Board input port has awful return loss - layout issue? - Page 1
Forum section: electronics / projects

author: jmw
author_rank: Frequent Contributor
timestamp: November 23, 2022, 04:45:05 am
content: I've been working on building an active probe using TI's BUF802; it's got impressive performance and is almost an all-in-one solution. After hacking TI's evaluation board to do most of what I wanted, I made my own test board to try a few other changes. The board terminates the input port to simulate probing a terminated line, but the return loss is horrible; S11 is about -2.5 dB at 3 GHz! By comparison, TI's evaluation board is about -12 dB, and the connector with the terminating resistor soldered together is better than -20 dB.The terminating resistor R11 is close, but the other signal paths via R1 + C1 and the voltage divider of R3 + R4 are connected by 5 mm and 10 mm traces, respectively, and come to think of it 10mm is pretty close to 1/4 wavelength in FR-4 at 3 GHz. Is this branching connection out from the input port the culprit?If I run a 50 Ω trace to R11 and place the other passives very close like below, will this improve performance? The connection to U2 is still through a long trace, but my thinking is that keeping the other loads right next to R11 will prevent most reflections.

author: octillion
author_rank: Contributor
timestamp: November 23, 2022, 05:35:15 am
content: The existing layout is certainly not very good, with stubs going to R1 and R3. As you point out, it seems one of them is hitting close to 3 GHz. You can bisect the problem and see if the R3 stub is the culprit by cutting the trace with a razor where it leaves R11.Your revised layout is much better. The path length between R3 and U2 is not very important as this only covers low frequency response. Keeping the signal path to the BUF802 short is more important (see first bullet under "11.1 Layout Guidelines" of the datasheet).

author: jonpaul
author_rank: Super Contributor
timestamp: November 23, 2022, 07:01:52 am
content: How did you measure RL at 3 GHz?What cables/adapters/connectors?Exactly what is the connector used for the BNC (?) J1?Not all are equal or rated at 3 GHz.Finally most work at those freq does not need such a probe, just a Zo probe on a 50 Ohm scope input.See very old 1980 Tektronix Probe Circuits Concepts book: Chapter on Zo probes.https://w140.com/tekwiki/images/6/62/062-1146-00.pdfEasy DIY, a 450 or 4950 ohm series R.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 23, 2022, 08:29:49 am
content: Why would you not put R3 right up on the trace? Its impedance is extremely high, it's not loading the trace it touches, and anything else is dead stub length. Yeah: second layout is far better!I don't know the input characteristics offhand of the BUF thing but if it's capacitive or RC, plus the 75 ohm into it, you may find its impedance dropping as frequency goes up. Which therefore should be balanced by an R+L or R || (R+L) for the terminator -- instead of just 50 ohms flat.600fF, is that a typo? Can you even get parts that low, what is FR-4 tempco going to do to it? Well, maybe I should read the BUF datasheet anyway.Oh---it's not much of an active probe, if it's 50 ohms, huh? Like, why not make a preamp instead, just toss in a MMIC or whatever, characterize it and apply compensating filters to flatten it back out (also, minimizing s11)? Not sure what the purpose here is, I must be missing something from earlier threads or something. Evidently it's not for a low-C FET probe sort of thing, and it's not matching 1M || some pF for a scope probe (which would beg the question of how to get 3GHz into such a probe in the first place(!!)).Tim

author: jmw
author_rank: Frequent Contributor
timestamp: November 23, 2022, 03:45:38 pm
content: Tim- oh this isn't the actual probe, it's a prototype board to characterize it under repeatable conditions. The real thing won't have a 50 Ω input termination, but we can assume it will probably be probing a low-Z terminated line, so R11 presents a fixed termination for hooking it up to test equipment. I spread things out in the layout since I expected to trim certain parts and didn't want everything packed together when I went in with tweezers - seems like a bad idea now. Without R11, the input impedance should be 2 MΩ || 500 fF (ish). TI's evaluation board has a RL termination for the reasons you mention, but I didn't include it because the probe won't be managing the termination in real use.Sub-1 pF capacitors are pretty common - this is the part I'm using for C1:GJM1555C1HR60WB01. Together with the 2.4 pF input capacitance of the BUF802, you have a 5x capacitive divider, and then with a doubly-terminated output you get a 10x probe. The low-frequency path through the R3 + R4 + U2 etc. is designed to match the 1/5 gain. It's been working well on both mine and the TI eval board except for this poor input termination is showing up as a lot of ripple in S21.jonpaul- I have a 450 Ω Z0 probe but wanted to try making a DIY active probe since this chip makes itveryeasy. The connectors on this board are surface mount SMA.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 23, 2022, 04:42:24 pm
content: Ah, OK.Seems I'm used to seeing rougher tolerances on 0805 caps, 0402 makes sense.Hmm lemme see here...https://www.ti.com/lit/ds/symlink/buf802.pdfYeah "2.4pF", buttypical. Who knows what tolerance is on that. You gonna trim that too?And who knows what ESR. Scopes for instance aren't "1M || 20pF" or whatever, there's ESR on that cap too (just as there's ESR on the probe cable itself). Quite possibly the same applies here, at higher frequencies but at smaller capacitances too. So you can end up with odd dips and peaks, probably still gradual (real poles/zeros) unless you've got some pesky trace resonances there, but basically the "hook" from ye olde probe design, just at higher frequencies.Tim

author: jmw
author_rank: Frequent Contributor
timestamp: November 23, 2022, 05:00:50 pm
content: Quote from: T3sl4co1l on November 23, 2022, 04:42:24 pmYup. On my board right now there's a dip around 2 GHz that goes away when I put a dab of flux or IPA on C1- but it's of secondary concern when there's so much S21 ripple I can't evaluate it for sure. Together with the part-to-part variation in the input capacitance, I'll have to keep plugging to see how robust this design is.

author: jonpaul
author_rank: Super Contributor
timestamp: November 23, 2022, 05:13:12 pm
content: Jmw, I wouldn't rely on the capacitive divider composed og the 700 fF series C and Cin of the amp, as the amp may have a complex Zin.Would check the S parameters of just the amp itself.Alt...decimate down to just the 700 fF C and amp, see the transfer function S12 and plot over desired band.At GHz freq, I think this type of probe is not useful vs a Zo. Try a 20, 50 or 100x Zo, we use the excellent Tektronix P6156, available tip R for all those attenuation, spec to 3.5 Ghzhttps://download.tek.com/manual/070643001.pdfEnjoy,Jon