[Boatanchors] Downward Modulation

[K2GKK D C_Mac_ Macdonald]

It is very easy got get "downward modulation" when
using an SSB transmitter on AM.  It is simply a case
of excessive audio applied to a carrier that is reduced
to 25% of the usual SSB PEP output to keep the AM
PEP equal to that of the SSB signal.

That 25% (25 Watts in most SSB rigs) will have its
PEP peaks at 100 Watts when fully modulated.  Any
more audio will attempt to drive the signal below the
actual output and that will cause distortion/trash on
the output signal.

Obey the manufacturer's instruction manual.  There
should be only very slight increases on the output
meter if you are operating correctly.  Easily seen if
you have a scope monitoring your transmitted signal.

73 de Mac, K2GKK/5
Licensed 30 Nov 1953
Oklahoma City, OK
USAF (Retired) 61-81
FAA (Retired) 94-10

________________________________
From: boatanchors-bounces at mailman.qth.net <boatanchors-bounces at mailman.qth.net> on behalf of manualman at juno.com <manualman at juno.com>
Sent: Sunday, April 24, 2022 00:46
To: wa3mck at gmail.com <wa3mck at gmail.com>
Cc: boatanchors at mailman.qth.net <boatanchors at mailman.qth.net>
Subject: Re: [Boatanchors] Downward Modulation

Actually it's not the same "downward modulation" that can be experienced
with boatanchor vacuum tube-type transmitters.
It's found in many Icom and Yaesu transceivers and some Kenwood rigs over
the last 20 to 30 years. Years ago, WA3WDR, coined the phrase "Reverse
Carrier Control" to describe what was happening.
For a detailed description, go to this link and scroll down to "Comments
from Bob, 'Bacon', WA3WDR":
https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.qsl.net%2Fwa5bxo%2Fasyam%2Faam3.html&amp;data=05%7C01%7C%7C038e98ed97e74794f6dd08da25b61195%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637863761154307932%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=4od8xtg1IGgLOsf26tml%2BP1VFO12KTX0sZPtJN1xEng%3D&amp;reserved=0

It's actually a good thing. With your IC-7300 you can drive your
amplifier to its maximum allowable drive and not worry that your voice
peaks on AM are going to exceed the plate dissipation of the amplifier
tubes. SO, you can drive your amp if capable, to a 1500 watt carrier,
modulate the carrier on AM, and not exceed the 1500 PEP limit. It also
protects the RF output and driver transistors in the transceiver from
being driven beyond their typical specifications.

Operators get upset when they have a forward reading power meter in the
feed line and when modulating on AM, the see the meter moving in a
downward direction. Put a scope in the feed line and see what's really
happening.

Also on AB4OJ/VA7Oj's web site, which is loaded with tons of Icom product
information, he discussed this years ago, and also ran some tests on the
IC-756 series which performs the same way.

You want to defeat it; it's simple. Actually, Kenwood published this info
for a different reason back in the 70's, but it works equally well in
this application.
You need to apply a variable small negative voltage to the ALC line of
the transceiver; or you can go into the ALC circuitry of the rig and
diddle with the ALC pot in the AM circuitry. Personally, I've never found
any advantage with the diddling. I've tried it several times on my IC-756
PRO II without telling the amateurs I was working and no one seemed to
notice any difference in my audio quality or power. And after 22 years of
running my 756 on AM, the rig is still running fine without any internal
diddling.

Pete, wa2cwa


On Sat, 23 Apr 2022 20:27:21 -0400 Submarine Radioman <wa3mck at gmail.com>
writes:
> Can anyone point me to a clear and concise explanation of downward
> Modulation? I�m experiencing it with my IC-7300 running about 20
> watts of
> carrier. I remedy it by lowering my carrier slightly and playing
> with my
> antenna matching unit. I remember having the same problem back in my
> old
> boat anchor days�
> But I don�t remember the cause or explanation.
>
Tnx de Howie Wa3mck