[AMRadio] Monitoring Modulation Accurately


K5MYJ macklinbob at gmail.com
Fri Nov 15 12:25:13 EST 2013


When using Controlled Carrier modulation PEAK POWER is important.

As an example a DX-60 has an idling carrier power of about 12.5W. But voice 
peaks can reach as high as near 60W. Controlled Carrier can not go to ZERO!

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: "W2XJ" <w2xj at w2xj.net>
Cc: <amradio at mailman.qth.net>
Sent: Friday, November 15, 2013 9:13 AM
Subject: Re: [AMRadio] Monitoring Modulation Accurately


>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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