Posted by
David Woolley (E.L) on
Jun 30, 2008; 7:13am
URL: http://elecraft.85.s1.nabble.com/Transmit-AM-through-FM-Filter-tp365954p365958.html
Joe Subich, W4TV wrote:
>
>
> Still, the FM filer response SHOULD be down some 70 or 80 dB
> +/- 30 KHz from the center. That would be more than enough for
Although probably still well down the skirts, you have to subtract the
maximum modulating frequency from this.
> most purposes - nobody is going to be trying to work AM stations
> at the noise floor <G>. With 70 dB of image rejection in the
I think the main concern is for transmit, where another operator may be
listening to SSB at the image frequency. FM tends to be used in
reserved band segments, where interference is handled by the capture
effect. FM transmissions also have infinite sidebands, and a normal
transmitter only filters these with the main LC filter.
> filter, even if the K3 were driving a 1500 W PEP amplifier the
> transmitted image would be less than .0005 milliwatts of carrier!
There is also any unbalanced first local oscillator at 15kHz offset.
Again, it may be far enough down the skirts not to be too much of a
problem. (However the suppression requirements are much higher than for
an SSB carrier.)
--
David Woolley
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