K3 AM TX - Questions and Observations

[Andy Wood]

I have recently been experimenting with AM on the low bands using the K3. The addition of Sync AM RX makes such a difference!

I have noticed on AM TX, use of compression, at any level, causes some distortion of the waveform at the zero crossing (when viewed on an oscilloscope) - not really an issue, but I wonder if anyone else has come across this? It also appears nearly impossible to exceed 100% modulation, regardless of the mic gain setting.

I also have a question regarding asymetric AM modulation and the K3 - is it possible for the TX to exceed 100% positive modulation whilst maintaining less than 100% negative? Being only new to the AM mode, I am not sure if it is possible with any TX that uses low level modulation. Any info greatly appreciated.

Andy  VK4KY

[W8JI]

I can't speak for details on the K3. Only generally.

> I also have a question regarding asymetric AM modulation
> and the K3 - is it
> possible for the TX to exceed 100% positive modulation
> whilst maintaining
> less than 100% negative? Being only new to the AM mode, I
> am not sure if it
> is possible with any TX that uses low level modulation.
> Any info greatly
> appreciated.

It is actually easier to have clean asymmetrical modulation
with low-level AM than it is with a tetrode in a plate
modulated circuit. It is also easier to have better
linearity with low level modulation.

http://www.w8ji.com/amplitude_modulation.htm

Someday I'll have to get an AM filter and try my K3. I guess
there is no way to use the transmitter without an AM
filter????

73 Tom

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[Dunc Carter - W5DC]

Using the FM filter for AM transmit has been requested but may or may
not happen.  According to Wayne,  this depends on whether it can meet
transmit specs and development priorities.  I'd like to have AM mainly
to work one friend who is running 1950s Harvey Wells gear but all the
filter slots in my K3 are filled already.  The FM filter does work fine
for receiving AM, at least for KCFR, one of my wife's favorite public
radio stations.

Dunc, W5DC
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[Andy Wood]

Thanks for the reply Tom - your information is appreciated. I have been to your website a few times but I hadn't visited the AM page before. Lots of good information there. I had to read it a few times to take it all in.

I have done some more research and it appears the K3 TX has a quite sophisticated, multiple loop ALC system. This is probably the reason it currently cannot be modulated in excess of 100% positive. Maybe there will be an update in the future?

Andy  VK4KY

[Joe Subich, W4TV-4]

Andy,

 > This is probably the reason it currently cannot be modulated
 > in excess of 100% positive. Maybe there will be an update in
 > the future?

You do not want more than 100% positive modulation with the K3.
If one assumes 25W of carrier, 100% positive modulation represents
100W peak power ... modulation greater than 100% positive would
result in limiting/compression in the final amplifier with its
attendant distortion.  To obtain greater than 100% positive
modulation (particularly in a low level modulation with linear
amplifier situation), the final amplifier must be capable of
PEP greater than four times the carrier power.

The only way the K3 could support more than 100% positive modulation
is if the carrier power were reduced and that would be rather
self defeating.

73,

    ... Joe, W4TV


On 5/11/2010 9:27 PM, Andy Wood wrote:

>
> Thanks for the reply Tom - your information is appreciated. I have been to
> your website a few times but I hadn't visited the AM page before. Lots of
> good information there. I had to read it a few times to take it all in.
>
> I have done some more research and it appears the K3 TX has a quite
> sophisticated, multiple loop ALC system. This is probably the reason it
> currently cannot be modulated in excess of 100% positive. Maybe there will
> be an update in the future?
>
> Andy  VK4KY
>

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[Andy Wood]

Hi Joe,

Yes, you are correct and I understand what you are saying.

However, reduced carrier power from the K3 TX is not an issue if driving an external amplifier to, say, 100 watts carrier. In this case, output from the K3 may be only 10 watts carrier (40w peak), leaving plenty of overhead to accommodate an increase in positive modulation well beyond 100%.

Andy  VK4KY

[Mark Stennett]

In AM analog broadcast, asymmetrical modulation is standard fare.

http://www.vorsis.com/pdfnfoto/Vorsis%20AM10%20InkJet%20090722.pdf

Output specs on page 7.

73 de na6m



On 5/11/2010 10:08 PM, Andy Wood wrote:

>
> Hi Joe,
>
> Yes, you are correct and I understand what you are saying.
>
> However, reduced carrier power from the K3 TX is not an issue if driving an
> external amplifier to, say, 100 watts carrier. In this case, output from the
> K3 may be only 10 watts carrier (40w peak), leaving plenty of overhead to
> accommodate an increase in positive modulation well beyond 100%.
>
> Andy  VK4KY

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[Andy Wood]

Gee, that is a nice "toy" Mark.

I wonder how many K3's you could buy for the price of one of those units................

Andy  VK4KY

[Brett Howard]

Yes I'm also hoping/waiting for AM TX to be allowed on an FM filter.
I've yet to try it but I'm sure if I lie to the K3 it'll work...  As to
how clean and w/in spec it'll be that would take some looking...

~BTH

On Tue, 2010-05-11 at 07:19 -0600, Duncan Carter wrote:

> Using the FM filter for AM transmit has been requested but may or may
> not happen.  According to Wayne,  this depends on whether it can meet
> transmit specs and development priorities.  I'd like to have AM mainly
> to work one friend who is running 1950s Harvey Wells gear but all the
> filter slots in my K3 are filled already.  The FM filter does work fine
> for receiving AM, at least for KCFR, one of my wife's favorite public
> radio stations.
>
> Dunc, W5DC
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>
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> Please help support this email list: http://www.qsl.net/donate.html

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