Poor performance of DIY SMA-M Precision Load - Page 1
Forum section: electronics / rf-microwave

author: W3AXL
author_rank: Regular Contributor
timestamp: June 29, 2024, 03:58:57 pm
content: Hi all!Decided I'd try and make myself a nice OSL test kit since I'm starting to accumulate more and more higher-frequency gear. I based my design off of several different articles showing various ways of making a precision 50Ω load that would work well into the multi-GHz range.The design I came up with, while crude as a first iteration, seemed promising. I found a bar of hex stock that would fit the 4-hole flange SMA connectors nicely. So I built one:The load is very simple - two 0.1% Panasonic 0805 100Ω resistors in parallel. This was a common design I found in many others' homebuilt loads, so I figured it would work well.Next step was to measure them. Fortunately at work we have a myriad of RF test gear, so I fired up the E8362 and set up a 10 MHz - 10 GHz sweep using our 85033C 3.5mm cal kit.I had on hand a set of LibreVNA and NanoVNA cal standards as well (the cheap ones they send with the units) so I figured I'd characterize those as well.And here are the results (note that the S-parameter tool I was using plots return loss as a positive number, so higher on the chart here is better):Sadly, and unexpectedly, my homespun 50ohm load was actually the worst performer out of the group! I'm curious what the forum thinks could be the culprit for the terrible response. It doesn't event hit 1 GHz before the return loss drops below 40 dB. I was hoping to at least have a usable load up to 3 GHz (where my home VNA caps out) but I may have to rethink the design significantly.

author: tszaboo
author_rank: Super Contributor
timestamp: June 29, 2024, 04:05:35 pm
content: 75 Ohm connector by accident?

author: W3AXL
author_rank: Regular Contributor
timestamp: June 29, 2024, 04:10:44 pm
content: I wasn't aware they made 75Ω SMA connectors - but as far as I can tell, these look identical to other SMA loads I've got at my desk.Here's a smith chart of the DIY load - I left the S1P files on my work PC so enjoy a terrible screenshot instead:Looks like I've got some capacitance on the lower end and some inductance above 5 GHz.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 29, 2024, 04:20:52 pm
content: Quote from: W3AXL on June 29, 2024, 03:58:57 pmWhat part number? Are they specified as "High Frequency/RF" smd resistors?Common smd resistors might not behave well at high frequency.

author: shabaz
author_rank: Frequent Contributor
timestamp: June 29, 2024, 04:56:40 pm
content: I think you're expecting too much from it. I wouldn't expect such a DIY load to be used beyond a few hundred MHz, perhaps 1 GHz at most. Also, I think most of the DIY loads use two 0603 resistors (0603 do fit in the gap between the SMA center pin and the outer conductor, measured from a reel of resistors to be as close to combined 50 ohm as possible, since sometimes just 1% thick film is used. Plus, it's a bit risky not securing the resistors. I don't know a good solution, but I have tried using epoxy glue in the past, covering all that area including the dielectric (not ideal again).However, reasonable 50-ohm loads are available ready-made at a few tens of $. Example: cheapest Huber & Suhner one should look like a dot on the VNA across your frequency range, and could be glued into the alu housing.

author: tooki
author_rank: Super Contributor
timestamp: June 29, 2024, 05:01:21 pm
content: Quote from: RoGeorge on June 29, 2024, 04:20:52 pmThe most beautiful SMD resistors I ever soldered wereVishay Sfernice CH seriesflip-chip 0603 resistors that my customer had specified by mistake, not realizing they weren’t in the signal path and thus didn’t need to be high-performance ones like those. They’re beautiful little white cubes that cost about $6 each. But they’re good to 50GHz, which is something an ordinary 0603 resistor can’t claim!

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 29, 2024, 05:05:06 pm
content: You can get a bit better frequency response by flipping the resistor over.See this article for more information on resistor frequency response at microwave frequencies:https://www.vishay.com/docs/60107/freqresp.pdfYou could also just buy these for a few bucks and select the best ones:https://www.mouser.com/ProductDetail/Amphenol-RF/132360?qs=V8V9vCxO5JtDrelx3RqrVA%3D%3D

author: W3AXL
author_rank: Regular Contributor
timestamp: June 29, 2024, 05:26:57 pm
content: Thanks for the great suggestions everyone!To those saying just buy a pre-made load, I absolutely could do that (and I've got a small pile of precision loads). But part of this adventure is manufacturing my own, and understanding all the complexities that go with it.I might throw a small quantity of some RF 0603s in my Digikey cart, to see how they compare. Good suggestion on mounting them upside down, I'll give that a shot as well.

author: tszaboo
author_rank: Super Contributor
timestamp: June 29, 2024, 05:45:56 pm
content: For antenna matching, that I make in-situ PCB OSL calibration standards, I've switched to the Vishay FC series resistors.Previously, I had a lot of trouble with antenna matching, because the simulated matching network just didn't work as expected, and I was spending easily an afternoon soldering 0402 parts and scratching my head why it doesn't work. The FC series is like 2-3 EUR each.I had matchings that worked on first try since.

author: szoftveres
author_rank: Regular Contributor
timestamp: June 29, 2024, 05:50:57 pm
content: I'm assuming that the SMA coax inner conductor - SMD resistor junction is a big capacitor and it just looks inductive at higher frequencies - at 10GHz λ/4 is ~7mm in vacuum (even shorter in teflon), so the capacitance could look inductive on the other side of the SMA connector - the clue is that the smith chart is orbiting around the 50Ω point.The bottom end looks more of a concern - the clue is 50.9Ω at 1GHz.I would suggest to characterize your DIY load between 100MHz and 500MHz - at these frequencies the Smith plot is supposed to show somewhat consistent resistance and reactance readings - e.g. 50.9Ω and some capacitance across the band. From this you can deduct an RC model, then calculate a delay, based on the frequency where the DIY load reactance flips sign (becomes inductive) and apply all this data when using your cal standard; or just use this data to reduce the capacitance and further improve your DIY cal kit.At very high frequencies mode discontinuity could be a problem, e.g. the homogenous TEM mode coming out from the coax all suddenly has to split into two, inductive SMD resistors. Usually a good load is a homogenous, lossy dielectric all around the center conductor, tuned for 50Ω. In the DIY world this only can be approximated by e.g. 4x 200Ω resistors in a cross pattern.

author: szoftveres
author_rank: Regular Contributor
timestamp: June 29, 2024, 06:47:36 pm
content: I'm able to approximate your measurement results with the below model - the only odd thing is the 18mm long SMA connector (QucsStudio could be using vacuum propagation speed by default, not sure what to change it to to match the SMA connector's dielectric)

author: G0HZU
author_rank: Super Contributor
timestamp: June 29, 2024, 07:50:13 pm
content: QuoteIt is possible to get fairly respectable performance using 100R 0805 resistors. See below for an old plot of my original SMA 50R load that I made for my old HP 8714B VNA. This was made using a really good quality SMA end launcher connector and two 0805 100R resistors.I made this the best part of 20 years ago and I spent some time selecting 100R resistors from various SMD manufacturers. Once I found the best ones, I selected two that gave the closest to 50R on a decent ohmmeter.You can see it managed 40dB return loss at 3GHz when using a decent Agilent cal kit as the reference. Sadly, it isn't quite this good today. It has some wear and ageing effects. I really should make another one.

author: shabaz
author_rank: Frequent Contributor
timestamp: June 29, 2024, 08:59:21 pm
content: That's a good idea to use a more suitable connector and 0805 (0603 probably wouldn't reach for those decent end-launch connectors with their tiny center-pins).

author: joeqsmith
author_rank: Super Contributor
timestamp: June 30, 2024, 01:32:28 am
content: Quote from: W3AXL on June 29, 2024, 03:58:57 pmWas the data collected with the unit assembled? Was the photo taken with the load off the hex stock to show how it was soldered up? With it having so much capacitive reactance from the start, I question the parasitics due to the hex stock. Sadly, you don't provide any details about how that was machined. Nor did you show the inside of the hex stock where it mounts.If you took the data with the load detached as you show in the photo, then something else is amuck.

author: DaJMasta
author_rank: Super Contributor
timestamp: June 30, 2024, 06:54:52 am
content: While I don't disagree with the recommendations already made, there could be a quick one to check: resolder the ground ends of those resistors. They don't look that well wetted onto the resistor terminals themselves (or, at least, there's a noticeable pinch point between the terminal and the connector body), and you may not be getting the ground connection you're aiming for even if there is some contact/adhesion being made.

author: W3AXL
author_rank: Regular Contributor
timestamp: June 30, 2024, 02:06:15 pm
content: Quote from: joeqsmith on June 30, 2024, 01:32:28 amI did have this suspicion, and I took measurements both with the load removed from the hex stock and without, and they were essentially identical. Maybe 1 or 2 dB different in spots.The hex stock just has a hole drilled about 1/4 deep and wide enough to accommodate the resistors and solder without pinching anything.Quote from: DaJMasta on June 30, 2024, 06:54:52 amNot a bad idea. I'm also planning on making a second load using an SMA-female flange connector to compare (less moving parts, etc). Plus, I'm looking at getting some better RF resistors to play with.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 30, 2024, 02:37:13 pm
content: Just an FYI, PNA was calibrated using an 85033E. Black is return loss for the cal standard I use for low frequency. It is 2X1206 100 ohm mounted to FR4. The load supplied with my original NanoVNA (yellow) works very well for the 300MHz the VNA is rated for.The benefit of the home made standard was to help compensate for some of the errors due to the PCB and connectors.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 30, 2024, 02:51:13 pm
content: QuoteBoth METAS and AppCad are available for free and plot the data as expected. Showing three different loads made with metal film axial parts and plotted with AppCad.One thing to keep in mind, if you use the low cost VNAs and the open source software with them, I ran into a problem with my software trying to import Touchstone files from QT where they write 0s for S21, which if you know your math, causes problems. I have not played with other software enough to know if there are similar problems but if they copy one another's work there is certainly that potential. These viewers can also have problems when presented with bad Touchstone files as well.

author: tomnut
author_rank: Newbie
timestamp: July 06, 2024, 12:09:12 am
content: I've made SMA kits like this for years. I would have expected slightly better than this, maybe -30 dB up to 6 GHz (I didn't go much beyond that).A couple of thoughts for getting higher in frequency though:- try four 200 Ohm resistors?- test it removed from the steel cavity, or enlarge the cavity behind the connector- don't worry about the resistor tolerance, 1% is like -50 dB reflection coefficient.- use Suhner / Telegaertner connectors instead of eBay cheapies- look very carefully at the Smith chart, and then start fiddling with the solder around the centre pin, and possibly the ground, to adjust it.For the last point, before you try to read the Smith chart, you need to port extend. To my eye your chart looks like it has a half-wave-at-10ghz of uncorrected cable.- First calibrate with a better cal kit than yours- then install your short, and adjust the port extension so that you get a clean short circuit on the screen. Cal kits have various reference plane positions, not necessarily at the back of the basic 4-hole SMA. I assume your short is made from copper tape flat on the back of the connector.- now try the load, you should see just one tail of L or C, not a spiral.- a spiral means you need a better connector.- it's probably inductive. Add a blob of solder, or a short wire perpendicular to the Rs, or bring some ground closer to the pin.- or add a small disk of something, to enlarge the pin to twice its diameter, before connecting the resistors (and switch to 0603, etc). We would do this with the centre pin of a PCB-mount connector, tweaking the size of the pad and the ground clearance for best VSWR.Thank you for fighting the good fight, plotting Return Loss as a positive number.

author: Co6aka
author_rank: Unknown
timestamp: July 09, 2024, 04:04:17 am
content: FWIW... Connector construction matters, and each resistor adds a tiny bit of capacitance (and inductance) so you might be better off with one quality 49.9-Ohm resistor.

author: eb4fbz
author_rank: Regular Contributor
timestamp: July 18, 2024, 05:15:33 pm
content: Try a better quality SMA connector and 3x 150ohm 0603 resistors mounted upside down.

author: virtualparticles
author_rank: Regular Contributor
timestamp: September 25, 2024, 03:12:33 pm
content: You'll see some lead inductance from the resistors. You also have a short post that sticks up from the middle. This has an inductance of about 20 nH per inch. You can still use this, but you would have to database it using a golden VNA and apply the database in the cal kit table of whichever VNA you use.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 25, 2024, 03:30:18 pm
content: nanoVNA-H can do GHz ?I would try to get LCR numbers of the small SMD resistors. Spec sheet might show it, but measuring it is a must.What does the terminator as-is show on an LCR meter?Then do the same on the bulkhead ground and pin from one end to the other to make sure the continuity is a good zero.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 25, 2024, 10:10:16 pm
content: Quote from: tooki on June 29, 2024, 05:01:21 pm0603, rated 400mW. Wow, it's a tiny package for 400mW (68 x 33 thou.in.), I wonder how hot it gets at 400mW in open air.

author: Randy222
author_rank: Frequent Contributor
timestamp: September 25, 2024, 10:20:43 pm
content: Just a side question.With the level of wanted "precision" in this DIY config, I wonder if some form of DIY laser diode spot welder, or resistance welding, would help?

author: Randy222
author_rank: Frequent Contributor
timestamp: October 01, 2024, 02:02:42 pm
content: Here's one I did using crap-ola 1% axial (axial measured 50.81OhmDC and 0.096uH on my 4w LCR). Since my cutoff for "usable" is 1.5vswr this poorly made item is good to about 463MHz.The center pin was sticking up about 1.2mm past my solder joint, so I filed it down as far as I could get it.Another perhaps better way is to get small PCB (board about same size as bulkhead flange) that you can slip over the center conductor, has impedance matching tracks and gnd vias, so that you can better easily solder quality smd chips.Trying to solder down low on the center post makes a small blob of solder, which changes the distance between post and gnd across the radius of the insulator, affecting Z over the bandwidth.Another note, make sure to clean after soldering, I use old electric toothbrush with rubbing alcohol, then a final rinse with alcohol and a spray out using can of compressed air. You be surprised of the solder splatter that makes it way all around where you don't want it.

author: virtualparticles
author_rank: Regular Contributor
timestamp: October 10, 2024, 02:03:55 pm
content: Quote from: Randy222 on September 25, 2024, 03:30:18 pmYou can pick the most appropriate configuration for VNA measurement using the attached article.

author: coppercone2
author_rank: Super Contributor
timestamp: October 11, 2024, 12:11:42 pm
content: hmm I bought those 50 ohm tiny ass leaded RF 3GHz resistors a while agoI should try this.

author: Randy222
author_rank: Frequent Contributor
timestamp: October 16, 2024, 08:21:56 pm
content: Here's idea for getting a PCB made to fit the bulkhead. The orange-ish areas are copper. To nail down solid gnd connection (so not 100% relying on metal-metal-contact), after soldering the post to board you can drill a small through hole near an edge/corner/pad and solder in a thin copper wire from pad to bulkhead. Also recommended to trim down the post before soldering, just solder the very tip of post to board so not to leave a solder blob. The outer pads have vias to the 2nd layer gnd plane. It's much easier and cleaner to solder SMD's to pads.Four 200ohm SMD's, 50ohm.Board made in Diptrace.

author: Randy222
author_rank: Frequent Contributor
timestamp: November 12, 2024, 08:17:23 pm
content: Testing out that idea.Just found some decent 0603's that will work out to about 6GHz.

author: tszaboo
author_rank: Super Contributor
timestamp: November 12, 2024, 09:30:18 pm
content: Quote from: Randy222 on November 12, 2024, 08:17:23 pmYou should metalize those mounting holes. Also for RF project like this, I started to throw "best practices" out, and stick vias into footprints, and making all the effort to get parasitics as low as possible.Quote from: Randy222 on September 25, 2024, 10:10:16 pmThey might get very hot, these are designed to dissipate most their heat into the copper on the pcb.

author: Randy222
author_rank: Frequent Contributor
timestamp: November 13, 2024, 03:51:24 pm
content: I did some light analysis of adding some via stitching and making the mounting holes just big vias, but there's not much transmission line between feedpoint and loads. There's some pF there between edge of each trace to the gnd pad.The OP also placed the load side into a gnd cavity which will impact response.I will test several ways, one 50ohm, two 100ohm, four 200ohm.Metal screws in mounting holes also have impact.The design was just an idea to accomodate 1, 2 or 4 smd resistors using pcb instead of trying to solder smd to a post that spans over the dielectric to gnd.

author: tszaboo
author_rank: Super Contributor
timestamp: November 13, 2024, 05:09:08 pm
content: Quote from: Randy222 on November 13, 2024, 03:51:24 pmOK. I usually work with CPW, this is microstrip. Let us know how it goes.

author: Randy222
author_rank: Frequent Contributor
timestamp: November 18, 2024, 10:53:51 pm
content: 4 x 200'sIt's very "bandy" "notchy".Runs ok below 1GHz, notchy above 1GHz. At around 1.6GHz the vswr goes past 1.5, which is my cutoff for practical use, but it is notchy, vswr does come back below 1.5 at higher frequency, but bouy bobs higher and higher until it stays above 1.5With or without the screws made no diff in tests.I reflowed the SMD's, the placed PCB onto SMA bulkhead and hand soldered the post.The results include the 8inch SMA transmission line.The 1st test was just the PCB on the SMA bulkhead, so the PCB bottom layer (gnd plane) is just being held against the gnd of SMA bulkhead.The 2nd test I soldered a strand of wire from gnd pad to the SMA bulkhead body (gnd), results seem to be the same.The PCB is teflon and the sizing of pads were made to be 50ohm(z) using PCB parameters.Results are no surprise.0603's. Vishay MCT06030C2000FP500I have 50ohm and 100ohm SMD's to test too.I have another idea to make just two "lines" on PCB where the 2nd one is 1/4wave of some frequency, so when some signal bounces back it combines 1/2wave out-of-phase and should cancel the reflection from the 1st line. This technically does not erase reflection, but it keeps the reflection out of the Tx port.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 19, 2024, 12:39:38 am
content: Quote from: joeqsmith on June 30, 2024, 01:32:28 amThat is the part I was going to question, too!Quote

author: szoftveres
author_rank: Regular Contributor
timestamp: November 19, 2024, 05:07:30 am
content: Try soldering the resistors directly on the connector; no need for big "center cross" capacitance and long leads.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 19, 2024, 01:04:42 pm
content: Quote from: Randy222 on November 18, 2024, 10:53:51 pmCompared with what?

author: Randy222
author_rank: Frequent Contributor
timestamp: November 19, 2024, 02:27:18 pm
content: Quote from: joeqsmith on November 19, 2024, 01:04:42 pmI have other dummy loads that I bought, all fairly low cost stuff, that are way flatter (more broadband) than my test board.This test was kinda just mirroring the physical setup the OP was using, except I went PCB route. 4x200's was to get some power ability, but 4x might not be good choice. I will try 1x50 and 2x100 just to see how those perform. I didn't fully understand the hex stock mounting, but I don't have that piece of it.Quote from: szoftveres on November 19, 2024, 05:07:30 amIt's something to try, but I would need to manually remove some Au-copper and they extend gnd pad back to the 0603 with wire. Not hard to do, but I can try that later.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 19, 2024, 02:46:01 pm
content: Quote from: Randy222 on November 19, 2024, 02:27:18 pmSame question applies. I remember fooling myself when I started to experiment in the >GHz region and am curious how you know their performance? I never had any luck making my own loads.

author: Randy222
author_rank: Frequent Contributor
timestamp: November 19, 2024, 04:25:07 pm
content: Quote from: joeqsmith on November 19, 2024, 02:46:01 pmI have VNA that can run to 6GHz.For attenutaors I evaluate using an SA that can run to 6GHz.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 19, 2024, 04:55:56 pm
content: Quote from: Randy222 on November 19, 2024, 04:25:07 pmI was more curious what you were using for standards but the VNA is certainly part of it.I have borrowed standards on occasions to characterize my own. Recently I attempted to tighten my measurements with a low cost LiteVNA, characterizing the cheap standards they supply (along with a load I sorted from a batch). Interesting was the cheap standards were not the limiting factor, but instead the VNA.https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5665155/#msg5665155

author: Randy222
author_rank: Frequent Contributor
timestamp: November 19, 2024, 10:39:16 pm
content: I am using no certified reference, no standard per say, and my gear is not the $60k VNA or SA.But I can compare tests of various DUTs, and that should be an ok comparison.The only reason why there's a + on my pcb is so that the pad could get over the distance of the coax dielectric radius (and to accomodate 4x 0603's). The pad spans over the PCB bottom gnd plane by a smidge, but I still used stripline math to derive 50z for the teflon PCB being used. I have plenty of parts to test with, so I will try soldeing the 0603's directly to the post, just need to mod a board to accomodate it, I can trim the post flush with PCB and then reflow the 4 0603's so that the ends of 0603's are touching each other at the center of the post.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 20, 2024, 02:15:20 pm
content: I did all the initial experiments with the original NanoVNA using the ideal model and supplied standards. For low frequency, this worked fine and I was able to verify the results making several measurements of various known circuits. Sometimes, making measurements without any calibration.Outside of wanting to know how far I could improve that T-Check measurement (overall accuracy), I really had no use.

author: szoftveres
author_rank: Regular Contributor
timestamp: November 20, 2024, 04:17:43 pm
content: Quote from: Randy222 on November 19, 2024, 10:39:16 pmThis math won't work at the center point where these traces meet - it'll be just a big capacitor there, unless the traces get narrower at the junction (FEM simulation can help designing the geometry). Also, in theory all 4 traces would need to be of 200ohm in order to add up to 50 when they're connected in parallel.Quote from: Randy222 on November 19, 2024, 10:39:16 pmWhy not just solder to the connector directly, as in the above picture? The PCB will make things way more complicated and less ideal due to all its parasitics it introduces.

author: Randy222
author_rank: Frequent Contributor
timestamp: November 21, 2024, 10:34:46 pm
content: Quote from: szoftveres on November 20, 2024, 04:17:43 pmDirect to bulkhead is like soldering a 0603 to 1kg hunk of aluminum. The idea of PCB was to be able to flow the tiny 0603's and then mount board to the bulkhead, no soldering to bulkhead body.There's no gnd plane under the pin, please see pic of the board I posted earlier.Here's some new testing. 2x100's.Runs a bit more flat out to about 3GHz. I then removed some side pad from the center +, and that improved peformance, 1.5 VSWR near 3.13GHz.Next test will be to remove all of the + and solder 0603's direct to post.

author: szoftveres
author_rank: Regular Contributor
timestamp: November 21, 2024, 11:15:56 pm
content: If you ever get to reproducing what I posted in the photo earlier, I'd be curious to see your measurement results. It can be done with an ordinary iron.

author: Randy222
author_rank: Frequent Contributor
timestamp: November 25, 2024, 09:40:28 pm
content: Did another run, but with some changes.1st, removed the + from the board, then flipped it over.2x100's, Vishay MCT06030C1000FP500Hand soldered (using a glue trick to hold the 0603's in place before soldering). Can probably get better solder with reflow.Two screws with star washers to bite into the board and bulkhead body.Performance is way better. Well below 1.25 VSWR, pretty linear to 1.25 VSWR @ 3.094GHz, and 1.5 VSWR just above that (pic).

author: Hamelec
author_rank: Regular Contributor
timestamp: November 27, 2024, 07:54:20 pm
content: thats all like reinventing the wheel..

author: tszaboo
author_rank: Super Contributor
timestamp: November 29, 2024, 10:40:31 am
content: I wanted to give this one a go, so here are my results from parts that I had around:Chinese end launch SMA connectorsGeneric 0402 resistorsRF splitter V2 PCB repurposedhttps://github.com/tszaboo/RF_SPLITTER_V2Both of these results are below. I was sweeping to 5 GHz.The reference was the LiteVNA calibration kit.Attempt 2 is better, 1.158 SWR. It's two resistors soldered to a CPW transmission line at the end.Since both of these have a bump at around 3 GHz, I could improve my footprint, or maybe a better connector can be used.While 1.15 VSWR is very good, 99.5% I would be very satisfied with that with an antenna.It's nowhere near the results from OP, some 15dB worse.

author: tszaboo
author_rank: Super Contributor
timestamp: November 29, 2024, 11:39:28 am
content: And a quick follow-up, some qucsstudio simulation of the circuit I have.It's an S parameter simulation, based on the parameters I got from the impedance calculator when making this PCB.It confirms my suspicion that placing the resistor closer to the end of the connector makes it better.Right now it's 4.2mm away, simulation placed this at -22dB, I measured -25.Placing it at 0mm, the situation would improve by -13dB.So it doesn't really like my generic controlled impedance FR4. Or maybe I have to follow the calculators to more digits for precision applications like this.

author: Randy222
author_rank: Frequent Contributor
timestamp: December 02, 2024, 06:20:13 pm
content: Quote from: Hamelec on November 27, 2024, 07:54:20 pmNot really. The only reason I selected a PCB in the footprint of the bulkhead, was to make soldering the tiny SMD's a tad easier.I am now working on a 50 x 1 on a modified board.

author: szoftveres
author_rank: Regular Contributor
timestamp: December 02, 2024, 08:05:29 pm
content: I see that everyone here is using the LiteVNA cal kit as the reference - is it really that good? How does it compare to an actual reference cal kit?

author: joeqsmith
author_rank: Super Contributor
timestamp: December 03, 2024, 02:18:32 am
content: Quote from: szoftveres on December 02, 2024, 08:05:29 pmAppears yours is the first mention of the LiteVNA cal kit in this thread. Define good? Wear? Mate cycles? Cost? Repeatable? Characterized?

author: szoftveres
author_rank: Regular Contributor
timestamp: December 03, 2024, 02:56:54 am
content: Quote from: joeqsmith on December 03, 2024, 02:18:32 amHmm, did I use the word "everyone", when actually there was only one mention in this thread of using the LiteVNA cal kit as a reference? I apologize for that!

author: joeqsmith
author_rank: Super Contributor
timestamp: December 03, 2024, 02:09:24 pm
content: Quote from: szoftveres on December 03, 2024, 02:56:54 amNone of the standards supplied with any of my low cost VNAs were characterized.All of these low cost standards have all been SMA, not a precision 3.5mm.None of the low cost short or open standards allowed a wrench to be used to lock the center conductor. Some of the thru standards did not provide a flat for a wrench. The thrus supplied with my LiteVNA64s have all had a flat. Short and open standards were modified.All of the low cost standards I have received were shipped in a bag loose, free to bounce around. I've seen many a ham treat connectors this way.The difference between the a virgin set of open and short standards supplied with the LiteVNA64 matches very close to the ones supplied with my V2Plus4. See the following:https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5666353/#msg5666353All the low cost loads I measured had poor return loss. I used one of them to create a low cost T-Check. The load I use for calibration was a hand selected terminator from Mini-Circuits.Characterizing all of that mess with my PNA (against an ECAL that was calibrated to a set of known Agilent standards). Using METAS VNA tools to derive the polynomial coefficients. Using Agilent's cal kit editor to create a new user cal for them. Then using these standards to calibrate the PNA, it would pass a T-Check within 10% over the entire frequency range (300k -9G). See:https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg5676077/#msg5676077Using these same standards with the LiteVNA64 yields poor results. It does not matter much if I use the METAS fit data, or the databased S-parameters. The same math can be ran with the Agilent PNA, it it works fine. My only conclusion is that if you were to put together such a set of standards, they would not be the limiting factor when making measurements with the LiteVNA64.How they wear over time, assuming you took very good care of them, hard to say. I doubt they are rated for very many mate cycles. The LiteVNA64 is a great system for the cost and very useful. But I don't think all of my low cost VNAs combined = the cost of one good short standard from Keysight.

author: Randy222
author_rank: Frequent Contributor
timestamp: December 03, 2024, 07:13:14 pm
content: Quote from: joeqsmith on December 03, 2024, 02:09:24 pmTrue.People buy what they can within reason, who would of thought a decent 4ch 200MHz 12bit digi o-scope can be had for $275(GBP). 20yrs ago that would have been $4k(GBP).Here's my next test, 50 x 1 done two times. Item "B" is better than "A", but both are fairly wavy on the VNA. I soldered a grounding band to the bulkhead.I cannot get "flat" reponse past 3.2GHz.The 100 x 2 setup was the best of the bunch.0603's are a challenge to solder, so I might try bigger SMD's.

author: coppercone2
author_rank: Super Contributor
timestamp: December 03, 2024, 10:40:21 pm
content: The steel ones might be easier to solder with the right flux, because they transfer heat less.I thought about how to do this properly alot. copper spot welding does kind of work. I thought to spot weld a copper shim to the body, so it can be bent into a U shape laying on the side, so you can solder a part to it with only a reasonable amount of heat, and minimum extra inductance, and after you spot weld it to the shim, bend and squash it into place along the center conductor. Spot welding copper to copper sheet does seem to work, but I have not tried thin to thick bonds. Or, spot weld a nickel strip, and then pen plate it with copper.It seems like the assembly of these loads is the hardest part, there is alot of options, but everything solders like crap.Another option is to maybe bolt a fabricated fairly thick copper shim with a hole in the middle to the conductor with alot of tiny bolts, and solder a resistor to it, so when you bolt it down you only need to solder it to the center pin.I imagine in almost all the cases, the resistors are severely stressed by the huge thermal mass of the connector, its not getting anything close to a proper solder profile. I wonder if it has a adverse effect on the RF properties of the part, like reduced stability.Or indium.The fact that the soldering profile is so out of whack makes me feel like all of the DIY standards are flawed compared to whatever magic they use for proper ones, but I think there is some solution that will solve this problem and let you confidently solder a expensive RF resistor down without violating its soldering spec.And if this was figured out, you could try to use whatever connector you please, not the one you have to find does not start spewing plastic during the soldering. For the higher frquencies, I think teflon might even be disturbed because of the giant thermal mass of the chassis connector, which is designed to be screwed in.

author: Randy222
author_rank: Frequent Contributor
timestamp: December 05, 2024, 10:23:23 pm
content: I have a decent spot welder for doing the nickel tapes to batts.Maybe I can flow solder to pad, and then try to spot weld a SMD to the pad.Some 0603's don't have a wrap-around tin end. The 50ohms I have have a sliver of tin on just one side, so that makes it hard to spot weld into place.I am also working on laser spot welding.But you gave me an idea for the other SMD's I have.