> > 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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of David > Woolley (E.L) > Sent: Monday, June 30, 2008 3:15 AM > To: elecraft@mailman.qth.net > 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> > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
