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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