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?
The lower the Q, the more IFTs required for a given amount of pass band and
attenuationout of band.
Within reason, for bandwidths typical of audio
receivers, you should be able to build a filter at 455 kHz that has
effectively the same response as a 2 MHz filter. There is no need to
throw out the 455 kHz IF just to get wide bandwidth.
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?
I didn't say I had rejected the flat topped critical coupled IF response.
That's what I use when my radio has the IFT coil distance adjust control set to
minimum BW, but when the coils are slid towards each other after tuning, the BW
becomes wider
without going rabbit eared, because the sum of the responses of the two IFTs is
still
flat topped. I found the mecanical slide method to be more predictable than any
other, and the IFTs don't
drift off Fo, and the rabbit ear shape in IFT1 is symetrical each side of Fo.
Commercially made communication receivers I have seen use the same method to alter
the coupling and hence BW of their three IFTs. There are no long wires going to a
complex switch.
Patrick Turner.
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/