Re: Receiver sensitivity and IF bandwidth??

In sci.electronics.design Tim Wescott <tim@xxxxxxxxxxxxxxxx> wrote:
billcalley wrote:
Hi All,

I keep reading that the high-gain front-end stages of a microwave
receiver almost completely sets the entire radio's NF and sensitivity,
and that the following stages (the I.F.) have little effect except to
amplify the signal and the noise equally to a higher amplitude for the
radio's detector. This doesn't make complete sense to me, because the
I.F. would have a HUGE effect on the receiver's signal-to-noise ratio,
and therefore its sensitivity, if we simply narrowed the IF's
bandwidth down from, let's say, 1MHz to 1kHz!! So, to me anyway, the
I.F. would have a gigantic effect on the receiver's sensitivity, even
if the front-end had infinite gain. Or am I missing something here?

(BTW: I fully realize we can't just narrow-down the receiver's
bandwidth below the bandwidth of the modulated signal, but I'm just
asking about all this on a theoretical basis to try and understand
"sensitivity" a bit better).

Thanks,

-Bill

Noise figure compares the receiver noise with the noise from a purely
dissipative element* at some temperature, usually 298K or 300K (i.e.
"room temperature"). Both the receiver noise power and the source noise
power are multiplied by the IF bandwidth, so it doesn't affect the noise
figure.

The implicit assumption is that you've already fit the IF bandwidth to
your signal of interest.


I think that's the pivotal point, Tim; An assumption in any receiver
design is that the IF bandwidth will be restricted to roughly the
bandwidth of the received signal. It's pretty much WHY heterodyning
exists - in order to be able to use a channel filter (IF filter) that
doesn't need to be tuned when the receiver changes channel.

If the receiver bandwidth is a given, then the receiver's NF is indeed
dominated by the NF of the first stage, provided that stage has a
reasonably high gain.

--
Rick

.

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