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February 6th 04, 01:26 AM

(Avery Fineman) wrote in message ...
In article ,
(gudmundur) writes:

My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses
of .8microseconds length, or about 1.25mhz. If all else remains the same,
and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F.
bandwidth at the 6db points, what increase in signal to noise ratio should
I see?

Signal to noise ratio changes as the _square_root_ of bandwidth
change. Wouldn't be much of an effect going from 1.25 to 1.5 MHz.

Um, he was starting with an 8MHz BW...

With 0.8 uSec pulses and a 1.25 to 1.5 MHz bandwidth (I presume
Mega Hertz, not milli Hertz), the output envelope will be very
rounded, almost Guassian or "cosine-quared" in shape. Rounding
happens because of the limitation of passing the harmonics of the
pulsed RF; all you have left is the carrier frequency.

Yes, the pulses will certainly be rounded when they come out of the
filter (though they may have started that way anyway). But depending
on the filter type, they may also incur lots of ringing, and if the
pulses follow one after another at the right spacing, the phase of the
energy in the pulse relative to the phase of the energy left in the
filter will matter a whole lot in what you see coming out. The
trailing edge of a rectangular pulse fed through a Chebychev filter
isn't very Gaussian looking!

As for the original question, the answer depends on the spectral
distribution of the noise...if it happens to be strongly peaked at the
carrier frequency of the pulses, the narrowing won't make much
difference; if it happens to be peaked at some other frequency, it may
help a lot. If it's uniformly distributed, AND you keep the filter
shape the same and narrow the bandwidth by 1.5:8, then you will have
1.5/8 as much noise _power_. You'll also have somewhat less signal
power, depending on the shape of the pulses. And of course, the
filter won't get rid of noise that's introduced after the
filter--fairly obvious but sometimes overlooked.

Cheers,
Tom