[time-nuts] WWVB PM Receiver

[Bob Camp]

Hi

The issue is dithering the input to the ADC. If you filter "to much" you get
to the point that you are not dithering very much. That actually reduces the
linearity of many ADC's (ie spurs go up). It very much impacts your ability
to pull out low level signals. 

Done that? yes lots of times. Got it working? yup. It's a very standard text
book approach that's been around for many decades. 

Can you carry dithering too far? Indeed you can. You do not want to overload
the ADC. Once it's clipping, you aren't getting anything useful out of it.
 
Things like broadcast AM and switching power supplies are both threats to
your receiver. If your loop antenna has a Q of 100 or so, it's going to have
a bandwidth of about 600 Hz. That eliminates the AM band pretty well.
Switchers can be anywhere, yes you might have one at exactly 60.0001 KHz.
Call up Javad for a brick wall filter in that case. 

Your 600 Hz wide antenna has taken out a lot of noise. More than likely,
your ADC isn't getting much into it. A 6KHz wide antenna would still kill
the AM band. The lower Q antenna also may give you less trouble over
temperature. Where between Q=10 and Q=100 you get reasonable dithering *and*
nuke your local crud *and* get a reasonably stable system.... TBD.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of J. Forster
Sent: Wednesday, September 26, 2012 9:18 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] WWVB PM Receiver

Have you actually tried it and gotten it working, except possibly in a
very strong signal area?

-John

================



> Hi,
>
> From: "Bob Camp" <lists at rtty.us>
>
>> A tuned antenna probably is going to provide all the selectivity you'd
>> need.
> Another possibility is to modify your old circuit like that:
> http://www.maxmcarter.com/rubidium/2012_mod/index.html
>
> Bye,
> Jean-Louis
>
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