AM Transmit Bandwidth

[Grant Youngman]

If the FM filter is assigned to AM transmit, is there any improvement in transmit audio bandwidth beyond 3 KHz,or does it remain DSP limited to this relatively restricted range?

Grant NQ5T
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[k6dgw]

I believe it is constrained to the normal SSB bandwidth of around 3 KHz
or less, unless using ESSB mode.  However, I'm certainly not the expert
here.

For US hams, 47CFR97.307 begins:

"(a) No amateur station transmission shall occupy more bandwidth than
necessary for the information rate and emission type being transmitted,
in accordance with good amateur practice"

which would sort of preclude SSB much wider than what the K3 normally
does.  However, I am neither a Commissioner nor a lawyer.

There were a few RCA Ampliphase 1KW AM transmitters made and they
sounded pretty good in a 10 Kc channel.  One or two might still be
around and available. :-)  Actually, I don't think RCA invented those.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015
- www.cqp.org

On 3/19/2015 7:24 PM, GRANT YOUNGMAN wrote:
> If the FM filter is assigned to AM transmit, is there any improvement
> in transmit audio bandwidth beyond 3 KHz,or does it remain DSP
> limited to this relatively restricted range?
>
> Grant NQ5T
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[Don Wilhelm-4]

Grant,

The AM audio bandwidth is limited by the DSP - both for transmit and
receive.  I don't recall the exact audio bandwidth limit, but it is 4kHz
or lower.

73,
Don W3FPR

On 3/19/2015 10:24 PM, GRANT YOUNGMAN wrote:
> If the FM filter is assigned to AM transmit, is there any improvement in transmit audio bandwidth beyond 3 KHz,or does it remain DSP limited to this relatively restricted range?
>
>

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[Grant Youngman]

I drug out the test gear and answered my own question.  

The answer is — regardless of transmit filter selected, the upper end of audio in AM transmit is a pretty hard limit at about 2.95 Khz give or take a bit where the output drops off to better than 10dB below the mid range of the audio passband.  At 3 KHz there’s virtually no audio at all.  So no, there is no improvement in transmit bandwidth by using the FM filter for AM transmit.

Which is a shame.  The Flex radios (for example, and ignoring the rest of their faults) if properly driven and adjusted can produce very nice AM audio with a reasonable transmit passband for this mode.  The K3 on the other hand generally gets a pretty bad (and perhaps deserved) rap on this in the AM community.  

It would seem since ESSB allows up to 4 Khz of audio bandwidth, that it might be possible to offer something in this range for AM and perhaps a bit more (adjustable to conditions) as well when using the FM filter for transmit.  It would make a big difference over the 2.9+ KHz currently available.

**** Please note, I am not trying to start yet another long thread on the whole bandwidth war discussion, or what the rules say or don’t or make anyone’s head explode.  But it sure would be a good thing if Elecraft could see a way clear to improve the transmit fidelity in AM.

Grant NQ5T


> On Mar 19, 2015, at 9:24 PM, GRANT YOUNGMAN <[hidden email]> wrote:
>
> If the FM filter is assigned to AM transmit, is there any improvement in transmit audio bandwidth beyond 3 KHz,or does it remain DSP limited to this relatively restricted range?
>
> Grant NQ5T

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[Bill-3]

I have found that careful adjustment of the TX EQ really helps how the
K3 sounds on AM. In fact, I actually do get some good reports - these
from people using vintage AM equipment.

It could be better, but I do not see AM being a priority with anyone
these days. I am just happy to be able to do AM in a reasonable fashion
at good power without having to have 400 lbs. of equipment stacked up on
my desk.

Bill W2BLC K-Line

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