Dealing with reflections from a 5-6GHz RF mixer - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: August 17, 2015, 07:00:36 am
content: I've stayed comfortably below 0.1GHz in the past, but I'm taking the plunge into 6GHz. I mean, it's less than 2 orders of magnitude more, how hard can it be, right?My plan is to use connectorized Minicircuits blocks (EDIT:Or maybe I'll try PCB after all), which are a bit pricey, but save me from worrying too much about EM simulation and all that, and gives me some shiny RF lego blocks to play with in the future. Anyway, I'm planning to make an I/Q demodulator with a bandwidth of 10-100MHz or so, and an LO of 5 to 6 GHz -- using direct zero-IF downconversion. I have some concerns that center around the poor VSWR of all the ports of the mixer (which I assume is standard fare for mixers?), and I'd like some assistance with my concerns for each of these ports:RFport: My plan is to make one of the cables ~11mm (actual number TBD) longer, which corresponds to a 90 degree phase shift at 5.5 GHz or so. This is marked as 90 degrees in the diagram. This achieves the 90 degree phase shift required for the I/Q demodulation, but also means that reflections from the mixer's RF port will be ~180 degrees out of phase by the time they return to the splitter, where they ought to get mostly dissipated? Is that a reasonable plan?LOport: I'm actually planning to make a 2 channel phase-coherent I/Q demodulator, which means that I can put a 90 degree phase shift on the two LO ports of the two mixers for the other channel. This will cause 2 reflections to be 180 degrees out of phase with respect to the other two, again resulting in the splitter dissipating the reflections? Is that a reasonable plan?IFport: This is a bit trickier. I presume the mixer will leak LO and RF (is this correct?), and even the upconverted signal up around 11-12 GHz (is this correct?). I'm not sure it's a good idea to feed this into my digitizer (likely to be a Picoscope 3000D series unit) -- will the 5GHz jump right past the band limiting circuitry? I considered using a low-pass filter as shown in the diagram, but those seem to reflect the stopband back at the IF port with almost no loss whatsoever, which seems like it could cause all sorts of spurious signals and nonsense (?). It seems weird to me, can't you buy simple RC lowpass filters that have R=50 ohm and therefore terminate the stopband, rather than reflecting it? Does putting the filter after my feed-through termination somehow help? Or is it OK to allow the leaked LO through to the digitizer, or reflect it back at the mixer?Just to say it again, my LO and RF will both be ranging between 5 and 6 GHz.(All VSWR & Isolation figures are the worst case within the 5-6GHz band I'll be operating in)AMP: Mini-CircuitsZX60-V63+: Output VSWR 1.7:1.MIXER: Mini-CircuitsZX05-73L+: L-I isolation 14dB, L-R isolation 22dB, Conversion loss 6.5dB, LO & RF VSWR both 3:1.2-way divider: Mini-CircuitsZX10-2-71+: 20.8dB isolation, output VSWR 1.1:1.4-way divider: Mini-CircuitsZN4PD-642W+: 24.4dB isolation, output VSWR 1.2:1.Lowpass: Mini-CircuitsSLP-200+or similar: Stopband return loss 0.1dB (!).90 degree"blocks": Just ~11mm lengthenings of the cable.Any other advice/gotchas to watch out for would also be greatly appreciated. Thanks!EDIT: Fixed broken image link.

author: tggzzz
author_rank: Super Contributor
timestamp: August 17, 2015, 07:56:43 am
content: You should not trust this reply, since I am only beginning to dabble in RF.I think you will find that the performance is dependent on getting exactly the right 0/90 degree angle, which will be difficult if you cut wires. If you can't find any references, a simple simulation will indicate the degree of the issue. Consider using a ready-built 0/90 degree splitter.A traditional workaround where poor VSWR causes problems is to improve the VSWR by inserting a resistive attenuator pad. Of course that "throws away" power, but amplification is relatively cheap!

author: KJDS
author_rank: Super Contributor
timestamp: August 17, 2015, 09:01:37 am
content: You need to use quadrature dividers, not the in phase ones you currently have considered.Put a 3dB attenuator after the mixer, or design a diplexer.

author: rs20
author_rank: Super Contributor
timestamp: August 17, 2015, 09:08:50 am
content: Thanks!Quote from: tggzzz on August 17, 2015, 07:56:43 amI had considered this -- even a "proper" 90 degree splitter will be a few degrees away from perfect, and phase inbalances and inconsistencies across the other splitters and nominally identical cables will add up as well. The end result is that getting exactly 90 degrees in the analog domain is impossible in any case.However, the good news is that the phase unbalance errors are easy to measure while calibrating, and can be corrected in software using an affine transform in the I/Q plane (think the "skew" operator in image editing programs).Quote from: tggzzz on August 17, 2015, 07:56:43 amCool, yeah I had considered that. I guess I'm wondering whether or not VSWR actually causes problems for my circuit, which I'd be keen to get people's opinion on.Quote from: KJDS on August 17, 2015, 09:01:37 amDoesn't the slightly longer cable (indicated by the 90 degree blocks in the diagram) take care of the quadrature for me?Quote from: KJDS on August 17, 2015, 09:01:37 amTo clarify, are you saying "the reflections on the IF portwillcause problems, youmustuse a 3dB attenuator", or "not sure if IF port reflections are a problem, but if they are, one way to improve VSWR is to use an attenuator"?

author: KJDS
author_rank: Super Contributor
timestamp: August 17, 2015, 09:27:14 am
content: If you're only using a very narrow band, then you can get away with a slightly longer cable on one path.You may get lucky and not need an attenuator, but usually without one then you'll get problems. Mixers really don't like spurious products coming back into them.

author: rs20
author_rank: Super Contributor
timestamp: August 17, 2015, 11:56:52 am
content: Thanks KJDS!A few additional questions for anyone:-- I'm thinking of making my own cables, since I need custom lengths and so on. Is crimping one's own cables a black art I should avoid? DoesUS$0.90 per SMA plug+US$2.20 per meter of KSR195sound reasonable, or am I sure to be getting bad performance/ripped off at that price? I'm slightly concerned by the lack of a datasheet with specs for the SMA plug, should I be expecting one?-- Can you buy SMA/BNC amplifiers that have >20dB of gain up to 100MHz or so for reasonably cheap? Or is that a bit of a niche thing? I'm OK with rolling my own if it comes to that.

author: KJDS
author_rank: Super Contributor
timestamp: August 17, 2015, 12:20:55 pm
content: If you were in the UK, thenhttp://www.gigatronix.co.uk/are the standard supplier for cables. Rolling your own is fun, but you'll need a way of measuring the electrical length to make sure, however I've got away in the past just by making plenty and swapping them round until the results were ok.Make your own amp, it's a fun little thing to design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2015, 01:24:15 pm
content: I'd suggest adding more pads and buffers. If you don't know the effect of crosstalk (or you can't predict it, because of wiring differences, say?), probably better to avoid it.Quote from: rs20 on August 17, 2015, 07:00:36 amNo, not quite -- the splitter is a lossless type (-3dB, not -6dB), so it won't attenuate anything, it will just pass it on to the source (and a little to the opposite port), where it will in turn bounce around (or be absorbed). The magnitudes will be less, but the energy doesn't go away.The amplifiers having near 1:1 SWR at those frequencies suggest they use either a resistive bias tee, or the HBTs have so much Early effect as to have the same outcome (I've heard some RF parts have this really bad, I just don't remember if it was HBTs or PHEMTs or what..). Or both. In either case, they should absorb energy not-too-badly, and having directivity > 0 means less of that reflected energy will leak out the RF port than the gain factor amplifies it by (if that was worded well enough to make sense..).It might be reasonable to put the splitters out front, followed by a pair of amps (assuming the amps are sufficiently phase matched -- not necessarily the case..).Don't forget a bandpass on the input, too. Better to avoid taking chances, and keeps strong interfering signals out of the amplifiers.The mixers will likely leak LO directly (isolation isn't the greatest), which will give you a zero beat (DC) to some extent or another. Of course, that'll be shunted by the transformers inside the mixer, and circulate internally. I don't know the impact of that (but to be fair, it's made to handle it, and there's no external way to observe or avoid it, so whether it exists or not, it's not of much consequence to you..). Harmonics and IMD and stuff should show up, which should be terminated appropriately; a lossy >6GHz filter might be used (while ignoring any close-in spurs or near-DC images, just for now anyway I guess).I don't think it should be a high priority to put a pad on the RF ports.QuoteAgain, not gonna happen; the splitter will reflect them as if shorted together. So they'll stay bottled up, and could cause problems (particularly if frequency needs to be adjustable, even if the range is modest?). Plenty of power and SNR should be available from the LO, so pads here (maybe 6dB for starters?) are fine.Interestingly, the LO port could actually develop DC (voltage), which suggests a lossy type HPF might be desirable. *shrug*QuoteYes, and harmonics, and IMD (give or take), and so on. LPF is a good idea.But note: the LPF you've picked isn't rated above 800MHz. You'll probably want one good for the whole range, or multiple cascaded. It would also be fine to cut it at maybe ~2GHz, if the RF port is already band-pass'd to the ~100MHz range desired.QuoteIndeed! So here's the trick for a "lossy L/HPF":Get a diplexer and terminate the unwanted port.Simple as that! I don't think you can construct an RLC filter that will be as sharp, unless it's designed in this manner to begin with. Certainly, the insertion loss will be much higher. So you don't normally have them available, I guess.That said, you might get a hunk of PCB with SMA connectors tacked onto it, and put an RC(RC?) filter on there -- 100MHz is enough below 6GHz that it'll at least have a good start on it.A diplexer is basically an HPF and LPF tacked together. But it's not quite sufficient to use just regular ones. When the designs are matched to system impedance, the filter impedances won't be complementary around the crossover region. Obviously, they have to be compatible types, too -- if you recall the standard filter topology (a ladder circuit), it starts with either a series or shunt element. If the input ports are wired in series (assuming common grounds aren't required..), shunt elements must face the input port; if wired in parallel, series elements. It won't work if it's half and half, which is easy to see.The filter profiles also get screwed up, again because of funny impedances. What's needed is an infinite impedance filter design (which can be chosen from any of the standard types, but the L/C values are slightly different from usual, to accommodate the intended port matchings).So you can't just tee a pair of COTS filters together, because if you did, the impedances won't be right, and you'd also have additional length between the two filters, so you don't know what part of the VSWR you're drawing power from (or reflecting it back into), in effect.So, diplexer. You can just as well get one, if they're not hard to shop for (I forget if Minicircuits has standard offerings).It might be reasonable to get only diplexers, and just use those for LPF/HPF purposes. With an open or shorted port on one side, it won't respond quite the same as a normal L/HPF, but it will be reflective. With a terminated port on one side, it will look like a normal filter, except input VSWR is always low.Tim

author: rs20
author_rank: Super Contributor
timestamp: August 17, 2015, 01:45:04 pm
content: Thanks Tim! You always have so much great info.Quote from: T3sl4co1l on August 17, 2015, 01:24:15 pmI thought a -3dB splitter was something like a Wilkinson divider*, which has a 100 ohm resistor in place:IIRC, that resistor does nothing when a signal incoming on P1 is split to P2 and P3. And if two signals are in-phase coming in on P2 and P3, again the resistor does nothing because it has no voltage drop across it.However, with my two 90 degree phase shifts, I've made sure that P2 and P3 are out-of-phase, which means that the resistor dissipates it? Or, put another way, if two out-of-phase signals arrive, how can theybothbe passed on to the amp?* Of course, whether the part I'm getting is a simple Wilkinson divider is unknown; I'm just debating the assertion that -3dB implies my idea won't work.Quote from: T3sl4co1l on August 17, 2015, 01:24:15 pmI had briefly considered diplexers, but assumed it was not the done thing after seeing Minicircuit'smeagre diplexer offerings. I will do some more searching buoyed by the news that the diplexer idea is more sensible after all.I'll continue to process your info tomorrow morning! It's late. Thanks!

author: LaserSteve
author_rank: Super Contributor
timestamp: August 17, 2015, 01:57:04 pm
content: One, when making phase delay lines, use an odd integer length multiple of 1/4th wave. This makes the trimming easier, or use a adjustable phase stretcher.. I'd still recommend a quadrature splitter.Second, about that coax, for short interconnects with mixer, splitter, etc, we'd use hardline micro coax. The solderable connectors are much easier to use and the losses are much lower. Crimping short cables is a no-no in microwave land above 2 Ghz. You can find piles of SMA terminated hardline jumper cables such as UT141 on ebay for cheap. Buy more then you need, as SMA is not rated for many insertions at Ghz.Also, what is the signal you want to look at? LO stability and phase noise become issues fast at these frequencies...I'd probably place a isolator between the LO and the splitter if I had enough drive. Reflections back into the LO cause all sorts of interesting effects. A pad will work, but an isolator is better.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2015, 01:57:13 pm
content: You've definitely not got a Wilkinson -- not for decades of bandwidth! It'll have to be some tiny transformer. Good point though, it probably does have some damping between split ports, to keep VSWR not terrible. Note it'd be in series from one port to the other, so it'll only absorb a few dB.Also, mind that my advice hasn't actuallybuiltanything at 6GHz, so experimentally I'm not much better off than you are. Hopefully the theory works out, eh?Tim

author: LaserSteve
author_rank: Super Contributor
timestamp: August 17, 2015, 02:17:06 pm
content: Tim, your darn close on the theory. I'm not seeing any nits to pick.In HAM practice we accept that uWave mixers are not perfect and we find that we can live with a lot of spurs when working with low bandwidth signals. As long as said spurs are not landing in our IF. We can also accept that on a good day, for most tasks, 3 to 4 degrees of phase error is not that bad, and that merely moving slight bends in the cable that error can come and go.LO drive level isveryimportant. Usually oscillators ship at slightly over or just at the spec power. So it is best to get an oscillator that is a little hot, and pad it down as needed. Pads are cheap, and they cover a multitude of sins..We have a saying, "GAIN IS CHEAP", MMIC amps cost 2-10$ a piece depending on band, power, and noise. They also act as a bit of a isolator if used correctly... We do find that we can tweek system spurs by slightly varying the operating current of the MMIC.For my ham gear, I've spent more money on microwave power meters then anything else. Even one of the uncalibrated 75$ kits from DownEast Microwave or a well biased home made diode detector is a must when balancing mixers LO power for the first time... Used bolometer based PMs are not inexpensive and maintain their value for a reason.IF the OP is doing this for a hobby, a 5.7 GHZ transverter kit has most of what he wants on already one PCB... Including the Wilkinson... If one wants something a little better, go with the Minicircuits for a more predictable IP3 and consistent performance for identical devices.Steve

author: rs20
author_rank: Super Contributor
timestamp: August 18, 2015, 04:14:46 am
content: Thanks everyone. Even though MMICs cost $2-$10, those same MMICs inside a shielded, connectorized case cost $50 or more. I'm starting to toy with the idea of making a PCB after all -- getting a Rogers 4350 prototype board made is very expensive, but not as expensive as all these isolated parts. I'm intimidated by the amazingly intricate shielding I see on these RF signal analysers, but at the same time if I do preamplification in external connectorized boxes (so that any interference on my PCB isn't boosted 40dB), and have separate shields for each channel, maybe I'll have OK performance? It doesn't really matter if the LO signal slightly leaks over to the RF line; the isolation of the mixer itself isn't that great anyway, and what's a little DC offset between friends? If I:-- pay close attention to keeping my transmission lines the right width(/i.e., impedance) and length and gap to top ground plane, and-- via stitch everything to hell, and-- pay close attention to the recommended board layouts for each part,Do you think I will be likely to succeed? Or is simulation with a $100k EM PCB design package just essential? Note that I'm not looking for a metrology grade instrument, the signal coming into this PCB will be preamplified, -10dBm typ, a -50dBm noise floor seems about right, and I really don't care if the odd spurious tone appears here or there. What I don't want is a completely brain-dead PCB that self-oscillates or something.LaserSteve, the signal I'll be looking is hypothetically (preamplified) 5.8GHz band Wi-Fi signals; and I'm looking for time correlation between the two channels. So I'm guessing that LO phase wobble is not such a concern for that particular usecase. I'm not actually looking to decode the Wi-Fi data, just time-correlated between channels and figure outwhich Wi-Fi channel is which(a pretty mild benchmark compared to what an Anritsu SA can do!)Tim, my divider has only one octave of bandwidth! The two-way one, at least.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2015, 04:29:07 am
content: You can't do any worse using FR-4 and 50 ohm microstrip (or CPW).The loss of FR-4 will be fairly small over short distances. Pack it together (closer than you'd be able to do with coax!), but not so close you can't work on it and you get screwed by proximity and crosstalk. A few cm between blocks would be fine (or you can try matching wavelength fractions, if you're feeling bold).Rogers laminate is really only handy for high performance applications, where waveforms need to remain crisp into the 10GHz range over long distances without compensation (because, the typical PCIe or LVDS signal over FR-4 gets pretty droopy, but those standards have wide thresholds, preshoot and compensation to deal with it), and where resonant structures need tight control of capacitance, impedance, frequency and Q (planar filters, etc.). Which you won't be using, because, good luck with that..You could build your own filters out of discretes, but at 5GHz, I suspect you'll have a hard time without 0402s. If you're okay with that, it might be worth a shot. You can always patch over it with some coax to use a commercial part. For verification, you could build a SPICE model of such a filter, incorporating the roughly expected values for first order parasitics (ESR, ESL, pad capacitance, trace inductance).Tim

author: rs20
author_rank: Super Contributor
timestamp: August 18, 2015, 05:04:10 am
content: I might try building some 2.4 and 5.8 GHz Wilkinson dividers on FR4 as a warm-up exercise. Would you consider that a "resonant" structure?Figuring out how close I can get without being screwed by crosstalk is one of the mysteries I need to figure out!Minicircuits sellsa surface mount diplexerthat look suitable, so I'd probably use that instead of coax cable or discrete arrangements.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2015, 05:48:51 am
content: Yes, in that it assumes wavelengths and stuff. The Q is low because of the resistor, but the matching will only be best at resonance. But if that's good enough, then who cares. Definitely one of the easier structures to look at.Tim

author: LaserSteve
author_rank: Super Contributor
timestamp: August 18, 2015, 02:06:59 pm
content: Sonnet Lite and the legendary HP Appcad are free... But Sonnet is probably overkill for this task.W1GHZ goes up to 3.5 FR4 with almost no concern:http://w1ghz.org/MBT/multiband.htmOthers go to 5 on thin FR4 with almost no concern:http://www.minikits.com.au/electronic-kits/multiplier-kitsThere really is no such thing as standard FR4 any more, modern laminates are quite different, and better...I hate designing interdigital filters, you have to do a lot of cut and try with copper tape for batch one, on the actual board material you will use when etched. That requires instruments at 5 Ghz, although Sonnet will get you darn close.. For what your doing you probably want the Minicircuits SMD or SMA filters.Keep the traces short, properly mitre your bends, use large amounts of plated thru holes with wires soldered into them, and add provision for soldered on tin plate shielding if needed for the IF.For lines less then 1/10th electrical wavelength, things are not very critical at all. I could make that argument until you start approaching 1/4th...I prefer to build my microwave stuff in many little tinplate boxes or in diecast boxes pulled from old gear. However I'm well aware of the resonances that can arise. I've torn apart many commercial microwave transceivers, and most older designers like modular functions with SMA connectors between boxes. Newer designers stuff it all on one or two boards, adding shielding as needed.You probably want a used copy of this:International Microwave Handbook - 2nd EditionEdited by Andy Barter, G8ATDand download this:http://www.n5dux.com/ham/files/pdf/RSGB-Microwave%20Projects.pdfThen take a look at W1GHZ' free stuff...I bet you can find a scan of the older RSGB Microwave Handbooks as well, the 1980s editions are a goldmine of old ways of doing things, many of which were edited out of the international addition for lack of space.Matjaz is the master of low cost, high performance, overkill... All of it on low cost board materials..I'm not responsible if you spend the next two days reading his site,http://lea.hamradio.si/~s53mv/Attached is a picture of a subharmonic mixer, just for discussion...Steve

author: rs20
author_rank: Super Contributor
timestamp: August 21, 2015, 07:25:07 am
content: Awesome, just playing with Sonnet Lite now. Double-sided 1.6mm FR4 gives quite a wide trace (3.2mm), and my particular source of four-layer boards has a 0.3mm - 1mm - 0.3mm layer spacing, so that kinda swings in the other direction (0.3mm dielectric leads to a 0.58mm = 22.9 thou trace). Any experience on whether 1.6mm or 0.3mm FR4 thickness is preferable? I'd like to lean towards 1.6mm as a preference as I have another board already in progress on two-layer.And how do dielectric losses factor into this?

author: KJDS
author_rank: Super Contributor
timestamp: August 21, 2015, 09:05:29 am
content: If it's up at 5GHz, then use the thin stuff or your via inductance will be huge.If it's for the 100MHz IF then you can get away with 1.6mm thick.Appcad should be able to tell you the dielectric losses.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2015, 09:32:42 am
content: Speaking of vias, consider their length as additive to the transmission line length. Use a row of them, if the trace is wide (e.g., 3.2 mm trace width could use 5 x 0.3mm i.d. vias, spaced 0.6mm on centers).Keep traces short and you don't have to worry about losses (give or take maybe 0.5 dB), or phase shift tweaks.Tim

author: rs20
author_rank: Super Contributor
timestamp: August 21, 2015, 09:37:57 am
content: Quote from: KJDS on August 21, 2015, 09:05:29 amThanks! I will go for the 0.3mm option then. But just checking -- a Wilkinson divider doesn't involve any vias, does it? I was under the impression that a microstrip is just fine on its own. Obviously you'd want lots and lots of vias when you start getting into shielded cans, but up until that point, you don't need any (apart from the end launch connectors)... right? (LaserSteve: I will look at your references, I know the answer to this is probably in there somewhere!)Quote from: T3sl4co1l on August 21, 2015, 09:32:42 amCan't I just not use vias? I was planning to make my 5 GHz signal path a first-class citizen on my PCB, absolutely no vias or other such shenanigans. Especially for my first PCB, which will be 3dB dividers only! If I ever do do a first PCB, 6 GHz network analysers are painfully expensive (I forgot the Australian dollar is so weak at the moment)Quote from: KJDS on August 21, 2015, 09:05:29 amAppcad seems to not know about dielectric losses?

author: KJDS
author_rank: Super Contributor
timestamp: August 21, 2015, 10:06:14 am
content: You could always put Hammerstad and Jensens formulas into excel

author: LaserSteve
author_rank: Super Contributor
timestamp: August 21, 2015, 01:41:16 pm
content: For Vias, we use large amounts of them closely spaced with large ground pours on each side. Vias then get wires soldered in them to reduce their resistance. Vias go around the edges of the pours. Edges of the board get a 3-5 mm ground pour all around. Copper tape wrapped around the edges and soldered. In very rare cases slots are cut and brass or copper strips are soldered plane to plane.All RF is on one side of the board. Ground pours, IF, power and LF decoupling can go on the bottom side.Use the thin board at Fop > then 3 Ghz... Thick board will have high radiation losses, it will try to transmit...Here is a board well designed for 5 Ghz.http://www.minikits.com.au/5800MHz-Mlt-1The pipe cap filter on the back side would be something for another thread.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2015, 09:47:09 pm
content: Well, you've still got to have groundssomewhere...If not involved on the traces themselves (like one-end-shorted stubs), then on the ground returns for the SMT parts involved at either end of the traces. And if CPW is involved, along those too.Consider the case for Kirchhoff: you can draw KCL for any single point cross-section of the circuit. So, where a transmission line enters a part, to ground, and back; currents sum to zero, and flow through the TL into the part to ground and back. Or in the middle of a TL, it goes up the TL and down the ground plane.It's often claimed that the speed of light violates Kirchhoff's laws, but I say those who think so are using them incorrectly. KCL/KVL are implicit in Maxwell's equations, and as necessary as conservation of charge and energy. Drawing an erroneously large and unclosed path (like from end to end of a trace, discounting the ground return) gets you into that sort of trouble. I find it very useful to consider a TL port (i.e., line to ground) as a Kirchhoff-able node, and this works for any TEM(00) mode (if the frequency is so high that fractions of a wavelength exist across the thickness or width dimension of the TL (i.e., the higher TE/TM modes), field lines terminate in space and a finer grained application of Kirchhoff becomes necessary).Anyway, at such a loop location, you want to ensure the ground returns are not seeing excessive length to those points. So where the TL enters/terminates into a port, the ground return of that port needs to be well grounded to the reference/ground/shield the TL flows over.Tim

author: AF6LJ
author_rank: Unknown
timestamp: August 21, 2015, 10:31:34 pm
content: Quote from: KJDS on August 17, 2015, 09:01:37 amI would have to agree.

author: AF6LJ
author_rank: Unknown
timestamp: August 21, 2015, 10:44:54 pm
content: Quote from: rs20 on August 18, 2015, 05:04:10 amThe bandwidth you want on those is small compared to FcenterIt is likely you will get enough usable bandwidth out of those.LaserSteve has some good advice, it is nice to see someone here who has worked in microwaves also.Albeit my time was three decades ago..

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 22, 2015, 12:03:29 am
content: You can just buy the whole thing on a chip:http://www.analog.com/en/products/rf-microwave/iq-modulators-demodulators/iq-demodulators/adl5380.html#product-overviewFor $12 from Digikey, by far the cheapest way to go.You can also get reflectionless filters (not sure if they are for sale yet):http://www.microwavejournal.com/articles/24825-reflectionless-filters-improve-linearity-and-dynamic-rangeQuoteActually, the OP was right. The reflected power would be 180 degrees out of phase and be terminated in the Wilkinson's resistor. Same thing if you used a coupled line type splitter. They all have termination resistors.

author: ywara
author_rank: Contributor
timestamp: August 22, 2015, 12:25:38 am
content: Quote from: rs20 on August 21, 2015, 09:37:57 amA traditional Wilkinson is made with stripline using single-sided board. There should be no issues doing it in microstrip, but keep in mind you want your line width and spacing to be conveniently bridged by your dump resistor(s). Lines that are only .020 (.5mm) wide may limit you to 0402 or smaller.End launch connectors don't necessarily need the top and bottom of the board joined by vias, although certainly solder pads are still a good idea. Outside of a very few special end launch connectors, they tend to create a dramatic mismatch regardless of whether or not all four grounds are tied together on the board. You will want some tuning features at this transition if you care about having low VSWR.

author: hendorog
author_rank: Super Contributor
timestamp: August 22, 2015, 04:07:32 am
content: QuoteYour design basically is a single port 6GHz VNAI've done something similar using minicircuit parts. I used hybrid couplers instead of splitters and phase shift lines.I've replaced those parts with the ADL5380 now, which was also mentioned.For ghetto filters I've experimented with using 3d printed PLA plastic as a substrate and 2mm solar panel tabbing strips for the stubs.