Hi Looking at what you *tell* the DAC to do is only part of the process. There are a number of issues that get into the mix that can cause the DAC to do something other than what you tell it to do. A Vref that changes is one example, there are an unfortunately large number of other possibilities …. In an AC ( like a sound card ) application a lot of these things really would not matter. In a DC application (like driving an oscillator) grubby details do start to get into the mix.
Bob > On Mar 6, 2022, at 1:39 PM, Erik Kaashoek <e...@kaashoek.com> wrote: > > Hi Bob > Good to hear. > As the internal logging does collect the DAC settings and the frequency of > the TCXO versus a Rb standard I'm lucky to have both. And full scale > linearity testing of the DAC's hopefully will show any issues there. They > should be linear within 0.2 step. > I've already build the single mixer+low noise 24 bit microphone input sound > card setup and locked a 4GHz PLL to a Rb and used the 400th overtone of 10MHz > from the TCXO so the lowest audio frequencies (20Hz) are in reality 0.05Hz, > low enough to connect to the 1Hz gate time measurement with the counter. > At first impression the phase noise is not terrible, now the calibration > needs to be done by comparing with a 10MHz source with known (bad) phase > noise. > The audio analysis SW used can do dB/log Hz. > Will be interesting to share the results here and hear your feedback. > Erik. > > On 6-3-2022 18:49, Bob kb8tq wrote: >> Hi >> >> DVM’s show up from a lot of places. Indeed some are nutty when >> shipped from here to there. The shopping process is always going >> to be “delivered price” based. >> >> The reason for looking at the dac output is: The frequency just jumped >> 2x10^-8 … was it from the dac or internal to the TCXO ? The oscillator >> drifted 5x10^-9 it one minute, was it the DAC / Vref? … >> >> Phase Noise: >> >> The “quick / simple” way to do phase noise is with a single mixer setup. >> You run both ports at “max” ( so 7 dbm on a 7 dbm mixer) and lock them >> in quadrature. A fairly simple audio amp based on any of a number of >> op-amps boosts the output to something an audio spectrum analyzer >> or sound card can “see”. You would like an opamp with something in the >> 1 nv/Hz vicinity in terms of noise. >> >> Once you get the setup going, it’s just a matter of calibrating things. There >> are a variety of app notes and papers on that part. It normally involves >> unlocking the loop and measuring the phase slope of the beat note with >> whatever you are using to look at noise. ( The op amp “preamp” normally >> gets switched out for this step). >> >> Not crazy expensive or hard to wire up. I’ve built the circuit a lot of times >> using “dead bug” sort of construction. They all seem to work. >> >> On a typical GPSDO design, you are looking at noise inside about 10 KHz. >> Sure there could be issues anywhere, but the most likely stuff is below the >> max limit on the typical sound card. Low end wise, it would be nice to get >> to 1 Hz. That isn’t going to be easy with the typical sound card. >> >> Yes that glosses over all the “joy” of tracking down ground loops and other >> local noise sources. You are measuring a *very* low level signal. Quiet >> supplies and good grounding are part of this. >> >> Bob >> >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an > email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
