[Elecraft] [K3] Transmit AM through FM Filter?
Dan Atchison
purchase ONLY an FM filter, can I also TX AM through it without issue?
I know RXing is not a problem.
Thanks
73,
Dan - N3ND (anxiously awaiting a K3)
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Don Wilhelm
The 'without issue' part is important here. While it could possibly
work, to work with spurs and other 'bad things happening' is not likely.
The 6kHz filter is for AM transmit. The K3 may even prevent AM
transmission if that filter is not installed, but I don't know that for
certain.
73,
Don W3FPR
Lyle Johnson
> 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
Don Rasmussen
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?
AD6XY - Mike
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.
Ron D'Eau Claire
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.
Dan Atchison
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
David Woolley (E.L)
>
> The 'without issue' part is important here. While it could possibly
> work, to work with spurs and other 'bad things happening' is not likely.
Did you, instead, really mean "without spurs..." here?
--
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>
Joe Subich, W4TV
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
AD6XY - Mike
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
--
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_______________________________________________
Joe Subich, W4TV
> 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
David Woolley (E.L)
>
>
> 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>
Joe Subich, W4TV
> > 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: elecraft...@mailman.qth.net
> [mailto:elecraft...@mailman.qth.net] On Behalf Of David
> Woolley (E.L)
> Sent: Monday, June 30, 2008 3:15 AM
> To: elec...@mailman.qth.net
> Subject: Re: [K3] [Elecraft] Transmit AM through FM Filter?
>
>