[time-nuts] Beating 20MHz was Shielding a DAC line

Robert Atkinson robert8rpi at yahoo.co.uk
Fri Sep 27 02:32:00 EDT 2013


Hi Bob,
You say you are not ready to go GPS. I assume you mean a GPSDO. If you have a 'scope you can use just about any GPS with a 1pps output. trigger the scope from the 1pps while monitoring the 20MHz sine wave and adjust for minimum drift of the trace. A timing GPS module will be better (you can get a used Oncore UT plus for under $20 on ebay) but anything will be better than beating an HF signal by ear.
 
HTH,
Robert G8RPI. (too many Bobs  :-) 
 

________________________________
 From: Bob Albert <bob91343 at yahoo.com>
To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Friday, 27 September 2013, 2:24
Subject: Re: [time-nuts] Shielding a DAC line
  

This is off topic but I'm unsure how to do it properly.

I am trying to 'discipline' a couple of sources.  I zero beat with 20 MHz WWV but can't tell the difference between fading and the beat, so I am stuck in the vicinity of 1 Hz possible error.  That's 50 ppb I think.

What can I do to take the next step to bring the oscillators in closer agreement?  I am not ready to go GPS or buy a rubidium standard.


Bob



________________________________
From: Bob Stewart <bob at evoria.net>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Thursday, September 26, 2013 6:13 PM
Subject: Re: [time-nuts] Shielding a DAC line


Hi Bob,

I should have mentioned that I added a new 5MHz output, and the coax ran within 1/8" of the single DAC wire going to the OCXO.  I don't think anything else changed, but of course there could be some flake of something on the DAC line that I missed.  I put on the RG-174 and I see that it's still locking high.  I suppose it could be just that it was a long power cycle to the oscillator.  Come to think of it, this thing does like to move to a new voltage sometimes when I have it off to mess with something.  So, maybe it's just the oscillator being cranky.  I haven't had it off for more than just a few seconds in a long time.  If I weren't in test and development mode where anomalies are good, I think I'd put the old one back in.

Thanks as always,

Bob





>________________________________
> From: Bob Camp <lists at rtty.us>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Thursday, September 26, 2013 7:54 PM
>Subject: Re: [time-nuts] Shielding a DAC line
> 
>
>Hi
>
>I suspect that you have a ground offset between the OCXO's ground return and the DAC's ground reference. The signal *should* be DC, Shielding it won't hurt, but it really should not help much.  If anything is an issue a simple R/C filter at the OCXO pin should nuke it better than coax will.
>
>Bob
>
>On Sep 26, 2013, at 8:09 PM, Bob Stewart <bob at evoria.net> wrote:
>
>> I made some minor hardware changes to my GPSDO today and I see that it's locked to a new DAC voltage about 21mV higher.  So, I was wondering about shielding the short run to the OCXO.  I have immediately available RG-174 and I'm putting that in.  But, should this be some sort of steel shelled semi-rigid coax?  Maybe it's a dumb question, but I thought I'd ask.
>> 
>> Bob - AE6RV
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