Looking for a detector circuit - Page 1
Forum section: electronics / rf-microwave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2021, 06:04:20 pm
content: I recall seeing an RF detector circuit, which I think was used in HP instruments or written up in an HP Journal somewhere, and that used two diodes and an op-amp to compensate for... might've been temperature, or just linearity, not sure. The diodes were in series and the signal was coupled in somewhere along them I think? I can't remember the topology...Anyone recognize that? Been searching all morning, can't seem to find it again.Tim

author: MasterT
author_rank: Frequent Contributor
timestamp: January 05, 2021, 10:35:14 pm
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author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2021, 03:16:44 am
content: The Tektronix SG503 and SG504 leveled sine wave generators use a pair of series connected diodes with their junction driven with RF for peak-to-peak detection. Temperature compensation is provided with separate diodes.I have used two diodes as part of a bridge with RF coupled into one side and the other side symmetrically RF grounded for very sensitive RF detectors. Here the other second provides temperature compensation. They could be seen as anti-series with their junction grounded.

author: M0HZH
author_rank: Regular Contributor
timestamp: January 06, 2021, 03:50:50 am
content: Does this arrangement ring a bell ?https://www.researchgate.net/figure/Simplified-equivalent-circuit-of-the-full-wave-balanced-detector_fig1_332702265Might be worth checking out the HP Journal describing the HP 8484A RF Power Sensor:https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-10.pdfEdit: The HP E4413A RF Power Sensor seems to be closer to what you're looking for:https://www.ab4oj.com/test/docs/5988-9214EN.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2021, 07:10:45 am
content: Nah, it wasn't a full-wave thing, at least I don't remember it being so.Tim

author: Circlotron
author_rank: Super Contributor
timestamp: January 06, 2021, 07:58:49 am
content: What RF frequency are you talking about? I have a circuit for an AM tuner that uses an opamp and a pair of diodes for 455kHz envelope detection.

author: chrisl
author_rank: Regular Contributor
timestamp: January 06, 2021, 08:43:27 am
content: This topology?

author: Circlotron
author_rank: Super Contributor
timestamp: January 06, 2021, 11:38:51 am
content: This one. I'll PM you the complete circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2021, 05:46:15 pm
content: Quote from: chrisl on January 06, 2021, 08:43:27 amHeh, that's more or less the bridge Dave described. Alas, it wasn't this.Quote from: Circlotron on January 06, 2021, 11:38:51 amThanks. Looks to be a basic precision rectifier though. Actually, wait, it has two split half-wave outputs, doesn't it? Weird... not even full wave.I think the circuit I'm after, was either using the amp as a follower, or in feedback with one diode, at ~DC, to compensate the other. So RF could be >> GBW.Tim

author: virtualparticles
author_rank: Regular Contributor
timestamp: January 06, 2021, 09:40:21 pm
content: I've used that kind of circuit in the past but I migrated to Linear Tech, now Analog Devices, power detector chips. You can get any sort of performance that you like including log. No point in using a pair of diodes any more unless you already have them in the junk pile.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2021, 10:58:03 pm
content: Quote from: virtualparticles on January 06, 2021, 09:40:21 pmSo, as it happens, this is for a personal project (and also the curiosity of trying to remember what it was I saw, if I'm remembering it right at all)...Guess I'll settle for the compensated diode circuit then (half wave rect with R || (R+D) load). Or just subtracting a diode drop (e.g. any of the differential configurations). Depending on whether I'm operating in square-law or linear amplitude territory, I suppose.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: January 07, 2021, 01:30:39 am
content: I am sure there are reasons not to go with the chip for maximum performance in specified areas? Usually I can find something in those AFE modules that is not to every ones liking, despite them being a general well rounded solution. The advertising with these modules is really strong though.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: January 07, 2021, 04:39:09 am
content: Quote from: T3sl4co1l on January 06, 2021, 05:46:15 pmThe attached circuit is from Horowitz/Hill "The Art of Electronics", 2nd. edition. The text on page 889 says that it is from HP Journal 10/80, but in my copy there is a handwritten correction to 9/80. I must have made that years ago, no idea if it is right. Horowitz and Hill make the following comment about it:"The designers of this circuit deserve a pat on the back (and three 'attaboys')."

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2021, 06:58:26 am
content: H&H, I didn't think to look in there!Hmm, and I only have 3rd ed. -- it doesn't have this circuit! That explains my foggy memory, I haven't read 2nd ed. in a while.Thanks!Tim