5 GHz. I have some supposedly 1.15 VSWR loads from China which are 2.92 mm, and I will have my friend measure them (they should be good up to 18GHz hopefully). Load 8 is a cheap calkit load for comparison.The NanoVNA software has no feature to import an S11 file, but it seems like the functionality can be easily implemented.

author: gf
author_rank: Super Contributor
timestamp: November 23, 2024, 09:18:28 am
content: Quote from: EggertEnjoyer123 on September 29, 2024, 12:24:23 amIt is not "wrong". There are two modes for VNA-based TDR, bandpass mode and lowpass mode. What this software does is bandpass mode. It still enables "time to fault" measurements w/o requiring low-frequency measurements down to DC.QuoteWhat you describe is lowpass mode. Untorfunately, it requires equally spaced frequency points from 0Hz (DC) to fmax, while many VNAs have a lower frequency limit.Quote from: joeqsmith on September 29, 2024, 03:17:52 pmThere is still nothing wrong with doing a IFFT in order to calculate the full time axis (possibly with zero-padding - after applying the window function - in order to get a higher (interpolated) resolution for the resulting time domain points).Consider the ICZT rather as a tool to calculate an arbitrary "zoom-IFFT" with interpolation. For example, if you want to zoom into a small range of the full time-axis, or if you want an arbitrary spacing for the resulting time domain points (independent of the frequency points).

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: November 24, 2024, 06:56:32 am
content: Quote from: gf on November 23, 2024, 09:18:28 amIt seems like you are right, but their previous implementation was still wrong, because they convolved the absolute value of the impulse response with the step function to try to get the step response, which doesn't work (you just get a stairstep up).I modified the nanovna-saver code again so you can select between bandpass and lowpass modes. I think the chirp Z transform is probably going to be too hard for me to implement, plus all the processing is done on a computer anyways which can easily do million point IFFTs in microseconds so there is no advantage to using it. (I'll have to admit that this is the exact kind of thinking that makes software slow nowadays)

author: gf
author_rank: Super Contributor
timestamp: November 24, 2024, 08:00:37 am
content: Quote from: EggertEnjoyer123 on November 24, 2024, 06:56:32 amIMO, only impulse response (magnitude) makes sense in conjunction with bandpass mode, and step response should not be offered in the GUI. I think other VNAs don't offer bandpass step response either.EDIT:So basically I see the following three choices:- bandpass impulse response (magnitude)- lowpass impulse response (real)- lowpass step response (real)

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: November 29, 2024, 01:23:32 am
content: I implemented bandpass TDR and importing S1P files for calibration in software. (Unfortunately, I don't have the loads that I sent my friend yet, so I can't get any accurate measurements)I also built a resistive splitter out of standard 18 ohm 0201 resistors. They are all soldered upside down to reduce parasitics. The S parameters look good up to 21 GHz (I need to get better amplifiers so I can go higher in frequency).After swapping out the old splitter with the new resistive splitter, I have 60dB of isolation for S21. I think I can improve it even more by fixing my DSP. I also measured a Beatty standard and a through, which were made by JLCPCB. I think I need to make the 25 ohm section longer on the Beatty. Also, it seems like my soldering is bad for one of the connectors on the through. (The reversed measurements were taken by flipping the board and measuring again). The theoretical reflection for the Beatty is 0.33.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: December 06, 2024, 01:59:20 am
content: I found another source of calibration loads.The loads here seem to be good up to 26.5 GHz at least (my friend can test them up to 40 GHz but it will take a while):https://www.aliexpress.us/item/3256807233299645.htmlI bought two of the <1.15 VSWR ones, and I got the following response (looks like the seller might be honest).The seller also offers 18GHz loads with VSWR <1.06 here:https://www.aliexpress.us/item/3256807162155931.html.I suspect someone is getting paid to measure and bin these. I haven't tested them yet, but I might buy a few once I get my loads back from my friends (so I can finally have a NIST traceable calibration set of dubious quality).The S88 load is from a calibration set that my friend has at his lab.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 06, 2024, 02:59:52 pm
content: Quote from: EggertEnjoyer123 on December 06, 2024, 01:59:20 amIf that is true (and would not surprise me), this is a very good deal. Owners of the low cost VNAs looking for a better load while not spending some major cash should be very interested in your find.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: December 10, 2024, 04:31:56 am
content: I got the measurements and it seems like the seller is honest. The measurement is different because a different cal kit was used (my friend said that someone else calibrated the VNA in the previous measurement so maybe they did it wrong)This is better than the last loads I got, where someone just shoved a 50 ohm resistor into a SMA.

author: prutser
author_rank: Regular Contributor
timestamp: December 10, 2024, 07:18:25 pm
content: From which of the loads you mentioned is this last plot ?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: December 11, 2024, 04:25:18 am
content: Quote from: prutser on December 10, 2024, 07:18:25 pmhttps://www.aliexpress.us/item/3256807233299645.htmlThis oneMy friend says they're good to 40 GHz as claimed.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: January 22, 2025, 08:04:38 am
content: I implemented windowing in my software. My code now uses a Hanning window on the ADC data before correlating it with the -1, 0, 1, 0, 1, ... sequence (to basically filter out signals at fs/4). This helps eliminate all the DC frequencies (the DC level changes every time the frequency of the oscillators changes, and as a result there is a lot of unwanted signals at low frequencies). According to my Python simulation, with the original filter there are some frequencies near DC which only experience -60dB attenuation, but with the Hanning window all frequencies near DC are basically eliminated. (The cost is obviously that the 12kHz "passband" is slightly wider, but it's not that big of an issue).Doing this seems to have eliminated a lot of the randomness with S21 measurements, and my isolation is now about -75dB with calibration (compared to my old results in post #53). Without calibration, it is still around -60dB (I probably need better shielding than aluminum foil). As you can see from the filter measurements, I am now able to measure a lower S21 (around -70 instead of -60dB). I'll probably have to redesign everything with better shielding to do better (there's only so much you can do with calibration).

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: January 22, 2025, 08:46:43 am
content: I also spent the last month looking at how to make a good resistive bridge, which should let me go down to 10 MHz. The balun seems to be the hardest part.I took apart a 10 MHz to 20 GHz mixer module to see how it worked. (To be fair it was already broken when I got it). They are using twisted pair instead of coaxial cable, along with a very small ferrite. The wire is 0.08mm diameter. I bought wire of the same diameter, but I got 50 ohms impedance with only 3 wires instead of 4. I tried making a balun using my twisted pair (more like triple) but I wasn't able to get it to work at all, so I'm probably doing something wrong. Maybe they are using special ferrite or something? I'm going to try again with 0.35mm microcoax from Aliexpress, which should hopefully be easier to work with. Also, twisted pair isn't TEM so I'm not sure how it would work in place of coaxial cable (which is normally used in these baluns). Hopefully someone else can explain (I suspect that the ferrite is probably special).From the image you can see the baluns, and the white objects are two diode rings and two resistors (there is one hiding behind the rings). The mixer is triple balanced, and the part number is T3-20GLCTG. The four wire "twisted pair" goes to the IF, and the other two are the LO and RF. There are five baluns: the two ferrites on top of the diodes have a twisted pair going through each hole (the LO and RF each connect to two baluns), and the IF balun has two wires connected to ground, and two wires connected to the IF output.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: January 25, 2025, 04:42:23 am
content: I'm doing some measurements while I wait for parts to try to make a lower frequency directional bridge.Here are the measurements of a bias tee made with some random chinese 100nF 0201 capacitor and a BLM15GG471 0402 ferrite bead from Murata. The loss seems to be at most 2dB up to 23 GHz. I measured a through and the bias tee and I got the results attached. This ferrite is recommended in the ADL9005 and MAAM-011100 datasheets.I wanted to see if there were any resonances (which could be the reason why my output power drops by a lot after 21 GHz). It seems like there are no resonances, so the issue is probably with the connectors I used after the amplifiers. Hopefully fixing that issue in a later revision will stop the VNA from drifting while the temperature stabilizes (measurements past 21 GHz only become stable if you wait for the PLL and amplifiers to heat up because the power going to the mixer changes, which affects the conversion loss when the power is low).

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: February 06, 2025, 07:31:35 am
content: I made some clones of the original mixer, and I am getting decent results up to 19 GHz. However, it seems like there is a huge resonance of some sort above that frequency which completely ruins my results. I am using 4 BAT24-02LS diodes (the cheapest I found was about $50 for 255 on winsource).I accidentally broke the PCB, so I don't have any good pictures of it. But I have the gerber files for the board, as well as an image I took of the back side of the board before I broke it. The capacitors are 22pF 01005.To test the conversion loss, I replaced a mixer in my VNA setup with my design. The VNA is uncalibrated, and the difference between the brown and blue line is the difference in conversion loss between the commercial mixer and my mixer. As you can see, my mixer is slightly lossier up to 19 GHz (which is partially explained by the fact that it is twice as large as the commercial design). However, after 19 GHz there is a huge drop in the mixer conversion loss, and I'm not exactly sure what is causing it. Changing the capacitors seems to have no effect. Touching the board also has no noticeable effect unless I touch the small wires connecting the diodes to the IF output. If I touch that, then the conversion loss drops even more after 19 GHz, so I suspect that I might have an unintended antenna somewhere with the pickoff for the IF. The wire which biases one end of the diode ring to ground seems to have no effect, and nothing happens when I touch it. In the commercial mixer, the wires are 5 mm long, but in my mixer, the wires are 8 mm long (so that could be a factor). Also I have no microwave absorber other than my finger.Also please do not order the gerber files without modifying the design! The copper gap between the vias is too small and it will bridge if you try to get it manufactured. I had to cut the bridges open with a knife. I'm using 0.6 mm FR4 from JLCPCB.In a later revision I'm going to add ferrites so the balun can work down to DC. My IF is 12 kHz (while the commercial mixer has to support up to 1 GHz IF) so I can get away with it.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: February 07, 2025, 07:50:21 am
content: Here's the commercial mixers for comparison.Adding some copper tape to make the IF pickup lines shorter did nothing. It seems like the issue might actually be with the capacitors, because swapping the LO and RF results in a much higher insertion loss (~5dB) at all frequencies, and the only difference between the LO and RF baluns is that one has capacitors and the other doesn't. Maybe SMD capacitors are just bad relative to the beam lead capacitors used in their design.

author: arlo_g
author_rank: Contributor
timestamp: February 22, 2025, 02:37:57 am
content: Thanks for posting the excellent photos of mixers.I had expected that Marki was using twisted strips of PCB (parallel strip transmission line) threaded through ferrite beads in their ultra-broadband mixers, but evidently not in the one that you autopsied.I seem to remember that Darrell Harmon had posted on his website, or maybe on Twitter, that he got excellent broadband performance from inexpensive multi-layer ceramic caps, but I can't find it again. It might be worth trying multi-layer (SMT) caps soldered on edge, too, as that can smear out the abrupt impedance transitions in the cap structure.Does anyone have any bright ideas for how to implement flex/rigid PCB on the cheap from flex PCB's at home? A lot of the commercial microwave mixer modules seem to be fabricated in thin suspended substrate held to a frame of metal (or maybe rigid PCB).

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: February 22, 2025, 07:39:15 am
content: Quote from: arlo_g on February 22, 2025, 02:37:57 amI already looked into the flex PCBs. They are too thin and a 50 ohm microstrip trace would be impossibly small on them (at least JLCPCB's flex is too thin). I'm probably sticking with 0.5mm FR4 which is extremely cheap, and probably not that lossy (if I make everything thinner so it's more in line with the commercial mixers). The cheapest way I can think of is 3D printing a case and covering it with copper tape. Certain plastics might be lossy enough but you can always add antistatic foam to kill resonances.I'm also going to switch my cheapo Chinese 0201 capacitors for better ones. (Maybe also use reversed 0204 capacitors for the mixer, since those have a lot less impedance). The CL03A line of 0201 capacitors have much lower impedance at 6 GHz (around 2 ohms for the larger ones in the microfarad range) which extrapolates to about 7 ohms at 20 GHz.Also Harmon did measure the 0402 ferrite bead that I'm using right now. Unfortunately he doesn't say how he measured it. I can probably use the bisection method to verify his measurements with a better test setup for SMDs. The plot is attached below. Coincidentally he has been working on a 20 GHz VNA for over 10 years at this point, but there aren't any details about it.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 22, 2025, 10:10:47 pm
content: I was in the team in 2003-5 that designed these 10 GB/s XFP fiber optic transceiversat Infernion fiber optics. Note the thick wire bracket over the flex cable next to thesolder joint. That clamp was the breakthrough. The laser and the optical receivermust swimm because the FO cable connector requires that. It took a huge amount ofHFSS simulations to get the ribbon work and produce nice eye patterns at 10 GB/s.I tried JLCPCB's el cheapo 4 layer process (not even the impedance controlled option)for my LMX2594 synthesizer. On a 100*100 mm2 minimum board is enough place for4 synths and some micro strip test structures.JLCpcb's impedance calculator says that 50 Ohm is 11.5 mil wide on top whenlayer 2 is GND. The FR4/Cu below gives mechanical strength. When tested withan Agilent 18 GHz TDR it turned out that the impedance calculator was right on spot.I needed another iteration for the SMA connectors. (Rosenberger)The center pad was much to large.regards, Gerhard

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: March 11, 2025, 03:24:49 am
content: Here's my latest mixer design. Still about 5-6dB short of the commercial mixer at high frequencies. (I am comparing the mixers by doing an uncalibrated S21 measurement and swapping the mixer out. You can ignore the S11 plot and only look at the S21 plot. The blue line is my mixer and the brown line is the commercial mixer.)Again, diodes are BAT24-02 and everything is done on 2 layer JLCPCB 0.6mm FR4.One thing I find weird is that if I swap the LO and RF of my mixer (the LO side has the DC block capacitors and the RF part has a short to ground), there is a very sharp resonance at 16 GHz and my insertion loss spikes to 30 dB. If I put a 3dB attenuator on the LO side, it fixes the issue somewhat. The resonance frequency is unaffected by what signal generator I use, so it is very likely related to something on my mixer board. Interestingly nothing happens at 16GHz when the LO and RF aren't swapped. The commercial mixer also has worse performance when LO and RF are swapped, but not by much.Design files are attached. Perhaps my mixer would get closer to the commercial mixer if I used a higher LO power (I think more power is lost across my substrate + capacitor). I also measured the mixer the right way around, but with a 3dB attenuator on the LO port. The results are exactly what you expect (last picture).

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: March 11, 2025, 03:27:35 am
content: I also measured the loss of my microstrip balun, which isn't great because FR4 was used.

author: ftg
author_rank: Frequent Contributor
timestamp: March 11, 2025, 08:44:54 am
content: Exiting progress!Just be aware that JLCPCB does not guarantee the FR4 material their for 2-layer boards.So the Er of the material used can change between batches.For 4-layer the FR4 material should stay the same between batches.

author: prutser
author_rank: Regular Contributor
timestamp: March 12, 2025, 12:24:45 pm
content: Never considered to use JLPC PTFE for mixer PCB ?It's only 2 layer and more expensive than 4L FR4, but still able to get 5 PCBs for approx $60,-

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: March 17, 2025, 12:49:44 am
content: Using a 0204 reversed style capacitor seems to get me a few dB improvement at high frequencies. (The part number is LLL153C80J104ME01E)I'm within 5dB of the commercial mixer now. I can probably get a few more dBs by optimizing the connectors.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: March 18, 2025, 04:10:37 am
content: I am getting a weird resonance after swapping out the solder bridges with a ferrite bead (BLM15GG471). The good thing is that the mixer now works down to 10 MHz (and probably lower). Conversion loss is about 8-9 dB at 10 MHz. I'm not sure if everything is still double balanced since the baluns don't work at low frequency, but I think that is easily fixable with ferrites.The commercial mixers also seem to exhibit the weird resonance effect (see bottom image). It looks like one of the commercial mixers is a lot worse than the other in my test. I think the only reasonable explanation is that my LO power is insufficient at high frequencies (since my original board has bad design and my amplifier is not designed for these frequencies). If you go to post 19 you can see that the output of my PLL + amplifier could go down to 0 dBm (my spectrum analyzer is also +/- 3.5 dB). This means the diodes aren't being driven hard enough, and look capacitive. The biasing inductors could resonate with them. When the LO is driven hard enough, the diodes look more like shorts than capacitors so we don't see the effect.I might have to upgrade the amplifiers in my next revision. I have a couple RF amplifier MMICs laying around that I could probably use (or I could buy the Mini Circuits LVA-273PN+). +15dBm is probably enough to drive all my mixers without breaking them. The Macom amplifier part (MAAM-011100) is good enough to get to about 20 GHz with no issues, but afterwards the amplitude drops significantly. The Macom part is about 4 times cheaper than the Mini Circuits part though.

author: rloc
author_rank: Newbie
timestamp: March 24, 2025, 08:49:38 pm
content: Good day!Quote from: EggertEnjoyer123 on January 22, 2025, 08:46:43 amMinicircuits recommends using a high permeability ferrite. I would guess that Marki used something similar to B62152A0015X030. Quote from one patent (dual output autotransformer from 30 MHz to 3000 MHz or greater):QuoteThe length of the LO/RF twisted pairs should be slightly shorter than a quarter wavelength of the mid-range, adjusted for propagation speed. The characteristic impedance of the LO/RF twisted pairs should be about 100 ohms, since they are connected in parallel on the port side.Quote from: EggertEnjoyer123 on January 22, 2025, 08:46:43 amCan you show in more detail the location of these resistors, what points they are connected to and the values?

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: April 12, 2025, 06:09:48 am
content: I tried messing around with OpenEMSIt seems like you can make a decent resistive splitter at 40 GHz. The "resistors" (white) are modeled by a thin sheet of material with finite conductivity, and some alumina on top (adding the alumina did not make a noticeable difference). Each resistor models 2 36 ohm resistors (01005) soldered upside down.I'm also trying to design a resistive bridge to replace the directional coupler in my design. So far my design works to maybe 15 GHz or so before my return loss suffers.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: April 16, 2025, 04:07:29 am
content: Quote from: rloc on March 24, 2025, 08:49:38 pmThe design is attached to this post. The resistors measure 22 ohms on a multimeter.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: April 16, 2025, 05:14:58 am
content: Also I recently noticed this:https://hforsten.com/designing-a-low-cost-high-performance-10-mhz-15-ghz-vector-network-analyzer.htmlIt looks like his bridge has low directivity though. If we look at the graph titled "Uncorrected S-parameters with short on port 1 and load on port 2 with EM gasket. 10 Hz IF bandwidth." we can see that the red and black lines cross, which implies negative directivity.I am able to get better results using the same design, but with 01005 resistors (I hate soldering those and I wasted so many) and 1mm rigid coax. My unoptimized results are attached to the post. The first picture shows the reflected output for a short and a load (you can see there is about 6-7dB difference). The second shows the insertion loss. My directivity sucks (and my return loss) but it is still positive up to 22 GHz which is promising. The coupling falls off at low frequencies because I had a soldering mistake where two conductors were very close but weren't actually soldered. I forgot to take screenshots but even after fixing the soldering, nothing changed at high frequency. Only the low frequency directivity got better. My return loss is also horrible and this bridge would probably be unusable because of that. One of the connectors is also missing, and that's because I intentionally did not solder it (otherwise it would be impossible to resolder the resistors). During testing I pressed it on. I have another bridge with the connector soldered, and it made zero difference.I think I can optimize the design to use fewer 01005 resistors, and also have a better return loss. I am currently trying to improve the design by simulating in OpenEMS.

author: rloc
author_rank: Newbie
timestamp: April 16, 2025, 07:30:46 pm
content: Quote from: EggertEnjoyer123 on April 16, 2025, 04:07:29 amThank you! This is amazing, I will definitely check it out.A long time ago I received a ZNB20/40 for repair with a burnt bridge. The ferrite material is 10K. The embodied thin-film resistors on a substrate had to be replaced with 01005.https://patents.google.com/patent/US9823284B2/en

author: rloc
author_rank: Newbie
timestamp: April 16, 2025, 07:48:41 pm
content: The DBM mixer in the high-frequency branch looks like a Giga-tronics 30037 with klopfenstein taper on the back of the PCB.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: April 17, 2025, 07:30:12 am
content: I wonder why they seem to have made no attempt to match the broadband capacitor (since the pad of the DC blocking capacitor is a lot larger than the trace width, which means some matching should be needed to keep the return loss low).I'm talking about the capacitor right below to the uppermost conical inductor on the first image you sent. It wouldn't have taken that much effort in an EM simulator to match the large pad to the thin microstrip trace. (Unless I'm missing a cutout on the bottom ground plane).Interestingly the Keysight VNAs seem to not care about matching too... I attached their specs below.

author: rloc
author_rank: Newbie
timestamp: April 25, 2025, 09:55:09 am
content: Quote from: EggertEnjoyer123 on April 17, 2025, 07:30:12 amA ground plane continuity issue seems like the most likely scenario.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 07, 2025, 03:16:29 am
content: I messed up my OpenEMS simulation for the splitter.My previous simulation is incorrect because I used a lumped port instead of a microstrip port. The simulation gives inaccurate results because of fringing fields, which effectively adds a capacitor to the port (making it not 50 ohms anymore). The proper way is to use a microstrip port in OpenEMS. The microstrip port is more accurate because the port extends all the way to the edge of the board. Updated results and a physical measurement are posted.Some tips (just so you don't waste hours like I did):1. Do not use a Dirac excitation. Use Gaussian excitation (otherwise you will run into stability issues with the PML boundary). Your simulation will also run much quicker.2. Ensure that the feed shift and measure plane shift are both large enough. If they are not, the feed and measure plane will be in the PML region which will screw up your measurements. (PML_8 means that the last 8 layers in that direction are PML).3. All microstrip ports must be terminated in PML, otherwise your code will not work. Microstrip ports should extend all the way to the end of the PML region (so if you have PML from x=-7 to -5, then your microstrip port should start at x=-7, and you should make sure your feed shift is larger than 2, and your measure plane shift even larger). Sides without microstrip ports can use MUR boundary condition for faster simulation.4. Set Feed_R to 50 ohms, otherwise your simulation will not work at DC (because the PML's effectiveness is dependent on the wavelength of the wave, and a large wavelength means the attenuation is smaller. This intuitively makes sense, as low frequency RF can easily penetrate objects while millimeter wave signals can't.) The Feed_R adds a lumped resistor which absorbs all the frequencies near DC.I still need to figure out how to simulate GCPWs. There's no "waveport" equivalent (as in Ansys HFSS), or if there is I don't know how to use it.

author: szoftveres
author_rank: Regular Contributor
timestamp: May 07, 2025, 06:17:02 pm
content: Quote from: rloc on April 16, 2025, 07:30:46 pmWRT the above patent, does anyone know what the advantage is of the resistive tee-splitter based bridge over a regular (resistive Wheatstone) bridge? It's unclear to me how this architecture helps extending the bandwidth further upwards and I don't seem to find an explanation in the patent text.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 08, 2025, 01:57:34 am
content: Quote from: szoftveres on May 07, 2025, 06:17:02 pmWith this design, as long as the microstrip connecting the resistive tees is 50 ohms, no reflections occur. This allows R&S to space apart the resistors (see picture of the actual design) without having to worry about reflections. As long as the resistors are electrically small everything else doesn't affect performance.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: May 09, 2025, 07:31:46 am
content: I attached my fixed simulation.I can probably design one with larger resistors (maybe 0201 or 0402) and order a test board once tariffs are settled.The FCSTD + ini file can be used to generate the STLs (using this program:https://github.com/LubomirJagos42/FreeCAD-OpenEMS-Export)The program splitter_openEMS2.py contains the actual simulation script (it is renamed so it doesn't get overwritten every time I update the STLs).The S21 program plots the S parameters after the simulation stops.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: May 20, 2025, 08:51:46 am
content: Back from a Two-Week Trip (Academic Exchange)After being away for about two weeks on an academic exchange, I'm finally back and getting back into my projects.During the trip and especially after returning, I spent a lot of time thinking about the VNA project I’ve been working on. To be honest, trying to build a full VNA as a one-person team is probably too ambitious right now. There are too many small details that could be missed. So I’ve decided to narrow the focus for May and June — my goal is to design and build a reliable directional coupler.Coaxial Directional CouplerI tested the coaxial directional coupler I designed, and unfortunately the directivity was quite poor. Around 20 GHz, it couldn’t even clearly distinguish between a load and a short. After some measurements, I found that the issue is most likely due to impedance mismatch.I checked the S11 of the RF input, and saw large deviations at high frequencies. The real part of the impedance dropped to about 10 ohms at around 8 GHz. This suggests there may be a problem with either the coaxial line or the PCB trace impedance.Here are a couple of issues I ran into:1. I contacted JLCPCB, and their support told me that choosing “20% impedance control”basically means no impedance control at all— that was very disappointing.2. I measured my coax cables with a VNA and found significant deviations at high frequencies. The supplier claimed their cables had ±2 Ω tolerance at 20 GHz, but in reality, it wasn’t even close. Unfortunately, there are a lot of misleading sellers here.Because of these problems, especially the limited directivity of the coaxial design.I’ve decided to abandon the coaxial approach.The Microstrip Directional CouplerFor a long time, I thought it wasn’t possible to get good low-frequency coupling performance using microstrip couplers, and that’s why so many designs use alternative structures. But recently, I came across a very helpful paper: IET-MAP.2011.0471, which explains how to design a “wiggly-line” microstrip directional coupler in detail.This gave me some new ideas, and I’ve decided to try replicating that design.My Plan (To-Do List)1. Develop a mathematical model – this part I’m quite comfortable with.2. Simulate and fine-tune the design – this may take time, as the layout is sensitive.3. Fabricate the coupler – this is the most difficult part. The design requires high precision, so I either need to figure out how to process alumina boards myself, or I’ll have to pay nearly $1,000 per prototype run, which is not feasible for me. (If anyone knows a more affordable option or is willing to help, I’d really appreciate it.)4. Once the coupler is working, I’ll go back to working on the VNA.If anyone has experience with alumina PCB fabrication, or has worked on wideband microstrip couplers before, I’d love to hear your insights or suggestions.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 26, 2025, 07:40:08 am
content: I read the patent for both the R&S and HP directional bridges. Each one relies on using a lossy material to attenuate the common mode of a differential signal.My new idea is to do everything on a PCB, and to use conductive filament as the lossy material (since it would be easier to shape). Soldering 1mm hardline coax is not fun at all, and it should theoretically be possible to do everything on a PCB. Also, I won't need to optimize the coax to microstrip transition too (which I have to do with the triaxial HP design).I think I was able to make a good 50 ohm termination in OpenEMS. Assuming my model of 2 100 ohm 01005 resistors matches reality, it seems like you can make a good termination past 40 GHz.The next step is figuring out how to make a differential pair in microstrip which has high common mode loss but low differential mode loss. My current idea is to use a thin plastic spacer plus a large block of printed conductive PLA (conductivity 7 S/m). The fields in differential mode tend to stay near the traces, while the fields in common mode go all the way to ground. The idea is that more of the common mode fields would go through the lossy conductive PLA material. In simulation, 4 microstrip ports are used to determine the differential and common mode losses (you can easily convert between 4 single ended ports and 2 differential ports using a bit of math). With the 7 S/m material, you can see that the common mode does get attenuated more than the differential mode, but with 70 S/m the gap is wider. There aren't many materials around that conductivity though.Another idea could be removing the ground plane of the PCB. Then I can have a sandwich with the traces in the middle and lossy material on both sides.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 27, 2025, 07:26:40 am
content: The solution seems to be to add a cutout of the ground plane, and then add more lossy material underneath (7 S/m conductivity).Now the common mode is attenuated significantly more than the differential mode.The only thing left is to figure out how to get the impedance to be constant for both differential and common mode. Probably need to do some tapering.

author: arlo_g
author_rank: Contributor
timestamp: July 01, 2025, 01:31:22 pm
content: Achieving 50 Ohm impedance in edge coupled lines usually demands challenging geometry.It might be worth considering a slow wave structure with a regular array of floating strips set under, and bridging between the differential pair. I think that that kind of structure should enable 50 Ohm differential impedance and a reasonably high cutoff (Bragg) frequency with geometry that is more feasible in low cost PCB processes - especially if built in a surface and adjacent layer of a 4 layer stackup.Although it should also enable pretty high common mode impedance, the slow wave structure that I described would not in itself provide a lot of common mode loss. It is tempting to think that a pour of surface mount resistors connecting up islands of ground plane on the opposite face of the PCB might deliver common mode loss analogous to R&S's lossy plane.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 23, 2025, 08:03:18 am
content: It's a bit late here, but I finally figured out what the T splitters do in the R&S design.The hardest part to design is the balun, which needs to have high common mode impedance and a consistent differential mode impedance. The balun is also the hardest part to get right, since soldering coaxial cables to a PCB and getting consistent results over 0 to 40 GHz is probably downright impossible.In the R&S design, the resistance of the T splitters dominate over the common and differential mode impedance. This means any discontinuities (like the PCB to coax transition) don't matter (which is why R&S can get away with just randomly soldering a coaxial cable to a PCB board at 40 GHz). The characteristics of the lossy material doesn't have to be perfect. The coupling won't be flat but it can be calibrated out. You can see in the simulation that a good match occurs even if the common mode and differential mode impedance are both at 1 ohm (which is a huge mismatch compared to 50 ohms). The only things that affect the match are the resistances of the T splitter resistors, which can be easily tuned with a laser.In the HP design, the design of the balun is very important, and that is why HP needs precision machining and a ton of adjustment screws on the balun. The ideal balun and non-ideal balun (with a finite common mode impedance and mismatched differential impedance) are shown). As you can see, the matching becomes worse than -15 dB if there are nonidealities in the design.Time to wait until October 2033 so I can copy the R&S design legally....... (Though I do think it is possible to achieve this with only 2 matching pads)The simulation is also attached so you can mess around with Qucs on your computer.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 23, 2025, 08:28:44 am
content: I am not a lawyer, but in the R&S patent, there are no matching pads on the balun (with one leg of the matching pad going to ground). I'm not sure if this was an oversight by the R&S lawyers.Anyways it is probably possible to make a design with only 2 matching pads. Each matching pad will have one leg connected to the balun and one leg connected to ground, and one leg connected to the rest of the circuit. (The claim section says there are three matching pads in the patented design, so I don't think it would apply to a design with only 2 matching pads. Also the matching pads are connected differently compared to the one in the picture). It's too late to do math, but it might be able to get away with 2 or 3 resistors in the rest of the circuit while still achieving a good match at the test port.

author: tszaboo
author_rank: Super Contributor
timestamp: July 23, 2025, 11:10:56 am
content: Few questions. What kind of coax do you use? I got myself some semi rigid RG405 and RG402 for experiments like these. I already used them as transmission line probes, and some other basic experiments.I'm using QUCSStudio. What made you use QUCS?I was also wondering about the matching pads. So maybe it's not on the patent, but they are still there in the design.The coax to PCB transition, I've seen some end launch parts, where you solder a coax in on one side, and the other side looked like an SMA connector. I think it's from TE but I cannot recall now.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: July 24, 2025, 01:31:03 am
content: Quote from: tszaboo on July 23, 2025, 11:10:56 amI have 1mm diameter hardline from the ewaste.I also use QUCS because I'm on Linux. I think Joey has tried to do directional coupler experiments in another thread, and he concluded that it was impossible to get consistent matching with a soldered PCB to coax. I think you can get away with it in the single digit GHz range.Anyways here's my latest design which should be legal to build. The matching is still pretty good even with both common and differential impedance at 1 ohm (which is probably way worse than reality).

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 05:10:36 am
content: Thanks for sharing EggertEnjoyer123.btw bcuz of the source matching have less weight, phase and output power can directly sampled using some resistor, it will cut down alot cost than using extra bridge.the following is the simulation result of my design. quite the same as EggertEnjoyer123, but I keep using the power divider not power spliter to cut down the weight of the source impedance.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 05:26:39 am
content: i came up with a good idea to get the coaxial matched. use some 1mm SMP wire as the balun and use some well matched SMP connector, I think everything will become easier.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 02:36:09 pm
content: I'm going to merge the directional coupler thread to this thread and all the new things will posted here

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 02:43:06 pm
content: I build a demo of eggertenjoyer123 modified version, still doesnt works that fine.The coupling degree is flat and same as the simulation, but the isolation isnt good unless I put my finger on the solder points of the coaxial, so it's still the coaxial to pcb mismatch……

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 02:48:17 pm
content: also I used 0402 Resistor here bcuz it doesnt effect to impedance that much at around 8Ghz

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 02:51:35 pm
content: Also, my differential line impedance matching is also really bad.I suspect its due to my manufacturing process. I will try using a better etching machine later.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 03:07:04 pm
content: I probably know what the problem is.My mclin width does not match the coaxial line diameter.im going to use rt6006 h=0.254mm the width of the mlin will become 357um. Well I'm tired.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 03:21:00 pm
content: I found a cable fit that really well.anyway im going to take a nap, see u guys later

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 24, 2025, 04:20:30 pm
content: also I found that the dielectric of the coaxial exploded when I solder it, so maybe I should use some silver glue or some low temperature solder.when it exploded the dielectric constant will definitely change. and this will cause a serious impedance mismatch.

author: arlo_g
author_rank: Contributor
timestamp: July 25, 2025, 09:51:09 pm
content: Re. T splittersI think that you'll find that T splitters or other attenuators on the port help source match at the expense of directivity and dynamic range. You can likely still get adequate _corrected_ directivity with a sufficiently good calibration, but reliance on heavy correction doesn't likely come for free.

author: Johnny10
author_rank: Frequent Contributor
timestamp: July 25, 2025, 10:07:08 pm
content: Place a marker

author: JoeyDeng
author_rank: Regular Contributor
timestamp: July 26, 2025, 04:42:30 pm
content: QuoteCan u explain why the directivity will decrease plsase?the main reason for the failure of my previous coaxial balun resistor bridge is that the balun was heavily mismatched, and the input impedance of the balun had heavy weight. If we use a t divider to reduce its weight, the directivity should be much better

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 02, 2025, 01:36:58 am
content: Here's my latest simulation. I haven't really tried optimizing anything here yet.In the first picture, port 1 is on the top, port 2 is on the bottom, port 3 is on the left, and port 4 is on the right. Port 1 contains the device under test, port 2 contains the reference load (basically a 50 ohm standard), port 3 is the input for the signal, and port 4 is the output of the balun.The second and third pictures show the "transitions" in the balun. The second picture shows the transition from a differential microstrip with a copper ground plane to a differential microstrip with a lossy ground plane. The third picture shows the differential to single ended conversion. I haven't tried to optimize anything (I only made the thickness of the differential microstrips higher because that led to more common mode loss and more directivity).Here, everything is ideal and symmetric before the balun, so we should expect the S43 to be zero if the balun were perfect and rejected all common mode. In reality, we are getting around -60 dB. (see 4th picture). The insertion loss (S31) is about 10 dB. Basically, the power from the source oscillator will be attenuated by 10dB when it reaches the DUT. In the 5th picture, we can see that the S43 is around -20 to -30 dB. So if all of the signal on port 3 were reflected, the signal would show up on port 4 with -20 to -30dB attenuation. So in total S43 is -30 to -40 in the case where port 1 is unterminated, and -60dB in the case where port 1 is terminated, so my design achieves 20 dB directivity. Also notice that the match is pretty good for port 1 over a pretty large range.I think I can achieve a higher directivity by adding more lossy material to the top, and also making the differential pair with lossy material ground plane longer. Also, the low frequency performance can be improved by using ferrites (not shown here). It should be pretty easy to snap a few of them around the differential traces.

author: JoeyDeng
author_rank: Regular Contributor
timestamp: August 11, 2025, 09:40:13 am
content: https://www.kechuang.org/t/90896https://sdr-deluxe.com/forum/3-2-3http://circuitsonline.net/forum/view/140339EggertEnjoyer123 check thisQuote

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 12, 2025, 04:04:35 am
content: I don't think those designs work at millimeter wave.I think the design I shared earlier should work up to 40 GHz if you tune everything correctly. It'll probably take a few cut and tries to get everything right though.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 09, 2025, 03:47:37 am
content: Harmonic mixing might be a solution to another one of my problems (which is that I can't generate enough power for the LO past 20 GHz).I programmed my device to use harmonic mixing past 13.3 GHz. In theory, a double balanced mixer can be ran at odd harmonics (if you look at the harmonics of a square wave). If you turn on the diodes hard enough, there should be enough 3rd harmonic to mix higher frequencies down.Based on my oscilloscope capture, it seems like the insertion loss increases by about 13 dB. That doesn't really matter though because the isolation is nowhere near good enough right now to see the effects of that.Anyways, if I do decide to try higher frequencies, harmonic mixing is probably necessary since getting the 15dBm or so at 40 GHz to drive the mixers is going to be challenging (but 15dBm can easily be generated at 13.3 GHz, even with FR4). The XX1000 chip is also interesting (it is a frequency doubler IC which fits nicely with the LMX2820 frequency range), and could be used to increase the range with harmonic mixing. The 13.3 GHz cutoff could also be moved up since I only see issues with LO power at 18 GHz.I also need to see if my homemade mixer design works with harmonic mixing too.Picture 1 shows the difference between non-harmonic mixing and harmonic mixing at 13.3GHz. The signal on the left is the IF at 13.3 GHz (no harmonic mixing). The signal on the right is the IF at a little higher than 13.3 GHz (with 3x harmonic mixing). The signal on the left is about 4.3 times larger, which translates to 12.7 dB.Picture 2 shows a measurement of a 30dB attenuator with harmonic mixing. Everything works.Picture 3 shows the isolation. It is impossible to tell where the harmonic mixing starts (unlike with old spectrum analyzers, where you can clearly see from the noise floor where harmonic mixing is used). This is because my setup sucks in terms of shielding, and there is a lot of leakage. I think I will have to redesign everything to make the shielding better, but that'll be for when I make version 2 of the VNA.Picture 4 shows the difference between a warmed up and cold measurement. Before, there would be 10dB difference at 23 GHz because the LO power is too low to drive the mixer diodes properly. Now, there is 1dB of difference between the warmed up and cold measurement. Also, I untwisted and twisted the cable, so there is probably some error due to that. To measure it, I calibrated the device while it was hot, and then I waited for everything to cool down. I quickly powered it back up and loaded the calibration file, and then I ran another measurement to get the difference (though I probably shouldn't have unscrewed the cable). I think I probably should buy a proper torque wrench someday and not just use my fingers.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 10, 2025, 07:09:28 am
content: My homemade mixer appears to do way worse when it comes to harmonic mixing.My theory is that the diodes have high capacitance (0.2pF for the one I'm using). This means that the amount of third harmonic generated by the same input signal is smaller. I think beam lead silicon diodes can be bought with <0.1pF capacitance, and the designers of the commercial mixer probably used better diodes. I will probably have to buy GaAs diodes to compete (like the MA4E1317, which is about 3 bucks each from Mouser).The commercial mixer achieves an uncalibrated S21 of +/- 2dB in this test setup, which means my mixer is behind by over 15dB at certain frequencies. (In normal mode I can get within 5dB of the commercial mixer).

author: JoeyDeng
author_rank: Regular Contributor
timestamp: September 19, 2025, 02:10:42 am
content: Now use EggertEnjoyer123's balnanced bridge design we can reach 20db directivity in 8Ghz and 14db in 20Ghz easily, the most important thing is to find the correct coaxial and correct ferrite. I costomized some good ferrite that works pretty well at high frequency.using a large resistance resistor like 1kOhm and a 50Ohm term to sample the reference is wise choise in this design, because there is 11db loss of the bridge ,so theres basically 11db directivity of the reference.My friends helped me to tear down some keys*ght PNA-Xs' bridges and finally I got to know how to match the coaxial of the unbalanced bridge and makes it work at k band. but the material needed is quite expensive I need time to collect the material and get the bridge fully work.