Logarithmic Detector. - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 16th Sep 2023, 9:18 am

Over the winter I will be playing around with some filters, and to properly assess them I really need a logarithmic Detector to use with a sweep generator and scope, so I can see the symmetry of the response and get an idea of whether my tweaking is improving the shape factor or introducing near-in spurious responses.

In times gone by I would have hired something from R&S at great expense but now I have no business accounts in which to bury the cost I need to do it myself.

So has anyone got suggested circuits for a logarithmic Detector working around 1.4MHz??

Jon_G4MDC · 16th Sep 2023, 9:25 am

This or the less capable AD8361 maybe?

https://www.analog.com/en/products/ad8362.html

Radio Wrangler · 16th Sep 2023, 11:36 am

ADI have plenty of log detector chips, but look out for their low frequency roll-off specs.

At that frequency you can also build your own out of a lot of discrete transistors and schottky diodes. Think of a chain of differential limiting amplifiers with feedback to set less than 10dB per stage, with a load of schottky diode detectors - one on each stage. Put the DC from each detector into a summing node via a high-ish value resistor (all equally-weighted) and you have a log detector.

Alternatively find an old spectrum analyser for that frequency range and build a tracking generator for it.

Second alternative, if that analyser is a storage type, just sweep an ordinary sig gen manually and watch it paint the picture.

I've done all these things over the years. The discrete logger was for a spectrum analyser being designed into what was planned to be a replacement for the HP MLA.

The ADI chips are likely the easiest approach.

David

G6Tanuki · 19th Sep 2023, 5:30 pm

Thanks for the suggestions; the ADI chips look interesting, and they're not exactly expensive either.

I'd love to give home to a storage-capable analyser but these seem to be either frighteningly expensive (even secondhand) or - like lots of top-end test gear - of an age where I would probably spend more time repairing it/paying to get it calibrated than actually using it.

Radio Wrangler · 19th Sep 2023, 8:28 pm

I built an over the top ATU some years ago and put a dual directional coupler in it. Two channels with AD640 loggers (Samples, thank you ADI) gave me logarithmic scaled power meters fore and aft. Simple subtraction gave a linear meter of dB return loss. Limited outputs went into a phase detector and gave a phase meter. Voila! a VNA running full transmitter power.

Shame I built the wrong sort of ATU for the antennae I use. Comes out for special events, though. 10mW to 1kW on a single meter scale.

David

Jez1234 · 19th Sep 2023, 9:17 pm

I'll bet you can no longer get the ADI IC's in a usable format now though. No doubt BGA or SSOIC only.

mhennessy · 20th Sep 2023, 9:48 am

I was set almost exactly this challenge, almost exactly 30 years ago. Looking at my log book, 30 years and 1 month, to be precise! It was a bit scary - I'd only just started my industrial year, and it was my first RF project after a life of audio.

Luckily, I spotted the NE604, which was a complete FM IF system that included an RSSI output. I ignored most of the chip and just used the log converter driving the RSSI pin. It worked better than the dedicated log amps the older staffers suggested I investigate, required much less support circuitry, and was a much, much cheaper part. It was pretty accurate (+/- 1.5dB) over a 90dB range.

This bit of lateral thinking (luck in reality!) made me a bit unpopular with some, but the final option was cheaper and better in every metric, so was put into production (a high end radio receiver that I'm not allowed to talk about!).

Although that part is not in production today, I'm sure there will be modern chips that do the same, and I wouldn't be at all surprised if, like then, they are a lot cheaper than the dedicated log amps. That's because one has a relatively large market, the other is rather niche by comparison.

Cheers,

Mark

G6Tanuki · 21st Sep 2023, 6:12 pm

I'd forgotten about the whole thing of RSSI circuitry in receivers; I guess I'm now going to be spending hours looking at the less-intensively-integrated chips used in phones and WiFi routers.

Probably there will be something available for a few pence per chip [provided you order half a million of the things]!

Jez1234 · 21st Sep 2023, 10:38 pm

IIRC there was an S/A design for the radio ham years ago which used a wideband TV front end and as the log converter it was as above with the RSSI of the NE604. Also IIRC... it's in one of Pat Hawkers "Technical Topics Scrapbooks", or Radcom of course.

Radio Wrangler · 21st Sep 2023, 10:53 pm

Roger, GM4PMK did the best known British one.

Modern highly integrated receivers seem to have all gone NZIF quadrature. No separate RSSI as such and you need a major corporation behind you and NDAs just to see a data sheet.

David

Jez1234 · 21st Sep 2023, 11:59 pm

That sounds familiar yes. In fact did it not use the NE602 as a front end? (and there were issues due to LO not covering the published range for many constructors?) There were a few in S/A ideas in Technical Topics over a few years and I think I may be confusing that one with the TV tuner front end one and a CA3189 used for its RSSI, maybe by a NZ amateur? Entirely going from dodgy memory here!

jhalphen · 22nd Sep 2023, 8:30 am

Hi to all,

Just a suggestion from reading the US thread below about using a logarithmic amplifier to display a wide dynamic range when adjusting receiver IF sections.
The chip is a AD8307 DC-500MHz 92dB Log amplifier by Analog Devices.
Quoted as a "low cost device" (see included spec sheet).

interesting part is 2/3 down the page:
https://antiqueradios.com/forums/vie...24123&start=20

i'm no RF design engineer so please don't shoot the pianist if not useful to solve the OP's original request.

Best Regards
jhalphen
Paris/France

16th Sep 2023, 9:18 am	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,465	Logarithmic Detector. Over the winter I will be playing around with some filters, and to properly assess them I really need a logarithmic Detector to use with a sweep generator and scope, so I can see the symmetry of the response and get an idea of whether my tweaking is improving the shape factor or introducing near-in spurious responses. In times gone by I would have hired something from R&S at great expense but now I have no business accounts in which to bury the cost I need to do it myself. So has anyone got suggested circuits for a logarithmic Detector working around 1.4MHz?? __________________ I played a blank tape at full volume. The mime-artiste who lives next door complained.

16th Sep 2023, 9:25 am	#2
Jon_G4MDC Octode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 1,985	Re: Logarithmic Detector. This or the less capable AD8361 maybe? https://www.analog.com/en/products/ad8362.html

16th Sep 2023, 11:36 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,246	Re: Logarithmic Detector. ADI have plenty of log detector chips, but look out for their low frequency roll-off specs. At that frequency you can also build your own out of a lot of discrete transistors and schottky diodes. Think of a chain of differential limiting amplifiers with feedback to set less than 10dB per stage, with a load of schottky diode detectors - one on each stage. Put the DC from each detector into a summing node via a high-ish value resistor (all equally-weighted) and you have a log detector. Alternatively find an old spectrum analyser for that frequency range and build a tracking generator for it. Second alternative, if that analyser is a storage type, just sweep an ordinary sig gen manually and watch it paint the picture. I've done all these things over the years. The discrete logger was for a spectrum analyser being designed into what was planned to be a replacement for the HP MLA. The ADI chips are likely the easiest approach. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

19th Sep 2023, 5:30 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,465	Re: Logarithmic Detector. Thanks for the suggestions; the ADI chips look interesting, and they're not exactly expensive either. I'd love to give home to a storage-capable analyser but these seem to be either frighteningly expensive (even secondhand) or - like lots of top-end test gear - of an age where I would probably spend more time repairing it/paying to get it calibrated than actually using it. __________________ I played a blank tape at full volume. The mime-artiste who lives next door complained.

19th Sep 2023, 8:28 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,246	Re: Logarithmic Detector. I built an over the top ATU some years ago and put a dual directional coupler in it. Two channels with AD640 loggers (Samples, thank you ADI) gave me logarithmic scaled power meters fore and aft. Simple subtraction gave a linear meter of dB return loss. Limited outputs went into a phase detector and gave a phase meter. Voila! a VNA running full transmitter power. Shame I built the wrong sort of ATU for the antennae I use. Comes out for special events, though. 10mW to 1kW on a single meter scale. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

19th Sep 2023, 9:17 pm	#6
Jez1234 Banned Join Date: Mar 2021 Location: Morpeth, Northumberland, UK. Posts: 936	Re: Logarithmic Detector. I'll bet you can no longer get the ADI IC's in a usable format now though. No doubt BGA or SSOIC only.

20th Sep 2023, 9:48 am	#7
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,193	Re: Logarithmic Detector. I was set almost exactly this challenge, almost exactly 30 years ago. Looking at my log book, 30 years and 1 month, to be precise! It was a bit scary - I'd only just started my industrial year, and it was my first RF project after a life of audio. Luckily, I spotted the NE604, which was a complete FM IF system that included an RSSI output. I ignored most of the chip and just used the log converter driving the RSSI pin. It worked better than the dedicated log amps the older staffers suggested I investigate, required much less support circuitry, and was a much, much cheaper part. It was pretty accurate (+/- 1.5dB) over a 90dB range. This bit of lateral thinking (luck in reality!) made me a bit unpopular with some, but the final option was cheaper and better in every metric, so was put into production (a high end radio receiver that I'm not allowed to talk about!). Although that part is not in production today, I'm sure there will be modern chips that do the same, and I wouldn't be at all surprised if, like then, they are a lot cheaper than the dedicated log amps. That's because one has a relatively large market, the other is rather niche by comparison. Cheers, Mark

21st Sep 2023, 6:12 pm	#8
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,465	Re: Logarithmic Detector. I'd forgotten about the whole thing of RSSI circuitry in receivers; I guess I'm now going to be spending hours looking at the less-intensively-integrated chips used in phones and WiFi routers. Probably there will be something available for a few pence per chip [provided you order half a million of the things]! __________________ I played a blank tape at full volume. The mime-artiste who lives next door complained.

21st Sep 2023, 10:38 pm	#9
Jez1234 Banned Join Date: Mar 2021 Location: Morpeth, Northumberland, UK. Posts: 936	Re: Logarithmic Detector. IIRC there was an S/A design for the radio ham years ago which used a wideband TV front end and as the log converter it was as above with the RSSI of the NE604. Also IIRC... it's in one of Pat Hawkers "Technical Topics Scrapbooks", or Radcom of course.

21st Sep 2023, 10:53 pm	#10
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,246	Re: Logarithmic Detector. Roger, GM4PMK did the best known British one. Modern highly integrated receivers seem to have all gone NZIF quadrature. No separate RSSI as such and you need a major corporation behind you and NDAs just to see a data sheet. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

21st Sep 2023, 11:59 pm	#11
Jez1234 Banned Join Date: Mar 2021 Location: Morpeth, Northumberland, UK. Posts: 936	Re: Logarithmic Detector. That sounds familiar yes. In fact did it not use the NE602 as a front end? (and there were issues due to LO not covering the published range for many constructors?) There were a few in S/A ideas in Technical Topics over a few years and I think I may be confusing that one with the TV tuner front end one and a CA3189 used for its RSSI, maybe by a NZ amateur? Entirely going from dodgy memory here!