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

[Lux, Jim]

On 7/7/22 8:55 AM, Bob kb8tq via time-nuts wrote:
> Hi
>
> Yes, you do need to know the system gain. Since we are talking about
> gain at audio, measuring the gain directly is not a crazy thing to do. One
> of the things that makes audio spectrum analyzers a nice tool for this that
> they eliminate the “variable gain to the sound card” issue.
>
> Some sound card setups are a lot easier to work with than others. If you
> are restricted to the sound input on your motherboard things can get a bit
> crazy. It is not unusual for folks to dig up a “pro” (whatever that means
> on a sound card ) card that has better drivers and more access to this and
> that.
>
> Given how fast the PC world changes, the board that was a wonderful thing
> last time somebody dove in, likely is long out of production by now. The drivers
> that made it work so well may have been “improved” and it no longer gives
> you the control it once did. This makes for a bit of trial and error to get it all
> going.
>
> Bob


Rather than a sound card, it might be better to pick a small singleboard 
like a Teensy that has a decent ADC, and make a "sampling engine" with a 
USB interface.

Or, in general, going to a USB interface sound interface might be good.  
You can get them with a lot of channels (at least 8) and they sample 
simultaneously, so the uncertainty in USB latency won't bite you.  
Google for things like the Focusrite Scarlett


I've not tried it for this kind of application, but it is likely to have 
better noise properties than a "inside the PC" card. Typically 24 bit 
converters and 192kHz sample rates.