[hpsdr] Companding in SDR RX Front End

[Guido]

Jeff, Koos,

Thank you for the response and the reference. I need some time to read the
details in the spec,
but I appreciate to understand what it could mean to the overall receiver
performance.

73, Guido
PE1NNZ

On Jan 6, 2008 5:05 PM, jeff millar <wa1hco at wa1hco.net> wrote:

> 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
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/attachments/20080107/f18935ed/attachment-0004.htm>