[time-nuts] Is this measurement for real?

wb6bnq wb6bnq at cox.net
Mon Dec 9 01:30:42 UTC 2013


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.

Bill....WB6BNQ


quartz55 wrote:

>Well, Bill, I just don't know.  I think that's why I asked.  However, I did measure it again over 2.5 hrs and I got a roughly 500uHz drift pretty much all in one direction.  I'm wondering if this is telling me I'm cabable of measuring 1mHz with some amount of success, I realize I will have to keep repeating this to see what happens in the long term.  I've never tried to measure these small increments before and was wondering if others have had success doing this or am I chasing a rabbit down a hole?  I did try using the WSPR software to measure freqs and that seemed to be all over the place, plus there's no way to record it over time except manually.
>
>I tried recording the WWV 500/600 Hz AM audio freqs with Speclab and it's so noisy it's hard to get a good plot, but from looking at the plots I did make it seems to be in the range of 20mHz variation, but pretty much centered on the freqs.
>
>I also measured some AM stations and I had drifts of 56 mHz for a 1030 KHz station, and I have measured the DSP drift as around 14 mHz.  Do the AM stations really drift around that much?  I thought they were pretty stable.  When I measure the service monitor locked to the Rb, I get much less drifts than actual stations, like I said, around 15 mHz and that seems to relate to the fan in the TS2000 going on and off.
>
>Dave
>N3DT
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