[time-nuts] Shielding a DAC line

[Bob Stewart]

Hi Bob,

If you've got a stable SSB receiver and an AM receiver capable of 20MHz, here's a trick to get an accurate beat:  Tune both to WWV, and listen to the WWV generated tone.  Adjust the frequency on your SSB receiver to exactly beat the tone on the AM receiver.  That way, you're matching a tone, and not a fading carrier.  Then adjust your 20MHz oscillator to zero-beat your SSB receiver.  If you have a good ear, it should get you down there to less than 1Hz of error.

But, if you are hooked enough to ask the question, you are probably hooked enough to eventually buy both GPSDO and Rb standard.  Ask me how I know.  =)  There are numerous GPSDOs out there that you can build or buy complete.  I'm currently rewriting much of Bert, VE2ZAZ's GPSDO code.  Another programmer is "joining the staff" as well.  With a case, a PSU, an OCXO, a board and chip from Bert to start with, and the usual misc parts, you can join the fun.  (I do not sell parts or code or programmed chips.  This is a hobby for me, not a money-making scheme.)  Or you could go the standard route with the more traditional bought parts which might turn out cheaper in the long run.  Cases etc are just so expensive these days.

I'm a newbie time nut, so take the above with a grain of salt.

Bob - AE6RV




>________________________________
> 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: Thursday, September 26, 2013 8:24 PM
>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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