Avoiding a back termination for a terminated output- special case or BS? - Page 1
Forum section: electronics / rf-microwave

author: jwet
author_rank: Frequent Contributor
timestamp: February 26, 2022, 03:48:03 am
content: I'm designing a simple high impedance probe for a spectrum/network analyzer. My network analyzer has an input Z of exactly 50 ohms (return loss >40 db). I'm looking at FET input op-amps like the OPA656 (leading candidate and I have some) which has input Z of 10^12 ohms in parallel with .7 pF. I'd like to get a 1M ohm input Z with a gain of 1 and less that 1 pF. I'm thinking I can put the op-amp out at the probe tip in a non inverting configuration with a 1M pull down to bias the tiny bias current and AC couple the signal in to give me a response to say 10 Hz. At the output, it would be simplest to just run the output into the coax and into the NA input and not back terminate it with 50 ohms. Back termination is generally advised but my logic is that the analyzer input is exactly 50 ohms so there will be no reflection at the load end. Back termination just tames things down for poor terminations which isn't my case. What this does for me is to avoid the 2/1 loss that a back termination would cost me. The OPA656 has a BW of 500 MHz with a gain of 1 but only 200 Mhz of BW at gain of 2. It seems like a shame to throw away this bandwidth for something that won't happen- the return loss at the SA is perfect. The OPA656 is capable of driving a 25 ohm load and will output +-3V into 50 ohms- is has a 1W limit- I will heat sink it probably. I will include the back terminator in the layout and the gain of two option but I don't think its necessary in this situation.There is one other variable that you might be thinking about and that is the characteristic Z of the coax line to the SA. I have some pretty nice flexible cable that is right on the button at 50 ohms or let's just assume it is for the purposes of the question. I don't know if I'd want to put this into production but it looks worth trying for a one off.Discuss amongst yourselves- flames welcome, be polite. Its something that occurred to me that is possible in this special case of a precision load.

author: bdunham7
author_rank: Super Contributor
timestamp: February 26, 2022, 04:18:26 am
content: Sounds like this project here:https://www.edn.com/high-impedance-fet-probe-extends-rf-spectrum-analyzers-usable-range/Yes, I think if everything is perfect your idea should work, but you'd have to try it as there might be unknown traps. It should be easy enough to build it both ways and test it, as you'd just have to omit or swap out R3,4,5 as required. Also, perhaps look at OPA659.https://www.ti.com/lit/ds/symlink/opa659.pdf?

author: jwet
author_rank: Frequent Contributor
timestamp: February 26, 2022, 04:32:01 am
content: Geeze- that's so close- I guess great minds think a like. I've read stuff by the author in the past.Thanks very much for the response.

author: jwet
author_rank: Frequent Contributor
timestamp: February 26, 2022, 05:20:58 am
content: After looking at this link a bit more, I'm less impressed and confused I guess- perhaps I'm missing something. He is trying to emulate the input characteristics of a standard scope input- this wouldn't be too bad, one could then use standard 10:1 type probes but if the idea is to make a true Hi-Z probe with the amp at the tip, its kind of strange. I don't understand why he would add C to the input of a real circuit level probe- this makes it look more like a scope input but that's not the problem. I like the op-amp obviously, same one that I found, but I don't understand the input circuit- adding 5 pF? I want to maintain all the goodness of that nice amp and will experiment with the gain of two and back termination. Realistically, I don't need a lot of bandwidth- I'd just like to use the vector voltmeter type of function for things like tuning feedback loops- compensation- kind of like a Ridley (sp?). Thanks anyway- thought provoking. I'll contact the author- thanks.

author: bdunham7
author_rank: Super Contributor
timestamp: February 26, 2022, 05:43:50 am
content: Yes, he clearly intends to duplicate a scope input so that you can properly compensate a probe, among other things. You could omit the capacitor and see what happens, but I doubt you will get a bandwidth or flatness anywhere near what you hope for, as you cannot eliminate the parasitic capacitance, only compensate for it as you do with a probe.If you just want an inexpensive or DIY active probe and don't mind AC coupling, have a look at this eBay seller's design based on the dual-gate . Unfortunately he is in Ukraine, so you probably won't be able to buy anything anytime soon. However, he has helpfully included a schematic.https://www.ebay.com/itm/332393676082https://www.nxp.com/docs/en/data-sheet/BF998.pdf

author: jwet
author_rank: Frequent Contributor
timestamp: February 26, 2022, 06:30:57 am
content: Thanks and god bless Ukraine.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: February 26, 2022, 06:46:08 am
content: I used some random online calculator, I'm too lazy to do the proper math, but according to that your noise density on the output, just from the amp would be ~33dB above thermal noise. That's noisy.This means that the passive 1kOhm resistive splitter approach with the 26dB loss is still better than this. Obviously it gets worse the higher the input impedance needs to be, but especially probing RF applications something along these lines should be enough. The passive probe will also have much higher bandwidth with a flatter response and needs no bulk in the probe nor a power supply.This is of course true if your SA is sensitive enough but any reasonably modern SA LNA should be much better than that. Let the analyzer work for you.Do I miss anything? What is your requirement for the input impedance other than 1M sounds enough?

author: jonpaul
author_rank: Super Contributor
timestamp: February 26, 2022, 09:02:57 am
content: Jwet, Bravo for the effort,Suggest you study the fine Tektronix FET and differential probes, designed for exactly the same use, P6201, P6202A, etc. Documents and manual available at Tekwiki, BAMA, etc.Also the fine Tektronix Circuits Concept book form1970s, Oscilloscope Probe Circuits.The exactly same design applies to SA application.Your SA (which one) may have 50.000 Ohm R, but must have some residual L, C, thus a matching systems is requiredEvery RF transmission system, instruments, Analyzers, have Zsource= Zload, NOT back term.bon chanceJon

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2022, 12:38:53 am
content: What you suggest will work to preserve the highest frequency response. Old oscilloscope vertical amplifiers which were working at the limit of their transistor Ft did exactly the same thing to avoid losing half of their gain-bandwidth product per stage.Besides performance limitations, the disadvantage of using a feedback amplifier is poor overload recovery.

author: jwet
author_rank: Frequent Contributor
timestamp: February 28, 2022, 04:30:26 pm
content: Thanks- the overload recovery is another benefit.I found Jim Williams talked about this in his AN47 about High Speed Amplifiers. He has an example and shows an optional sort of series resonant RC snubber to ground at the transition to the cable that can be used to tame any residual reflections.Good Network analyzer inputs are kind of a special case- they are precisely 50 + J0 ohms by design and I can measure it with a second instrument.I have some other ideas for this circuit- one is to float the circuit on its own output, kind of bipolar bootstrapping. This should make the input Z very, very high.Thanks All- I'll report back.

author: jwet
author_rank: Frequent Contributor
timestamp: February 28, 2022, 06:10:16 pm
content: The goal is to use the NA at low frequencies to look at control loops etc- usually in op-amp level circuits or power supplies feedback loop. My rationale is below.The instrument that I have (HP4195a) goes down to very low frequencies- 10 Hz SA, .001 Hz NA but has 50 ohm inputs. They're aren't many instruments that give you accurate phase information directly -getting phase from an O Scope is pretty imprecise especially if you want sweeps. There are instruments like Dynamic Signal Analyzers (old HP and Stanford Research) that have high Z inputs and do complex FFT's but those instruments top out at high audio frequencies around 100KHz. I'd like to lightly load fairly high impedance signals (1k to 10k) line level signals at frequencies up to about 5 Mhz with some headroom for harmonics- say 50 Mhz as a stretch goal. A scope 10x probe is a pretty good approximation of what I'd like though I could probably live with a DC impedance of a 300K or so and reactance of 20 pF or so and would like the lower noise. I know that if you take these impedances and noise densities and integrate them over wide bands, noise looks bad but I'm not looking at RF. I'll be looking at 1v'ish signals. I have an HP85024 RF FET probe for probing high impedance RF stuff- its fine except that its lower F limit is 300 Khz and its pretty fragile and low level- very easy to fry doing casual trouble shooting. Repairing these requires a custom $300 hybrid when you can find them. The 950 ohm direct probe is almost good enough. I'm going to just try a 9950 ohm - 100 x probe. The analyzer has about 100 dB of dynamic range- so your point is well taken, I could throw some of this away though getting to 100K would throw an awful lot of DR.My main intent with the post that I thought was kind of thought provoking was that back termination is not strictly required. I've satisfied myself that this is true and I wanted to bounce it off of others to get their opinions. I've found a couple of references (below) where this is done. I think NA inputs represent a somewhat unique case in that their inputs are precision 50 + J0 ohm inputs.Thanks all for your inputs.