[Discuss-gnuradio] DBS Rx gc1 gain

[Lee Patton]

This one's probably for Matt.

I would like to be able to estimate the average power at the SMA input
to the DBSRx board.  I believe the signal processing chain looks
something like this:

         _____________     ____________________________     ________
       | MGA-82563   |    |          MAX2118           |    | AD8132 |
SMA -> | apx. 13.4 dB| -> | gc1 gain, gc2 gain, dl gain| -> | 0 dB   | 
        --------------     -----------------------------     ---------

      _______    ______
     |  PGA  |  | FPGA |
---> [0-20 dB|->| 0 dB | -> USB
     --------   -------

Am I missing any major gains/losses?  


Attached are two plots. The first (gc1_as_is.jpg) shows that setting gc1
according to the formula in db_dbs_rx.py does not result in expected
gains.  I did my best to calibrate my board, and the results are shown
in the second figure (gc1_cal.jpg).  The final equation to calibrate the
board was: gc1 = int( (-0.0198*gain + 2.5854)*4095/3.3)

The captured data in both cases was a -26.13 dBm signal at 1.823 GHz.
During the capture, gc2 was set to 31 (0 dB gain), dl was set to 0
(disabled), and pga was set to 0dB.

All gains were calculated relative to the USRP output when gc1 was
commanded to 0 gain. For the "as is" case, gc1 corresponds to 2.6005
volts for a 0dB command, and in the "calibrated" case gc1 is 2.5852
volts for a 0dB command. 

The (max) gain at 0.75 V is 60 dB min and 69 dB typical according to the
data sheet, but both plots show it to be apx. 48 dB. Any insight into
this?

I've also attached the mfiles used to make the plots.

Thanks,
 - Lee

gc1_as_is.jpg
Description: JPEG image

gc1_cal.jpg
Description: JPEG image

calcpwrs.m
Description: Text Data

delta_gain.m
Description: Text Data