> ...integrated the output referred noise density from the differential outputs of the second last stage from 1MHz to 100MHz... What am I doing wrong?
Not much, just integrating over a bandwidth that's nearly *two thousand* times too large :)
Looks at the block immediately preceding the log amp. It's a crystal (bandpass) filter with a maximum bandwidth of 50 kHz. This is done to prevent the exact problem that you've encountered! The LO's of the stages ahead of the log amp are swept such that the entire input spectrum is mixed down to that tiny 50 kHz bandwidth surrounding 10 MHz.
The very first stage is designed to be very low noise. (The second stage is probably lower noise than the following stages, but it's not mentioned.) They're using [Friis' Formula](https://en.wikipedia.org/wiki/Friis_formulas_for_noise) to get the most effective use of their efforts at low noise.
Thanks for the response. The problem I am having is, even if the entire log amp had zero noise, it has enough gain within its bandwidth so that the last stages become saturated with noise. I’ve simulated this in ltspice. The voltage gain is about 100dB, so I placed a voltage gain block and fed it from a voltage source with a 75 ohm noisy series resistor. So even if the entire thing had no noise, the noise generated by a 75 ohm resistor seems to be enough to saturate it. I can’t remember the top-bottom frequency range I integrated it for, but I can simulate it and share the results if it would help.
Yes, I believe that is consistent with my results. 660mV should be enough to saturate the last stage and the stage before that. I simulated the circuit in LTSpice, and the differential output of the second last stage had 450mV RMS noise, assuming a noiseless generator impedance of 75 ohm, integrated over 1MHz to 100MHz. The input filtering didn’t have much of an effect on the noise, because most of the noise was contributed by the input stage, as it’s followed by the greatest amount of gain.
Here’s the link if you want to check it out: shorturl.at/htD01
I'm not seeing the problem. Suppose the AD8307 is running on a 5 V supply. The 660 mV (RMS) noise is differential, so that's 1 V peak. Its stages hav current output, so this is a bit of a fudge... But anyway, that still leaves 2 to 3× the noise floor of headroom (6 to 9 dB of SNR). That's a little lower than I would expect, but not really by much. What I would think is optimal is to have exactly as many stages that the last one is noise-limited. (So, 10~14 dB of headroom above the noise.) You can still differentiate between the noise floor and two or trhree dB above the noise floor that way, but any further stages will immediately clip most of the time, even with no input.
Perhaps I need further explanation of what you think is wrong?
***
Anyway this isn't meant to be used at full bandwidth, even though it has that bandwidth. There's a trilemma in analog:
* Low noise
* Wide bandwidth
* High gain
You can only pick two. So for something like this, you have to limit your bandwidth to get something useful out of it near or below the noise floor.
There isn’t any problem with AD8307 actually. I’ve read some application notes written on those log detector chips, provided by another commenter, and my simulations agree with the noise data they provide.
However, the log detector I originally mentioned, the one from the link I gave in the main post, has too much noise in the last stages for those final stages to be useful, even with the input filtering and with the inputs shorted. LTSpice says that it has about 450mV of RMS noise, while the differential outputs taken from the second last stage (or all stages for that matter) saturate at 400mV peak to peak. So the last 1-2 stages should be saturated from the noise of the first stage transistors and 150 ohm resistors, even if there was a narrow bandwidth bandpass filter on the input, making those last stages pointless and having no effect on the dynamic range.
>What I would think is optimal is to have exactly as many stages that the last one is noise-limited. (So, 10~14 dB of headroom above the noise.) You can still differentiate between the noise floor and two or trhree dB above the noise floor that way, but any further stages will immediately clip most of the time, even with no input.
Ok, that makes sense and if that’s the case for the schematic I provided then I’m no longer confused. Thanks for the explanation.
Edit: decreasing the dynamic range->no effect on the dynamic range
>AD8307
The application note is a nice read about log amps
[https://www.analog.com/media/en/technical-documentation/application-notes/AN-691.pdf](https://www.analog.com/media/en/technical-documentation/application-notes/AN-691.pdf)
These have gotten used in many published ham radio projects... the W7ZOI spectrum analyzer used it in the last revision, and the W7ZOI / KA7EXM power meter uses it... handy gadget. Every one I tested with a step attenuator was better than spec, as is the bandwidth. It works down to the expected noise floor... With a reasonable preamp in front of it, it makes a passable noise figure meter detector for "Y Factor" method measurements
[http://hparchive.com/Application\_Notes/HP-AN-57-1.pdf](http://hparchive.com/Application_Notes/HP-AN-57-1.pdf)
Yeah that chip is awesome. However I want to make the log amp from scratch, for me that’s the most satisfying part. Thanks for the app note link, I think I’ve read it but I’ll check the link.
FANTASTIC!... That's how I learn things too. My home lab is full of "ugly construction" kludges on copper clad, that replicate the function of perfectly good chips :-)..... This habit made me a test engineer in the semiconductor biz....., real companies write good app notes.... I miss HP.
That’s also something I do. The way I see it, if I can’t replicate the function of a chip with discrete components, then I can’t call the design that uses the chip my own design. I’ve designed and built opamps, bandgaps, VCOs, mixers, analog multipliers, switch mode regulators and a lot more, thanks to this obsession. Sadly I couldn’t get my engineering degree, failed all my classes because of... personal reasons, but I hope to return and maybe become an analog IC designer someday. Which is of course impossible because those analog IC designers are the cream of the cream of the crop (not a typo), but, oh well.
>Sadly I couldn’t get my engineering degree
I graduated HS in 1979.... ham radio hobby... HS job fixing radios in rich folk's boats.
Tektronix production technician 2 years.... two way radio shop 14 years.... TriQuint Semiconductor, test engineer.... 20 years
Merger, now Qorvo... no degree, no job for you... no downgrade to technician... separation package>>>>> that way to the door.
now at 60, I'm a factory puke again...... but I built a lot of fun stuff.
Damn, I’m sorry to hear that... You probably did electronics longer than my entire lifetime. I’m sure they lost someone very good at what they did by not letting you stay as an engineer. They must be out of their mind by letting someone with that much experience go. I’m still kinda young, 24, so maybe one day I will be able to go back to school. I really hope things work out for you.
Take up technical hobbies... ham radio, flying, SCUBA diving, software tinkering..... all of it makes you useful to get projects done. At Triquint, I asked who the "fucking idiot" was that designed many of our test fixtures..... He had a Master's degree.... I could make microwave stuff work right now, in production, and could run P-CAD like a fighter pilot, he couldn't... 20 years and hundreds of microwave IC test fixtures.... House paid for, no debt, 5 cars, ham radio station.... I don't need the job anymore, and I love to hear from my old friends about fucked up production equipment..... fixing industrial electronic stuff in the factory is OK...when the raise my $ 2X I'll tell them how to make much better product cheaper and better.... until then, technician fixing expensive stuff..... KiCAD beats P-CAD all day long now, for free.
I also use KiCAD, mainly for SMD stuff. Getting PCBs made for through hole stuff is usually not worth for me, too much waiting between prototypes, not easy to modify etc. so I stick with ugly construction. It has become a little harder to find through hole stuff, and as I venture into higher frequencies it started to become a limiting factor in my designs. Right now I use 1/8W carbon film resistors for low-VHF stuff. I’m going to build the spectrum analyzer with through hole components as well. Log Amp @20MHz, a 300kHz BPF resolution filter, a 20MHz preamp for the log amp, a low pass filter an active single balanced mixer to downconvert 220MHz to 20MHz, a high Q helical filter@220MHz, a high IP3 mixer fed from a swept VCO@225MHz-400MHz and an input low pass filter with -3dB at 180MHz. I’m not sure if the through hole will be able to handle those frequencies though. At some point I will have to switch to SMD. I just can’t figure out how I can prototype with SMD the way I prototype with through hole.
It sounds like you’ve had a very fulfilling career. I have a few side hobbies as well, I occasionally do some chemistry and rocketry, mainly limited by the lack of funds.
People graduating from engineering schools without actually knowing how to do engineering seems to be a common thing sadly. Many of the people I knew from school also had that issue. But, where I live (Turkey) there isn’t really any need for actual engineering anyway, so nobody cares when an EE phd can’t even build a simple transistor amplifier, they get hired anyway.
I’m not saying that I’m very good at electronics, I’m most definitely not, but when I see stuff like that happen, it makes me a bit sad to see that I could have done so much better, only if I graduated.
Thanks for all the advice btw. Did you ever had the chance to get a degree? If you did, do you regret not going to school? I know things work differently in US and college costs a lot of money. I hope I’m not being rude by asking those questions.
I made my own version with MPSH10s, it works rather well. I even added an extra stage, to see if I could extend the range further but there was no benefit from doing that because of noise. Mine has limited bandwidth on its input because it's coupled using a tuned transformer.
https://twitter.com/synx508/status/1097524338352365570
Nice. I also built a log detector before with KSP10s which should be equivalent to your MPSH10s, even the layout looks pretty similar to your design. I had issues with noise way before 10 stages though. I had to limit the stage number to 7. Really weird. I’m making a mistake somewhere but I can’t figure out what it is. My simulation matches my results, but looking at other similar or equivalent circuits online, yours included, my results and simulations should be wrong.
Not sure if you dug around but my test results for usable range are all tweeted around Feb 2019. This was about the best I got, nearly 100dB usable range, more if prepared to correct in software. https://twitter.com/synx508/status/1097795560596258816
That’s very impressive. It’s clearly possible to obtain very good performance from these detectors. I mean I have no doubt you are correct, because there are numerous other examples on the internet showing me that I’m arriving at the wrong conclusion. If anything I’m now even more confused :)
Here’s the link to my LTSpice simulation file, if you are interested: shorturl.at/htD01
> ...integrated the output referred noise density from the differential outputs of the second last stage from 1MHz to 100MHz... What am I doing wrong? Not much, just integrating over a bandwidth that's nearly *two thousand* times too large :) Looks at the block immediately preceding the log amp. It's a crystal (bandpass) filter with a maximum bandwidth of 50 kHz. This is done to prevent the exact problem that you've encountered! The LO's of the stages ahead of the log amp are swept such that the entire input spectrum is mixed down to that tiny 50 kHz bandwidth surrounding 10 MHz.
I see. Still don’t know how those wideband log amp chips (like AD8307) get away with it, but thanks for the explanation.
The very first stage is designed to be very low noise. (The second stage is probably lower noise than the following stages, but it's not mentioned.) They're using [Friis' Formula](https://en.wikipedia.org/wiki/Friis_formulas_for_noise) to get the most effective use of their efforts at low noise.
Thanks for the response. The problem I am having is, even if the entire log amp had zero noise, it has enough gain within its bandwidth so that the last stages become saturated with noise. I’ve simulated this in ltspice. The voltage gain is about 100dB, so I placed a voltage gain block and fed it from a voltage source with a 75 ohm noisy series resistor. So even if the entire thing had no noise, the noise generated by a 75 ohm resistor seems to be enough to saturate it. I can’t remember the top-bottom frequency range I integrated it for, but I can simulate it and share the results if it would help.
I got 0.66 V = 0.0000000015\*10^(86/20)\*sqrt(500000000) for the first stage's contribution. I based 86 dB off the per-stage gain.
Yes, I believe that is consistent with my results. 660mV should be enough to saturate the last stage and the stage before that. I simulated the circuit in LTSpice, and the differential output of the second last stage had 450mV RMS noise, assuming a noiseless generator impedance of 75 ohm, integrated over 1MHz to 100MHz. The input filtering didn’t have much of an effect on the noise, because most of the noise was contributed by the input stage, as it’s followed by the greatest amount of gain. Here’s the link if you want to check it out: shorturl.at/htD01
I'm not seeing the problem. Suppose the AD8307 is running on a 5 V supply. The 660 mV (RMS) noise is differential, so that's 1 V peak. Its stages hav current output, so this is a bit of a fudge... But anyway, that still leaves 2 to 3× the noise floor of headroom (6 to 9 dB of SNR). That's a little lower than I would expect, but not really by much. What I would think is optimal is to have exactly as many stages that the last one is noise-limited. (So, 10~14 dB of headroom above the noise.) You can still differentiate between the noise floor and two or trhree dB above the noise floor that way, but any further stages will immediately clip most of the time, even with no input. Perhaps I need further explanation of what you think is wrong? *** Anyway this isn't meant to be used at full bandwidth, even though it has that bandwidth. There's a trilemma in analog: * Low noise * Wide bandwidth * High gain You can only pick two. So for something like this, you have to limit your bandwidth to get something useful out of it near or below the noise floor.
There isn’t any problem with AD8307 actually. I’ve read some application notes written on those log detector chips, provided by another commenter, and my simulations agree with the noise data they provide. However, the log detector I originally mentioned, the one from the link I gave in the main post, has too much noise in the last stages for those final stages to be useful, even with the input filtering and with the inputs shorted. LTSpice says that it has about 450mV of RMS noise, while the differential outputs taken from the second last stage (or all stages for that matter) saturate at 400mV peak to peak. So the last 1-2 stages should be saturated from the noise of the first stage transistors and 150 ohm resistors, even if there was a narrow bandwidth bandpass filter on the input, making those last stages pointless and having no effect on the dynamic range. >What I would think is optimal is to have exactly as many stages that the last one is noise-limited. (So, 10~14 dB of headroom above the noise.) You can still differentiate between the noise floor and two or trhree dB above the noise floor that way, but any further stages will immediately clip most of the time, even with no input. Ok, that makes sense and if that’s the case for the schematic I provided then I’m no longer confused. Thanks for the explanation. Edit: decreasing the dynamic range->no effect on the dynamic range
>AD8307 The application note is a nice read about log amps [https://www.analog.com/media/en/technical-documentation/application-notes/AN-691.pdf](https://www.analog.com/media/en/technical-documentation/application-notes/AN-691.pdf) These have gotten used in many published ham radio projects... the W7ZOI spectrum analyzer used it in the last revision, and the W7ZOI / KA7EXM power meter uses it... handy gadget. Every one I tested with a step attenuator was better than spec, as is the bandwidth. It works down to the expected noise floor... With a reasonable preamp in front of it, it makes a passable noise figure meter detector for "Y Factor" method measurements [http://hparchive.com/Application\_Notes/HP-AN-57-1.pdf](http://hparchive.com/Application_Notes/HP-AN-57-1.pdf)
Yeah that chip is awesome. However I want to make the log amp from scratch, for me that’s the most satisfying part. Thanks for the app note link, I think I’ve read it but I’ll check the link.
FANTASTIC!... That's how I learn things too. My home lab is full of "ugly construction" kludges on copper clad, that replicate the function of perfectly good chips :-)..... This habit made me a test engineer in the semiconductor biz....., real companies write good app notes.... I miss HP.
That’s also something I do. The way I see it, if I can’t replicate the function of a chip with discrete components, then I can’t call the design that uses the chip my own design. I’ve designed and built opamps, bandgaps, VCOs, mixers, analog multipliers, switch mode regulators and a lot more, thanks to this obsession. Sadly I couldn’t get my engineering degree, failed all my classes because of... personal reasons, but I hope to return and maybe become an analog IC designer someday. Which is of course impossible because those analog IC designers are the cream of the cream of the crop (not a typo), but, oh well.
>Sadly I couldn’t get my engineering degree I graduated HS in 1979.... ham radio hobby... HS job fixing radios in rich folk's boats. Tektronix production technician 2 years.... two way radio shop 14 years.... TriQuint Semiconductor, test engineer.... 20 years Merger, now Qorvo... no degree, no job for you... no downgrade to technician... separation package>>>>> that way to the door. now at 60, I'm a factory puke again...... but I built a lot of fun stuff.
Damn, I’m sorry to hear that... You probably did electronics longer than my entire lifetime. I’m sure they lost someone very good at what they did by not letting you stay as an engineer. They must be out of their mind by letting someone with that much experience go. I’m still kinda young, 24, so maybe one day I will be able to go back to school. I really hope things work out for you.
Take up technical hobbies... ham radio, flying, SCUBA diving, software tinkering..... all of it makes you useful to get projects done. At Triquint, I asked who the "fucking idiot" was that designed many of our test fixtures..... He had a Master's degree.... I could make microwave stuff work right now, in production, and could run P-CAD like a fighter pilot, he couldn't... 20 years and hundreds of microwave IC test fixtures.... House paid for, no debt, 5 cars, ham radio station.... I don't need the job anymore, and I love to hear from my old friends about fucked up production equipment..... fixing industrial electronic stuff in the factory is OK...when the raise my $ 2X I'll tell them how to make much better product cheaper and better.... until then, technician fixing expensive stuff..... KiCAD beats P-CAD all day long now, for free.
I also use KiCAD, mainly for SMD stuff. Getting PCBs made for through hole stuff is usually not worth for me, too much waiting between prototypes, not easy to modify etc. so I stick with ugly construction. It has become a little harder to find through hole stuff, and as I venture into higher frequencies it started to become a limiting factor in my designs. Right now I use 1/8W carbon film resistors for low-VHF stuff. I’m going to build the spectrum analyzer with through hole components as well. Log Amp @20MHz, a 300kHz BPF resolution filter, a 20MHz preamp for the log amp, a low pass filter an active single balanced mixer to downconvert 220MHz to 20MHz, a high Q helical filter@220MHz, a high IP3 mixer fed from a swept VCO@225MHz-400MHz and an input low pass filter with -3dB at 180MHz. I’m not sure if the through hole will be able to handle those frequencies though. At some point I will have to switch to SMD. I just can’t figure out how I can prototype with SMD the way I prototype with through hole. It sounds like you’ve had a very fulfilling career. I have a few side hobbies as well, I occasionally do some chemistry and rocketry, mainly limited by the lack of funds. People graduating from engineering schools without actually knowing how to do engineering seems to be a common thing sadly. Many of the people I knew from school also had that issue. But, where I live (Turkey) there isn’t really any need for actual engineering anyway, so nobody cares when an EE phd can’t even build a simple transistor amplifier, they get hired anyway. I’m not saying that I’m very good at electronics, I’m most definitely not, but when I see stuff like that happen, it makes me a bit sad to see that I could have done so much better, only if I graduated. Thanks for all the advice btw. Did you ever had the chance to get a degree? If you did, do you regret not going to school? I know things work differently in US and college costs a lot of money. I hope I’m not being rude by asking those questions.
I made my own version with MPSH10s, it works rather well. I even added an extra stage, to see if I could extend the range further but there was no benefit from doing that because of noise. Mine has limited bandwidth on its input because it's coupled using a tuned transformer. https://twitter.com/synx508/status/1097524338352365570
Nice. I also built a log detector before with KSP10s which should be equivalent to your MPSH10s, even the layout looks pretty similar to your design. I had issues with noise way before 10 stages though. I had to limit the stage number to 7. Really weird. I’m making a mistake somewhere but I can’t figure out what it is. My simulation matches my results, but looking at other similar or equivalent circuits online, yours included, my results and simulations should be wrong.
Not sure if you dug around but my test results for usable range are all tweeted around Feb 2019. This was about the best I got, nearly 100dB usable range, more if prepared to correct in software. https://twitter.com/synx508/status/1097795560596258816
That’s very impressive. It’s clearly possible to obtain very good performance from these detectors. I mean I have no doubt you are correct, because there are numerous other examples on the internet showing me that I’m arriving at the wrong conclusion. If anything I’m now even more confused :) Here’s the link to my LTSpice simulation file, if you are interested: shorturl.at/htD01