Re: [Discuss-gnuradio] SDR question

[Andrew Rich]

Thanks Marcus

So you can go outside the useable bandwidth, you just need to understand 
that you will loose something as you move to the next chunk of RF ?

I saw an image of several MHz and a little decode window, but I guess that 
is a decoding window, smaller than the SDR sampling window.

I want to use SDR for satellites and packet radio

Does it meet a tnc / analogue radio specs ?

- Andrew -


----- Original Message ----- 
From: "Marcus D. Leech" <address@hidden>
To: <address@hidden>
Sent: Saturday, October 29, 2011 7:16 AM
Subject: Re: [Discuss-gnuradio] SDR question


> 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.
>
>
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