Debugging RF filters + LNA, I'm at a loss. - Page 1
Forum section: electronics / projects

author: aaronhance
author_rank: Regular Contributor
timestamp: December 21, 2024, 10:27:21 pm
content: Electronics hobbyist here, I've designed a board with 137Mhz bandpass, FM band stop and LNA. I've made up two of these boards, the first I manged to get some I would say reasonable results on my nanovna, but after messing about with it I think the board developed a short, my latest board I get nothing out of it, S21 reads the same with LNA on or off, I'll provide the graphs. Now when my first board was showing promising results, I tried it and the signal, a NOAA weather satellite LRPT, was almost invisible, I had better reception without my board. I initially didn't use a dc block into the nano VNA though, with my first board and initially with my second, only recent realised it says it needs a dc block, which I've whipped up a pretty bad one on some proto board, made no difference. I'm just not sure what to check from here. Thanks for any help!schematic and layout:Simulation:Best test from first board:Latest results:test setup:

author: shabaz
author_rank: Frequent Contributor
timestamp: December 21, 2024, 11:46:38 pm
content: Hi,I don't know the answer, but as a suggestion, it could be worth taking advantage of the fact that everything is 50-ohms by doing things bite-size and testing individual building blocks.For instance, the photo shows a much smaller building block, just an amplifier, that can be tested in isolation. This is more versatile, too, since you can reuse the building block for any other applications as your requirements increase or change.A filter would be another building block, which could first be tested on a small scrap of copper-clad board and again tested in isolation.Regarding how to test, maybe a bit unorthodox, but I'd suggest that using a VNA (or at least, using vector measurements) is a bit hard to start off with since it requires some additional knowledge and calibration up-front, and some of that is subtle and easy to get wrong initially and end up quite confused. It's probably way easier to initially just consider scalar testing, either with the VNA, or just a spectrum analyzer plus signal source (ideally a tracking generator function).You'd then test your amplifier, and it should be very easy to confirm you're seeing the gain you're expecting across your frequency range of interest. You certainly should have a DC blocking capacitor as part of your amplifier building block; for instance, in the photo, you can see C48 at the output and C47 at the input.28dB is a lot of gain for one stage; depending on end application, you might prefer to split into (say) two 14 dB building-blocks, so you can optionally remove or switch one out if desired.You'd also test your filter in isolation, get that functioning and then chain it all together for testing your system.If/when you wish to test with a VNA, you'll then at least already know that your building-blocks are to a good degree already functioning.

author: jwet
author_rank: Frequent Contributor
timestamp: December 22, 2024, 01:38:42 am
content: You've provided a lot of data but its not so simple to sort it all out. As shabaz said, the best way to attack this stuff is by breaking it down.I would start by putting an SMA connector stub on the input side of the MMIC and looking at it. (you'll have to do a dummy calibration with something similar or use a port extension- does Nano support that?) Your LNA looks cool but is barely specified for these frequencies- its much more comfortable around 1.5G. You were probably impressed with it NF, etc. but these are also up at higher freq's. I would get a handle on its input Z at 137M. The data sheet graphs shows an S11 of 0 at low freq's, that something like a short or an open- not so good. Measure this and turn this into complex impedance if you can- the LNA might not be suitable. Once you get a handle on this- you can get a decent match from your filter, might involve tweaking filter. Are you assuming Zin of this amp is 50 ohms- its not anywhere near that at your freq's.I think you can get this to work but I would probably look at mini circuits for something that's designed for VHF. At these frequencies, you also could make a pretty simple common base stage and follower with a decent pair of VHF transistors (MPSH10, etc). Bias them pretty hard for lowest noise consistent with re you'll get on the input. What MDS are you hoping to get? What NF do you think you need? Would like to hear more.

author: aaronhance
author_rank: Regular Contributor
timestamp: December 22, 2024, 04:01:25 am
content: Quote from: jwet on December 22, 2024, 01:38:42 amAh, yes I see now, looking at the s parameters the input impedance of the LNA is way off at 137Mhz, S11 of -4dB (202.3 + j59.1 Ω? vs my 63.8−j10.4Ω? (estimated)), well I guess that explains a thing or two. In my defence I designed this board 8 months ago, when I didn't know any better. I'll give trying to measure the output of the filters a go, I might also try bodging in a matching network, if I can find the right components. Those things should hopefully tell me if that's my only problem, but I think there must be other issues or losses? Yes nano VNA supports port extension. Thank you!

author: aaronhance
author_rank: Regular Contributor
timestamp: December 22, 2024, 04:58:15 am
content: The first test yielded some fruitful results.

author: jwet
author_rank: Frequent Contributor
timestamp: December 22, 2024, 05:11:02 am
content: Shorten those wires on your VNA connect and make sure that they are calibrated out. This stuff even at VHF like 137M, (2M+), wire lengths like that turn into phase shifts and move things around a smith chart many degrees- its no joke- its changes everything. Keep it tight or calibrate it out.You're on your way I think. When you design an amp or whatever into your system- get something that is centered on your target frequency. A lot of companies will specify the widest range imaginable but its often a fantasy. At these low VHF frequency, MMIC don't really apply.it's always amazed me how much a small mismatch matters. The real problem is it logarithmic and gets bad fast.....

author: RFDx
author_rank: Regular Contributor
timestamp: December 22, 2024, 10:44:05 am
content: The output impedance of the GRF2133 is close to 50 Ohm, the input unfortunately is not. A matching network for 137 MHz is needed. An example with standard component values for the amp biased at 100mA or 70mA is attached. I used a small 4dB attenuator at the output to improve S22 and get the huge gain somewhat down. You are missing a DC block (1nF or similar) at the output. Don't forget to remove the 50 Ohm shunt resistor placed at the amplifier input.

author: jwet
author_rank: Frequent Contributor
timestamp: December 22, 2024, 09:04:56 pm
content: The software reminds me of some old software I used to use- RFSim99. Is it still around? Are there modern versions that run on Win10, etc? I used to love that program!

author: aaronhance
author_rank: Regular Contributor
timestamp: December 23, 2024, 01:15:52 am
content: It's still around, it's what I use.

author: fourfathom
author_rank: Super Contributor
timestamp: December 23, 2024, 02:12:59 am
content: When you said that your first layout mostly worked, but after messing with the board it is essentially dead, is that a new layout or cuts/jumps/value changes on the original board? I'm not sure what problem you are trying to solve...Probably unrelated, but in the layout below why are the input capacitors (C1 and C2) placed at the end of short trace stubs? This will add series inductance, providing notches and reducing the ultimate high frequency attenuation (much like an elliptic or Cauer filter). The short stub on L2 is less of an issue, but it will add some inductance. At even higher frequencies these stubs will act as shorted or open transmission lines, further affecting the filter response.Quote from: aaronhance on December 21, 2024, 10:27:21 pm