[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[Bob kb8tq]

Hi

Well, obviously it’s now time to go get some better OCXO’s :) :) :)

One further note: It is tempting to go to multi section lowpass filtering
if you are trying to cover a range of frequencies. You also might try to push
the range of the noise measurement up to higher and higher offsets. There
is a point that all of this gets nutty. Peaking in the audio range is often the
net result. Spice is one way to watch out for it. Sweeping the circuit is 
obviously a better way. 

Bob

> On Jul 13, 2022, at 3:46 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Ed,
> Thanks for the many good advice.
> I've tried to incorporate as much as possible, updated schematic can be found here: http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG
> 
> For audio into the PC I'm using a professional balanced microphone to USB input with a noise level of -130dBc/Hz and no spurs.
> Using a 7805 and 1500uF capacitors I tried to create a solid reference instead of the buffer op amp output and that did make a difference.
> Further the input of the first opamp has been change to have identical resistors at both the + and - input to reduce common mode signals.
> None of the capacitors (5.6pF) I tried to reduce high frequency gain improved the results. Most of the time it got worse.
> Removal of a ceramic capacitors eliminated the microphony.
> 
> I've added a switch to select between 0dB and 20dB gain so I can calibrate the level by offsetting the DUT frequency while keeping the drive to the mixer.
> To calibrate the effective noise BW of the FFT I create a test signal combining a -70dBm 10.001MHz signal with a -90dBm/Hz noise signal.
> The 20dB power ratio was confirmed using a calibrated spectrum analyzer. The FFT length and sample rate at the PC where then changed till the PC FFT showed the same power ratio.
> 
> Noise level at 1kHz is now -150dBc/Hz and -155dBc/Hz at 10kHz which is the spec of the DOCXO used so no need to go any lower.
> Also the 50Hz spurs and its harmonics are greatly reduced.
> 
> I'll have to invest in better coax cables as the current cable seem to leak RF.
> Erik.
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