RE: Transmit AM through FM Filter?

Posted by Joe Subich, W4TV-3 on Jun 30, 2008; 6:36pm
URL: http://elecraft.85.s1.nabble.com/Transmit-AM-through-FM-Filter-tp365954p365959.html



> > 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.

The difference between +/- 30 KHz and +/-26 or 27 KHz is probably
not enough to notice.  The FM filter should still have a shape
factor no worse that 2 ... a realistic 8-pole filter should be
in the 1.6 to 1.8 range at that bandwidth.

> > 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.)

Unbalance would effect all modes - even with a narrow filter.  I
have not checked but would expected the circuit to use a balanced
mixer specifically to maintain a wider spurious free passband.

73,

   ... Joe, W4TV
 

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of David
> Woolley (E.L)
> Sent: Monday, June 30, 2008 3:15 AM
> To: [hidden email]
> Subject: Re: [K3] [Elecraft] Transmit AM through FM Filter?
>
>
> 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
> "The Elecraft list is a forum for the discussion of topics related to
> Elecraft products and more general topics related ham radio"
> List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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