[Discuss-gnuradio] DBS_RX/USRP warmup stability

[Marcus Leech]

One of the things that's quite important to RA types is the stability of 
the receiving equipment over longish
time periods--particularly over periods that are the same order of 
magnitude as the signals they're
 trying to observe.

I happened to have shut down my USRP/DBS_RX stack today to do some 
enclosure upgrades, so it had
 cooled down to room temperature (20C).  Once I brought it back up, I 
noticed the baseline drift upwards
 over a period of 20 minutes, and then it stabilized.  It drifted 
0.275dB in those 20 minutes, and after reaching
 operating temperature, seems to be stable to roughly 0.025-0.05dB, 
with random excursions occurring over
 periods of a few minutes.

Something that's interesting is that the system can detect when I'm 
sitting at the computer, but not necessarily
 typing anything.  Since my feedhorn is pointed out the window, it must 
be seeing a reflection of
 my blackbody radiation.  I confirmed that my feedhorn was able to see 
me, by placing my hand
 in front of the feed--sure enough a significant rise in the detected 
baseline resulted.  That means
 that the front-end LNA and feedhorn is working properly in that 
they're able to see my
 blackbody radiation. Humans make good weak-signal microwave sources :-)