[time-nuts] Close-in phase noise measurements

[Bruce Griffiths]

John Miles wrote:
>> 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.
>
>   
That would be the AD7760 then?
>> 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
>
>   
John

Unlikely, the dual mixer time difference technique has been around so 
long that most patents like:

United States Patent 5128909 (issued  7 July 1992, expired on 7 July 2004).

which is an extension of the technique to multiple channels, have expired.

Bruce