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
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