Re: [Discuss-gnuradio] USRP2 Spectrum Sensing Help, (variant for FunCube Dongle?)

[Marcus D. Leech]

On 03/19/2011 11:06 AM, devin kelly wrote:
> I've only started using the AGC block very recently.   I started using 
> because I wanted some way to control the AGC.  This makes me think of 
> the question, is the AGC on by default?  That is, if I don't have an 
> AGC block will the AGC be doing anything?  Also, in the first two or 
> three ICs in the WBX there is a digitally controlled attenuator and an 
> LNA, those are part of the AGC correct?
There is no automatic gain control in the WBX hardware.  The step 
attenuators are set when you set the hardware RF gain of the daughter-card.
   The AGC block within Gnu Radio simply applies a multiplicative 
coefficient to the data stream within Gnu Radio, based on smoothed
   historical signal power estimates.

AGC is useful for certain types of signals, and not for others, which is 
why there's no automatic AGC in the hardware.  Now various
  daughtercards have *manual* gain control, and you can certainly 
implement AGC that's distributed between software and hardware,
  but that's up to the software.  In general, in SDR, you want the 
hardware to implement (borrowing from X11 for a second here)
  "mechanism, not policy".   Consider precision RF measurement 
applications, for example.  You don't want the hardware implementing
  an AGC policy of any kind.  Just like you don't want the hardware 
implementing "the one, true demodulation mode".


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org