[time-nuts] Is this measurement for real?

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 8 21:10:06 EST 2013


Bill,

On 12/09/2013 02:30 AM, wb6bnq wrote:
> Hi Dave,
>
> My question was more centered on determining your expectations.  I ran
> into an even worse condition with the cheap sound card, in my shop
> computer, I used for the Frequency Monitoring Tests (FMT) ran by
> Connie (K5CM).  Being in a space with no temperature control at all,
> the sound card had a 7.0 Hertz variation over a few minutes.  Clearly,
> it was a crystal going wild.
>
> As an experiment, I decided to rip out the crystal and replace it with
> the output of a HF synthesizer dialed to the proper frequency.  My
> synthesizer, and other LAB equipment, is locked to my house standard
> which is monitored (not controlled by) with GPS.
>
> As expected, the results were spectacular !  I ended up with a
> measurement process that had a resolution of 120ns, and maybe somewhat
> less.  At 1000 Hertz that is an uncertainty of +/- 1.2e-10.  I did not
> try to account for ground loops or other anomalies; and the sound card
> was some cheap $18 item with no spectacular ratings in and of itself.
>
> I am now finishing up on a project to replace my expensive commercial
> synthesizer so it can return to test equipment duty.  If you are
> interested in what I am doing in that regard, email me about it off list.
>
> As for the TS-2000 radio, I have not studied it, per se.  But like a
> lot of these modern radios there are several possible error points
> within their design that could cause offsets and drift that may affect
> the outcome depending upon your application.
>
> However, if you are using a common analog detection type radio in the
> "AM" mode, then the radio does not matter to the outcome.  The radio
> only serves as a mixer, albeit an expensive one.  For example, when
> comparing an approximate 10 MHz unknown signal, the mixing action
> provides four (4) more decades of resolution if the output of the mix
> is 1000 Hertz.  If using a modern DSP radio in the "AM" mode you may
> have to account for possible slight errors in the internal codecs
> (A>D/D>A).
>
> The only two error points that matter (using "AM" in the above
> example) is the local signal generator used to beat against the
> unknown incoming signal and the computer's sound card stability.  If
> both are tied to your "House Standard," then it is totally up to the
> quality of your local standard's stability and accuracy.
There are dirt cheap DDS-modules to buy from China, based on AD
DDS-chip. If you need help with the reference frequency, you might want
to use that TAPR module for clock-stepup. I don't think the TAPR module
does the frequency you want straight away.

Just to give you some ideas to follow up. It will be much cheaper and
compact than your RF-generator today, and considering you already is
listening to a stepped down signal, the purity is good enough for the
purpose.

Cheers,
Magnus


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