[Discuss-gnuradio] software controlled antenna

[Chuck Swiger]

This weekend's project was pretty easy and useful. My indoor antenna is a loop
with pretty sharp Q which requires touching up, manually, even if receiver 
frequency
is changed only 50kHz. I have to get up, walk over to the loop and turn a 
365pf variable
cap to 'peak' the signal.

I just found out about MVAM109 varactors, which go from about 40 - 400 pf with
0-9vdc bias, so it was fairly easy to use the gnuradio/usrp aux DAC 'D', 
the sigma
delta, which is brought out on J17 on the basic RX board, and makes 0-3.3 vdc
with 12-bit precision, a 2N2222 transistor dc amp with a 12v supply to boost it
up to 0-9vdc. Put two MVAM109's back to back on the loop fed thru 100k 
resistors
(they're very high Z) and now I can adjust and peak the antenna with a 
slider on the gui ;))

--Chuck

http://partsandkits.com/MVAM109.pdf
src.write_aux_dac(0,3,value)      0<value<4095