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
RE: [K3] [Elecraft] Transmit AM through FM Filter?
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
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--tp18161074p18183029.html Sent from the [K3] mailing list archive at Nabble.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
RE: [K3] [Elecraft] Transmit AM through FM Filter?
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: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of AD6XY - Mike Sent: Sunday, June 29, 2008 1:06 PM To: elecraft@mailman.qth.net 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 4p18183029.html Sent from the [K3] mailing list archive at Nabble.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 ___ 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
Re: [K3] [Elecraft] Transmit AM through FM Filter?
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. -- View this message in context: http://www.nabble.com/Transmit-AM-through-FM-Filter--tp18161074p18163177.html Sent from the [K3] mailing list archive at Nabble.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
RE: [K3] [Elecraft] Transmit AM through FM Filter?
We on this reflector have often beat to death the American FCC Amateur service rules about bandwidth since they give no numbers about what is acceptable as long as the emission stays within the Amateur band. Unlike commercial frequencies, where specific technical standards avoid co-channel interference with other services, the rules for Amateur emissions are vague in keeping with the basic philosophy about not having rules where no rules are required. From a technical standpoint, the signal-to-noise ratio drops as the bandwidth increases. The best signal to noise ratio for a given effective radiated power is found at the narrowest bandwidths. That's why CW works with signals that can't be heard at all on SSB, and why SSB gets clear copy with signals unreadable on AM. Even if the receiver filter is tailored to match the signal, the amount of noise power increases in direct proportion to the bandwidth. The effect upon the listener is that the noise gets louder and the signal weaker as the bandwidth is increased. So, without trying to argue the law, that is a solid technical reason to maintain narrower bandwidths whenever possible. In free society, such as the USA, begins with the concept that citizens can do *anything* that pleases them but, to avoid injury to others, we must adopt laws prohibiting specific behavior. The objective is to have as few laws as possible, and to write the laws to prohibit exactly the behavior that is damaging. Of course we Yanks didn't invent the concept. One predecessor was the Magna Carta. Sure, some Americans don't understand that or they choose to freeload, taking advantage of anything they can get away with without respect to the law. That injures the rest of us, then we're further put out by having to pay to keep them locked up when they are caught. But the vast majority of Americans I've known over my lifetime are what we call honest, law-abiding citizens, in spite of what the movies suggest. Indeed, like all good story tellers, novels and movies dwell on the unexpected and unusual, not the commonplace. Ron AC7AC -Original Message- 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. ___ 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
