[ntp:questions] Dual Mixer Time Difference (DMTD) instruments sought

David L. Mills mills at udel.edu
Tue May 13 05:26:49 UTC 2008


Joseph,

I took a look at the instrument instruction manual to see what is going 
on. In typical todayspeak, Symmetricom doesn't say how the gadget works. 
I make it what used to be called a Costas direct-conversion receiver. 
The test signal is connected to two mixers; the reference oscillator is 
connected to the other mixer inputs in quadrature. The mixer outputs are 
digitized and filtered, the Q signal is shifted 90 degrees from the I 
signal and combinted. The result is a baseband SSB dignal which is then 
Fourier transformed for display. Is this what you have in mind?

Dave

Joseph Gwinn wrote:

> In article <joegwinn-01533B.10412407052008 at comcast.dca.giganews.com>,
>  Joseph Gwinn <joegwinn at comcast.net> wrote:
> 
> 
>>I may need a Dual Mixer Time Difference (DMTD) instrument, to measure 
>>picosecond changes in electrical length in a coax plus amplifier time 
>>reference signal distribution system with total delays in the hundreds 
>>of nanoseconds, currently operating at 10 MHz (sinewave), but with 100 
>>MHz likely at some future date.
>>
>>What DMTD instruments are commercially available?  A google search was 
>>not successful - all noise no detectable signal, probably because DMTD 
>>instruments are not that common, and many people build their own.
> 
> 
> The silence, the silence.  I have not found too many commercial DMTF 
> units, but I have found one, although the maker does not market it a 
> such:
> 
> The Symmetricom 5120 
> <http://www.symmttm.com/products_pn_adev_test_sets_5120A.asp> is at 
> heart a digital DMTD instrument, and will make all the usual DMTD 
> measurements, although it is marketed primarily as a phase noise test 
> set.
> 
> What else is available?  
> 
> 
> Joe Gwinn




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