On Thu, 22 Aug 2019 07:35:06 -0700 "Richard (Rick) Karlquist" <rich...@karlquist.com> wrote:
> It is easy to homebrew a phase detector using a high LO drive mixer > (eg JMS-5H+ $12.95) followed by an LT1028/LT1128. > Then all you need is a low frequency spectrum analyzer. > If you don't need to go below 20 Hz offset, you can use > software that makes your PC sound card into a spectrum analyzer. > Depending on what you are doing, you may need to add an op amp > after the LT1X28 to make a phase locked loop. If you are going the soundcard route, I would not recommend to mix down as low as with DMTD systems. The ADCs used in soundcards have very poor DC performance. They don't need to have one anyways, as the lowest most people can hear is in the 20-50Hz range. Due to that, I would suggest to have the IF frequency between 100Hz and 1kHz and then perform the rest of the analysis in software, maybe even including another down-mixing step down to DC. As you have always two channels, I would recommend to use two mixers fed with an LO that is 90°C out of phase to get I and Q components. This will further reduce noise, at least to some extend. While the isolation between the ADC inputs in soundcards is pretty darn good, the noise isolation isn't. So there is some correlation there that cannot be removed (easily). Another important thing to know with audio ADCs is, that they are almost always some sigma-delta variant. This gives them a high linearity and low noise with moderate cost. But that also means that the higher the bandwidth you are using, the higher the noise floor becomes, as you are "averaging" over less and less samples. I.e. while an audio ADC might be spec'ed to be sampling at 192kHz, you will be only able to use up to 20kHz with the nice low noise floor that is specified. If you go beyond that, you will have a corresponding increase in noise. In a mail on this ML pretty much three years ago, I wrote the following: Subject: [time-nuts] ADCs for phase noise measurement (was: windows for FFT measurements of phase noise) Date: Sat, 11 Jun 2016 15:33:30 +0200 ---schnipp--- Alternatively, I would suggest using one of the modern sigma-delta or SAR ADCs, which can deliver increadibly high ENOB and SNR at astonishing sampling rates. Good candidates might be: AD7982, 18bit 1Msps AD7984, 18bit 1.33Msps LTC2378-20, 20bit 1Msps LTC2368-24, 24bit 1Msps Eval boards for these are available (between 100 and 200€) and interface with SPI. You can either use an USB SPI dongle (between 5 and 50€) or use a small uC board to interface with the PC. Saving the samples in a wav file and using one of the many FFT tools shouldn't be a problem. ---schnapp--- And in another mail: Subject: [time-nuts] measuring noise of power supplies (was: For those that insist on using switching power supplies) Date: Sat, 15 Oct 2016 00:53:25 +0200 ---schnipp--- The mid frequency range is mostly influenced by the telecom noise requirements, which for historical reasons cover the 10Hz to 20MHz range. It is probably the easiest region to measure with homebrewn instruments. A decently fast ADC with a low noise voltage reference (like the LTC6655) are all you need. Depending on how accurately you want to measure the noise, it makes sense to further split this range into a lower range up to ~500kHz and an upper range above 500kHz. The reason is that there are today several high resolution ADCs available that support sampling rates of up to 1Msps (and some beyond),eg: AD7982, 18bit 1Msps AD7984, 18bit 1.33Msps AD7960, 18bit 5Msps LTC2386-18, 18bit 10Msps LTC2378-20, 20bit 1Msps LTC2368-24, 24bit 1Msps These would allow to accurately measure the noise range that is IMHO most interesting for most applications. Interesting because a lot of applications are insensitive to noise below 1Hz or even below 10Hz and noise above several 100kHz becomes easy to filter out using inductors, ferrit beads and ceramic capacitors. When choosing an ADC for this range make sure you check the actual SNR/SFDR performance as it a higher output resolution not necessarily corresponds to the actual performance delivered. This becomes especially pronounced when going higher with the sampling rate to cover the higher noise frequency ranges. Beyond 5-10Msps 16bit is the best you can get and conversly the SNR is limited to something around 90dB-95dB. ---schnapp--- Especially the AD7960 and the LTC2386-18 are interesting in this application, due to their high sampling rate, which allows to keep the IF frequency far out of the flicker-noise region and at the same time does not require aggresive anti-aliasing filters, making the measurment more stable. An important part in this setup is to have two (or four) low noise mixers that are driven coherently with the same LO signal, and more importantly, the same LO noise. Designing the mixers such they can cope with the wide range of input power, input frequency and still maintain a low noise figure (and low flicker noise) is an art in itself. Unfortunately, I have very limited experience in this and thus have to refer you to more knowledgeable people. As most of you know, the state of the art is using direct sampling of the input signal and doing all the processing in the digital domain à la Timepod and Timepod v2.0 (aka Jackson Labs PhaseStation 53100A). For this you need higher sampling frequency ADCs. But, even though you can get 16bit ADCs with 210MHz sampling range, their SNR is limited (the LTC2107 has ~80dB). Another problem with this approach is to have the aperture jitter to be correlated among all ADCs. While the sampling clock's noise is the largest contributor to aperture jitter in high-speed ADC applications, it is not the only one. And at the performance we want to achieve, the jitter induced due to voltage fluctuations inside the ADC due to its own circuitry switching is significant. Unfortunately, the best I can tell you here is that (some?) dual-ADCs have a quite correlated aperture jitter, which might help in lowering the instrument noise if used properly. But why exactly the aperture jitter is correlated is not exactly known (people have been tossing theories around, but nobody has actually proven any of them). Attila Kinali -- <JaberWorky> The bad part of Zurich is where the degenerates throw DARK chocolate at you. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.