Re: [Discuss-gnuradio] capturing a broad range of frequencies

[Eric Blossom]

On Sat, Oct 01, 2005 at 02:12:14PM -0400, address@hidden wrote:
> hello all,
> 
>        I'm not sure that this question has been asked or not.
>        is it possible to record or capture a broad range of
>        frequencies for example from 55MHz to 550MHz all into
>        on file?
> Neal

I'm assuming that you mean that you want to capture *everything*
between 55MHz and 550MHz at the same time, with nothing left out.

The Nyquist criterion says that you need to sample at at least twice
the the bandwidth of interest.  Reality says it's more like 2.2 times.

In the example above, the bandwidth of interest is 550M - 55M = 495M.
Thus you'd need to sample at a minimum of 990M samples/sec.  Call it
1GS/s.

1GS/second A/D's are available, typically with on the order of 8-bits.

The other downside is that if you're covering 55M - 550M, you're going
to have lots of very weak signals and lot of very strong signals in
that range.  Thus you're going to need *lots* of dynamic range if you
want to see them.  Way more than 8-bits.

Of course you'd also need a raid array that was capable of sustaining
writes at 1GB/sec.  A good SATA drive will do about 50MB/s, so if you
striped 20 of then together, and had a system that could sustain
1GB/sec of i/o, you'd have that requirement covered ;)

In theory it's possible.  In practice very hard.

Eric