[time-nuts] WWVB: measuring local 60 KHz noise

[Charles Steinmetz]

Alberto wrote:

> If you have a sound card capable of sampling at 192 kS/s, you don't need an SDR
> to receive a signal at 60 kHz... just connect the output of an active antenna, like
> e.g. the mini-whip, directly to the Line-In of the sound card  *  *  *
>
> I used this method in the past to successfully receive the SAQ transmission at 17.2 kHz

You have to be careful about this -- the vast majority of computer sound 
cards are designed to handle the audio spectrum up to around 20-22kHz. 
Cards that have sampling rates in excess of about 48kS per second most 
often DO NOT process input signals higher than ~22kHz.  The higher 
sampling rate is used only for oversampling and noise shaping (look 
these up if they are unfamiliar terms), and the digital signals are 
noise-shaped and decimated on-card from 96kS, 192kS, 384kS, etc. down to 
48kS and lower.

So, yes, a sound card designed for signals up to ~22kHz should handle 
SAQ at ~17kHz, but sound cards that can digitize signals above 22kHz are 
rare.  Some "professional" sound cards handle signal frequencies up to 
~40kHz, but very, very few handle signal frequencies higher than that. 
There are also digitizers designed more for instrumentation and data 
acquisition than audio that may meet your requirements.

SO:  If you want to digitize frequencies > ~22kHz, read the 
documentation for the cards you are considering VERY carefully.  I 
haven't been shopping for extended-frequency audio cards recently, so 
I'm not up to date on what is available. Perhaps others will have 
particular suggestions for digitizing WWVB.

Finally, be aware that sound cards use sampling clocks that are 
invariably MUCH worse in terms of jitter and drift than the WWVB carrier 
(even as it is received over the air), so you need to deal with that if 
what you are after is a 60kHz reference that is as stable as WWVB.

Best regards,

Charles