Re: receiver sensitivity, harware vs. software filter?
- From: robert bristow-johnson <rbj@xxxxxxxxxxxxxxxxxxxx>
- Date: 3 May 2007 17:30:15 -0700
On May 3, 4:22 pm, "Thomas Magma" <somewh...@xxxxxxxxxxxxxxxxxx>
wrote:
They say that a receivers sensitivity is proportionate to it's bandwidth.
that's what i heard.
The narrower the crystal IF filter the more sensitive the receiver becomes.
now which is it? proportional and inverse-proportional is not the
same.
I'm talking about a traditional radio and as an example let's say I'm just
trying to detect a CW with a extremely low bit rate. They say you can gain
about 10 dB of sensitivity from going from a 12.5kHz wide IF analog filter
to a analog roofing filter (500Hz wide).
okay, maybe S/N increases as the BW gets smaller to a limit that has
something to do with the bit rate. if the "roofing" filter (not
completely sure what the terminology is) is an LPF of 500 Hz you can
get something like 1000 bit/s. if it's a 500 Hz wide BPF operating on
a complex IF signal, it would be about the same 1 Kb/s.
So if I sample the IF with the
12.5kHz wide filter in and apply a software filter with the same bandwidth
and shape factor as the 500Hz roofing filter, will my software radio gain
similar over all sensitivity?
ideally, if your bit rate is limited to 1 Kb/s, and your digital
filter can be a really sharp brick-wall (if you don't mind the delay
and you can throw an FIR with as many taps as you need at the problem
of designing the 500 Hz filter (whatever "roofing" means ... maybe you
mean "ceiling" which sorta has the same meaning as "brick wall" in
this context).
Simply put, can a narrow software filter replace a narrow hardware filter to
produce the same overall sensitivity of a radio?
yes, but the best word to use is *selectivity* instead "sensitivity".
but if "sensitivy" is S/N and you have a fixed and limited bit rate,
increasing selectivity increases sensitivity. and you can make *very*
selective digital filters, if you're willing to put enough taps in it,
or maybe an IIR LPF like a zillionth-order Butterworth, either way if
resources is not the issue then delay is what kills you.
r b-j
.
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- From: Thomas Magma
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