John Byrns wrote:
In article , Patrick Turner
wrote:


John Byrns wrote:


7. It has been suggested that using a 2 MHz IF frequency would allow
wider bandwidth than the standard 455 kHz IF frequency. I fail to see why
this should be true.

Because for the same Q value, the pass band would be 4 times wider


Where is it written that the same loaded Q must be used for both filters?
If you can change the center frequency, why can't you change the loaded Q?

Admittedly it might be a bit more difficult to achieve the same Q at 2
MHz as 455 but we are talking about homebrewing and experimenting here.



Its difficult to make a 455kHz typical old IFT produce a nice flat topped
20 kHz wide BW. Its either pointy nosed, undecoupled, or flat topped, critical
coupled,
or over critical or rabbit eared.
I have tried all that.


So you have tried all that and rejected the "pointy nosed", "flat topped",
and "rabbit eared" response curves. I am left to wonder what sort of
response curve you were looking for? Why not settle for a nice "flat
topped" response curve and be done with it?

Here's a wacky idea that I'll toss out just to see if it flies...

Could one use two garden variety 455kc xfmrs in series, one tuned at
center plus and the other tuned center minus? Impedance matching would
be an issue but maybe such a scheme offers a not so glamourous method of
achieving the wider bandwidth and maintaining the flatness with little ado.

Re wider bandwidth as a whole. On AM sets that I have owned with
excessively wide bandwidth they all tend to sound like crap. I don't
have a wealth of local stations that might be enhanced by the wider
width but on weaker stations the amount of noise and all those AM
"artifacts" seems to go way up making it very unpleasant to listen to.

As a result I think it would make the most sense to use a switchable or
continuously variable bandwidth scheme so as to not be left with an all
or none scenario after so much effort.

John B, you may remember one of my Tandberg receivers that had 4
positions of bandwidth ganged with a switch that somewhat tailored the
audio accordingly. To the ear (or rather to my ear) this seemed very
effective.

-Bill M