The AM Forum

Hi

At 100 percent symmetrical modulation, should the carrier observed on a average wattmeter or the plate, collector current meter in the rig move forward or stay still ??

my power meter on a linear amp output stays still.

the answer is that on an averaging meter the observed reading will remain steady for a modulation index of zero to one.  An index higher than one is asymmetrical modulation and will move upwards.

examples:      50% symmetrical modulation is a modulation index of 0.5

                     150% asymmetrical modulation is a modulation index of 1.5  yielding an upward movement

Depends on the type of meter, power supply regulation, final amp tuning, etc. In theory (everything perfect, including a perfect average power reading meter), no shift will occur. In the real world, a slight kick up or down is probably not a problem. Checking your signal with a scope and a spectrum analyzer will reveal any problems.

In general, I would say that downward carrier shift at 100% modulation suggests a problem that could need attention. It's a matter of degree; a small drop could be nothing, while a big drop could mean bad distortion. Many factors involved, so check it out.

It could be caused by bad things like poor voltage regulation on any of the tube elements, (both RF and modulator in linear stages) soft tubes, ALC action on riceboxes or the many causes of IMD.

Also, ever see the effects of a linear amplifier loaded too lightly or a class C stage without enough drive - both under heavy modulation?  I don't know if that qualifies as downward carrier shift, but it sure has similar distortion symptoms.

It's like having a pain somewhere and having the doctor check it out. May mean nothing or something to be fixed.

As HuzMan says, runs some tones thru, look at all the voltages for stability and see what the scope and spectrum analyzer looks like. If you haven't done this already, chances are you will be able to improve the rig with careful tests - even unrelated to the carrier shift symptom.

BTW, in my case, all of my plate modulated rigs show steady grid, screen and plate current under full modulation - and even a slight upward power swing (pos carrier shift) when hitting the audio hard. After optimization of all other parameters, this is how the rig usually acts when it is running well and tuned right.

Tom, K1JJ

A prior discussion on this very topic:

http://amfone.net/Amforum/index.php?topic=44929.0


Phil-AC0OB