[Discuss-gnuradio] Implementing Dicke switching

[Marcus Leech]

I'm starting to think about adding Dicke-switching support into my 
gr-radio-astronomy continuum
application.

I (once again) need to understand how to use the auxillary TTL I/O bits 
on the DBS_RX daughtercard, for
driving the hardware switching apparatus.

But there's another issue that I'm struggling with (a little, anyway).  
In Dicke switching, the front-end is
switched alternately between the "sky" and a cold load or calibration 
source, with the actual detected power
being the difference between the "sky" and calibration source.   
Ideally, you do this many times per second.
What this does is to remove gain stability from the equation, since any 
system gain change that affects
the "sky" measurements, will also affect the calibration measurements.  
When successful, this technique
removes system-induced gain variations by a significant amount--an 
order of magnitude is not
uncommon.

In order to do this, I need to know which samples correspond to "sky" 
and which samples correspond to
"calibration source" as I'm processing them.  One thought I had was to 
synthesize a pair of identical channels,
one called "Sky" and the other called "Calibration", and have the code 
that flips the hardware switch set
a multiplier on each of these channels to either 0 or 1.  That is:

   when the hardware is switched to "Sky":

   skychan *= 1.0
   calibchan *= 0.0

   when the hardware is switched to "Calib":

   skychan *= 0.0
   calibchan *= 1.0

With the two channels being low-pass filtered separately, then one is 
subtracted from the other to produce a
synchronously-detected result.

How can I make sure that the chanels are properly synchronized with the 
hardware state of the switch?

--
Marcus Leech                            Mail:   Dept 1A12, M/S: 04352P16
Security Standards Advisor        Phone: (ESN) 393-9145  +1 613 763 9145
Strategic Standards
Nortel Networks                          address@hidden