Joe Gwinn wrote:
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.
The time-difference between the two sampling points could be minimized
in such an approach as the phase could be shifted arbitrarilly in the
post-processing such that the effective phase difference between the two
chains reduces to near zero and hence the correlation between the
channels for the transfer oscillator would be better in phase and cancel
the transfer oscillator out better.
The postprocessing would then slowly tune the I/Q phase and keep a phase
adjustment track such that post-correlation could turn it back for
proper phase-trace.
An alternative approach is to use the Costas tracking loop as Bruce
suggested.
Regardless this first stage of digital processing can be done in a FPGA
frontend and bring the resulting signal bandwidth into very reasnoble
rates, just as for a GPS receiver.
Cheers,
Magnus
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
