Lux, James P (337C) wrote:
On 7/24/09 6:32 AM, "Stephan Sandenbergh" <[email protected]> wrote:
Hi,
Many sources and papers refer to the actual DMTD mixer system (and
zero-crossing detector etc.). But, I've seen very little on the phase
shifter circuits, that are used in conjunction with this system.
I was hoping some of you might have experience, and perhaps a circuit
example. If it is similar to what is used in audio applications, the single
opamp, 0 - 180deg, RC-all pass filter, it is simple. However, I assume there
are more to it than that. It seems such is the case with all precision time
related things :)
For RF in coax, it's probably more like a mechanical line stretcher or
trombone line. For 10 MHz there are some variable transformer sorts of
schemes.
One can also do a 90 degree hybrid with a variable power combiner on the
output, or, essentially the equivalent, a I/Q or vector modulator. There are
also "linear phase modulators" based on diodes. With anything using an
active device, one is going to worry about the usual things: noise, aging,
etc.
All of the "electronic" phase shifters will require some form of
calibration to get better than a few degrees accuracy. In some measurement
schemes you don't really care what the phase shift is, just as long as you
can get it to quadrature.
For the DMTD, aren't you really looking at timing of the beat notes, not the
absolute phase?
For DMTD you use a local oscillator which is detuned to produce a beat
frequency of say 10 Hz. That means that you will scan through all phase
relationships 10 times every second. For 10 MHz that means about 1
milion different phase-relationships. Now, the phase relationships
producing a beat voltage going through zero will choose themselfs. Thus,
no need for a phase-shifter when doing DTMD as such. However, it can be
wise to shift the second input such that it does not occur neat the same
time, since that way time-correlation could be reduced. However, such
phase-steps does not need to be made in linear steps, but rather crude
steps approximating 0, 90, 180 and 270 degrees will suffice.. however
other means to achieve the same thing can be used to create the separation.
Cheers,
Magnus
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