[time-nuts] Phase shifter circuit for DTMD

[Lux, James P (337C)]

On 7/24/09 6:32 AM, "Stephan Sandenbergh" <stephan at rrsg.ee.uct.ac.za> 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?