[time-nuts] Close-in phase noise measurements

[John Miles]

> Most, but not all, sound cards have a low frequency cutoff of 20Hz or so.
> Some (but not all) sound card ADCs can dc coupled.
> A high resolution dc coupled ADC may be more effective for frequencies
> below 20Hz.

True; I'm assuming that anyone using a sound card for these purposes is
either going to bypass the coupling capacitor in front of the ADC, or
calibrate out the highpass response by adding an inverse function.  (A QEX
article gave an example of the latter technique not too long ago.)

I'm still hoping to get a 24-bit, 2.5-MSPS ADC chip hooked up via USB 2.0
fairly soon.  That will solve a multitude of problems, eliminating the need
for both a sound card and an HF analyzer.  I have C code on the PC that's
talking to the FPGA, but haven't yet tried to bring the ADC up with it.

> Sound card support appears to be something of a minefield, baudline
> thinks my 16 bit 48kHz motherboard sound system is a 24 bit 192kHz system.
> This probably means that the frequency scale and consequently FFT filter
> noise bandwidths are unreliable.
> However with a low frequency noise calibration source and set of marker
> harmonics derived from a crystal these calibration issues can be resolved.
> Windows software fares little better and some crashes when set to sample
> at 192kHz (the windows machine has a sound system with a 192kHz 20 bit
> ADC system).

Yeah, I think it'd be better not to even use the sound-card drivers if
possible.  At 10 million bytes per second (32 bits/sample at 2.5 MSPS) they
won't be an option for the hardware I'm looking at.

> A dual mixer time difference system can have a lower noise floor than a
> single mixer system.

I wonder if those are still covered by patents in various corners of the
world, the way the TSC's dual-ADC architecture appears to be...

-- john, KE5FX