[AMRadio] Monitoring Modulation Accurately


W2XJ w2xj at w2xj.net
Fri Nov 15 15:43:40 EST 2013


On most scopes there are 5 minor graticules for each major graticule. If the scope is calibrated so with carrier only plus and minus 2 major graticules are used, plus minus 1 minor graticule would be minus 90 percent. Alternately, run the mod to 100% with a tone and back the level back one DB, mark the scope to more easily see the 90% point.

Some cautions..... Pretty much any amateur or military rig will have over shoot unless modified. Almost all BC rigs will work well if the frequency response is kept to something reasonable like 3 to 5 kilohertz. Many home brew PDM type transmitters will also work well. The key is to have a phase linear path through the transmitters, no audio filtering, limiting or processing. A good test is how well it can pass a square wave up to 3 KHz which requires fairly decent 15 kilohertz response.  Filtering, limiting and clipping should be external to the TX. Most SSB transceivers where the carrier is merely re-inserted to make AM make pretty band AM units. 

Sent from my iPad

> On Nov 15, 2013, at 10:32 AM, Bry Carling <bcarling at cfl.rr.com> wrote:
> 
> But how are you measuring that 90% please? Just trying to learn here.
> 
> Bry Carling
> http://af4k.com
> 
>> On Nov 15, 2013, at 12:54 PM, W2XJ <w2xj at w2xj.net> wrote:
>> 
>> I would respectfully disagree. First, if one is at all interested in following FCC rules, peak power is all that counts. The widely touted 375 watt limit for AM applies only to symmetrically processed waveforms. An asymmetrical waveform requires a reduction of carrier power if one chooses to follow the rules. I know that a number of hams with converted BC rigs run 1KW carrier and I am not going to debate various operating practices, to each their own. I do not see a reason to go to extensive efforts in measuring power. Either one follows the rules and sticks to 1500 watts peak, easily measured on any number of available meters, or power is totally irrelevant so why bother? 
>> 
>> Positive modulation does not cause distortion in even poor detectors unless the following audio stage somehow overloads. Many broadcast stations routinely run 145% positives and sound good. Back in the days of the modified Harris BC5P and later the BC5 HA stations sounded good running 200% or more positive peaks and it did indeed improve penetration. It is negative peaks that causes a diode detector to clip and I never go past 90% negative. 
>> 
>> A properly phased audio chain sounds best when everything is aligned for positive peaks. I have built many BC stations with modified audio processing that only responded to negative peaks. So the more the positive flasher flashes, the better. This all really depends on the phase linearity of the audio path and the TX. With a typical SSB transceiver all bets are off for many reasons.
>> 
>> Bottom line, for legal operation, peak power is all that counts and controlling negative modulation makes for a clean operation. If one chooses to operate otherwise, I would suggest negative modulation is all that matters. 
>> 
>> Sent from my iPad
>> 
>>> On Nov 15, 2013, at 9:13 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
>>> 
>>> 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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