[time-nuts] Measuring phase shift between 1 Hz DMTD signals by I+Q processing

Joe Gwinn joegwinn at comcast.net
Sat Jul 25 14:26:01 UTC 2009


It occurs to me that there is a possible alternative to the ZCD-chain 
approach typical in DMTDs, if one is willing to provide two mixers 
and two ADCs per channel, with a 90 degree phase offset between LO 
signals provided to the mixers of a channel.  The output of the four 
ADCs will be a pair of I+Q signals, one pair per DMTD channel.

The key observation is that if one has two signals, one being a time 
delayed replica of the other, if one multiplies one signal by the 
complex complement of the other signal, the result is Exp[j(phase 
difference)].  This is true whatever the waveform of the signal, so 
long as the only difference in signals is a delay.  The mathematical 
argument function of this exponential is the desired phase.

In practice, one will sample far faster than 1 Hz, say 1 MHz, and 
will heavily average the resulting stream of products.

Now I have not gone through the math to estimate performance compared 
to the traditional ZCD approach, but the complex multiply and average 
approach should be quite robust against noise, and is easily 
implemented in a DSP or FPGA.

Joe Gwinn



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