Transmit AM through FM Filter?
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As far as I know, the K3 filters are not intelligent,
the K3 has no way of knowing what the filter is other than by your setup. If the FM filter was defined as a 5 khz filter, the K3 would probably send the signal through it and the signal would probably be spectrally purer than the vast majority of the gear that's being used on AM today. But the only way to find out would be to put it on a spectrum analyzer. Elecraft has spoken of removing the firmware block at some point in the future. There is also a block that keeps you from transmitting out of band, which is annoying when you're connected to a dummy load. [Elecraft] [K3] Transmit AM through FM Filter? Lyle Johnson kk7p at wavecable.com Fri Jun 27 13:13:42 EDT 2008 Previous message: [Elecraft] [K3] Transmit AM through FM Filter? > Checked the archives, but unable to find an answer. If I was to purchase > ONLY an FM filter, can I also TX AM through it without issue? The current firmware blocks the use of any foilter for AM Tx other than 6 kHz. 73, Lyle KK7P _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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Of course, the FM filter can be used for AM transmit. All you need to do is tell the radio it is an AM filter. It will not know otherwise.
There is really no reason not to transmit wide band AM, just like there is no reason not to transmit wideband SSB. The modes are perfectly legal in most of the world. For voice, it is a waste of spectrum but for digital modes, especially on the higher bands having more audio bandwidth may be useful. I think it might be that, in the USA it is not legal to transmit 15kHz wide AM or SSB. This is a difference between the USA rules and the rules elsewhere. In the USA, people are expected to do foolish things unless they are told not to, hence the limitations on transmitting out of band. Here in Europe, we have just as many fools, but it is considered their responsibility. We suffer greatly because of this but it also gives us a lot of freedom. |
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Re: Transmit AM through FM Filter?
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In reply to this post by Don Rasmussen
Thanks for all the answers. To set the story straight, the reasoning
behind the question was not to determine if I could transmit "hi-fi" AM, it's simply a matter of cost. At $125 a clip for an AM filter and the fact that I may only transmit AM once or twice in a lifetime, I couldn't see the extra cost to add the AM filter if TX through the FM filter worked. Since others have already identified that a filter can be "fooled" into thinking it's a different bandwidth, I do have a plan that I'll try somewhere down the road. Looking at the schematics it appears to be an easy solution that I can use an empty filter spot and diode switch (add two diodes) that position into an FM filter consumed location. That empty spot would be designated as the AM filter spot. Yeah, I know, it's only $125, but I'm a cheapskate. 73, Dan -- N3ND _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
RE: Re: Transmit AM through FM Filter?
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Since the AM transmit function is implemented in DSP, it should be clean and band limited anyway. I don't see why it would not be possible to enable the 13 KHz filter for AM transmit. As far as image rejection goes ... even if the FM filter has a shape factor of 2 (and it should be 1.5 or so) the image response would be down more than 60 dB unless there is a lot of circuit leakage (filter blow-by). 73, ... Joe, W4TV > -----Original Message----- > From: [hidden email] > [mailto:[hidden email]] On Behalf Of Dan Atchison > Sent: Saturday, June 28, 2008 7:51 AM > To: [hidden email] > Subject: [Elecraft] Re: [K3] Transmit AM through FM Filter? > > > Thanks for all the answers. To set the story straight, the reasoning > behind the question was not to determine if I could transmit "hi-fi" > AM, it's simply a matter of cost. At $125 a clip for an AM filter and > the fact that I may only transmit AM once or twice in a lifetime, I > couldn't see the extra cost to add the AM filter if TX through the FM > filter worked. > > Since others have already identified that a filter can be > "fooled" into > thinking it's a different bandwidth, I do have a plan that I'll try > somewhere down the road. Looking at the schematics it appears > to be an > easy solution that I can use an empty filter spot and diode > switch (add > two diodes) that position into an FM filter consumed location. That > empty spot would be designated as the AM filter spot. > > Yeah, I know, it's only $125, but I'm a cheapskate. > > 73, > Dan -- N3ND > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > 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: [hidden email] 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 |
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In reply to this post by Dan Atchison
It is not only $125. It is $125. This is a lot of money for something essentially unnecessary to you. That $125 could be much more usefully spent on a better CW filter or if you do not need a CW filter, $125 donated to charity would go a long way to making you feel good compared to knowing your rarely sent AM signal is exceptionally clean. What do most radios with only 15kHz roofing filters do on AM? 73 Mike |
RE: Transmit AM through FM Filter?
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> What do most radios with only 15kHz roofing filters do on AM? It's not a matter for the filter bandwidth. The first IF filter is a significant factor in controlling image rejection. With a 2nd IF at 15 KHz, the image only 30 away from the desired signal. 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 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 filter, even if the K3 were driving a 1500 W PEP amplifier the transmitted image would be less than .0005 milliwatts of carrier! 73, ... Joe, W4TV > -----Original Message----- > From: [hidden email] > [mailto:[hidden email]] On Behalf Of AD6XY - Mike > Sent: Sunday, June 29, 2008 1:06 PM > To: [hidden email] > Subject: Re: [K3] [Elecraft] Transmit AM through FM Filter? > > > > > > Dan Atchison wrote: > > > > > > > > Yeah, I know, it's only $125, but I'm a cheapskate. > > > > 73, > > Dan -- N3ND > > > > > > It is not only $125. It is $125. This is a lot of money for > something essentially unnecessary to you. That $125 could be > much more usefully spent on a better CW filter or if you do > not need a CW filter, $125 donated to charity would go a long > way to making you feel good compared to knowing your > rarely sent AM signal is exceptionally clean. > > What do most radios with only 15kHz roofing filters do on AM? > > 73 Mike > > > -- > View this message in context: > http://www.nabble.com/Transmit-AM-through-FM-Filter--tp1816107 Sent from the [K3] mailing list archive at Nabble.com. _______________________________________________ Elecraft mailing list Post to: [hidden email] 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: [hidden email] 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 |
Re: Transmit AM through FM Filter?
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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: [hidden email] 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 |
RE: Transmit AM through FM Filter?
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> > 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> > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > 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: [hidden email] 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 |
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