Re: [Discuss-gnuradio] SDR question

[Marcus D. Leech]

On 28/10/2011 5:08 PM, Andrew Rich wrote:
> I have a question about software defined radio
>
> I saw a pass band the other day on a screen which prompted me to ask
>
> The Software defined radio has a specific bandwidth ?
>
> Does it "scan" across the band very quickly to form the passband, or 
> is the bandwidth already that large it just appears as a chunk of MHz ?
>
> I am trying to make the connection between how a Software Defined 
> Radio would be different from an analogue system.
>
> For example decoding packet radio using an SDR, is there any 
> performance degradation due to the way it works ?
>
> Would the SDR "sweep" and miss some of the signal ?
>
> - Andrew VK4TEC -

A typical SDR hardware front-end (just taking the RX view for now) has a 
tunable direct-conversion down-converter that converts a swath of
  bandwidth at a desired center frequency into a complex (I,Q) baseband 
signal that "straddles" from  -BW/2 to BW/2,
  with "DC" in the middle.

That swath of (analog) bandwidth is sampled by an ADC and FPGA, and then 
decimated for delivery of a lesser bandwidth (again, in complex
  baseband form) into the host computer for further processing. The 
decimation also acts as a filter, so that there is strong alias suppression
  in the delivered bandwidth.  It is usually the case that the FPGA 
decimator is configurable with respect to the amount of bandwidth
  delivered towards the host.

Bandwidths of several MHz into the host are achievable these days, with 
all demodulation, etc, happening on the host.

That is not to say that you couldn't implement a sweeper for doing 
SIGINT and spectral estimation, etc.  In fact, there are Gnu Radio
  applications that do just that.