[AMRadio] Monitoring Modulation Accurately


Rob Atkinson ranchorobbo at gmail.com
Fri Nov 15 12:13:50 EST 2013


I agree with some of this but not all of it.  Peak power may be
everything if you are a slopbucketeer, but in ham AM, it's a bogus
power indicator--it's difficult to measure accurately, and gives a
false idea of your transmitting effectiveness because AM has a complex
waveform over time and frequency spectrum.   For AM I measure dead
carrier into a 50 ohm pure resistance with a Bird 43 and thermocouple
RF amp meter (you can also use a VTVM with a HP RF probe tapped off
the dummy load) and then under modulation let the chips fall where
they may, only making sure the negatives don't clip the carrier.
Others may obsess about peak power or PEP but I think it's a waste of
time.

Other reasons why obsessing over high positive peaks and peak power
are inadvisable:  If you go wildly asymmetric on the positives, you
may be technically clean if you limit negative to 95%, but you'll
still distort in a lot of receivers not equipped with sophisticated
detectors.   Extremely asymmetric AM has been shown to suffer more
from selective fading.   And, it doesn't really do anything for
getting through on the other end.  Instead, what really matters and
should be pursued as a goal, (if the operator cares about any of this)
is _high average_ audio power.  This is achieved with a combination of
compression and peak limiting.

The next time I work an AM op and he tells me his PEP I'm going to
tell him I'm only interested in PEP when I'm going to Pep Boys for car
parts hi.

The one useful thing about the positive and negative peak flashers on
a monitor is that if you see the positive flashing a lot more that the
negative, you know you need to flip the phase on the audio.

73

Rob
K5UJ

On Fri, Nov 15, 2013 at 10:17 AM, W2XJ <w2xj at w2xj.net> wrote:
> With proper processing it should not be a problem. I always trust a scope before a mod monitor. Properly clipped negative peaks are easy to see and the absolute value of positives are not important. In broadcast it is common practice to modulate as much positives as the TX can take and in amateur operation a peak power measurement is the only thing that really counts.
>
> Sent from my iPad
>
>> On Nov 15, 2013, at 5:15 AM, Steve WA1QIX <wa1qix at piesky.com> wrote:
>>
>> Using a scope to monitor your modulation gives you a good indication of the shape of your waveform, but the human eye is just not fast enough to catch the peaks nor can you really determine accurately your actual percentage of modulation unless you are using a very modern peak storing scope.
>
> Sent from my iPad
>
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