[hpsdr] Companding in SDR RX Front End

[jeff millar]

National describes a receiver gain control system in their GSM A/D and downconverter chip
	http://www.national.com/ds/LM/LM97593.pdf
See section 6.0 on Page 35.

The approach monitors the output of the A/D and when it approaches clipping, logic switches in 
digital attenuation (within a few clock cycles) in the RF/IF path and simultaneously adds gain in 
the digital path to compensate.  The signal processing algorithms don't see any net change in gain. 
  This approach allows the noise figure of the receiver to rise rather than clipping or 
intermodulation products.

Software algorithms see a much larger dynamic range in the numerical values out of the combined A/D 
and AGC algorithm.  The National chip grows from a 12 bit A/D to a 21 bit output range

This algorithm has a number of advantages
	- Self contained within RF, IF, ADC.  Software can ignore it
	- Fast and automatic
	- Prevents clipping and IMD at expense if noise figure

jeff, wa1hco


Guido wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> 
> Companding increases the dynamic range of a channel. So, could this 
> advent the performance of our SDR receivers ?
> 
> Applying companding just in front of the ADC could increase the dynamic 
> range of the overall system. I understand that companding goes at the cost worse 
> distortion for small signals, but is this effect really noticable (the harmonic 
> products of a small signal are also small) and scarifying the receiver performance ?
> 
> Any thoughts?
> 
> 73,
> Guido
> PE1NNZ



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