Hello,

I was wondering what happens to the signal quality of the demodulated audio signal when a carrier envelope is overmodulated?

 

take a look at these ... 100 percent modulation, 105 percent and 120 percent.

 

umm what about the sine wave that comes out from demodulator? amplitude of a sine wave clips?

 

Hello,

I was wondering what happens to the signal quality of the demodulated audio signal when a carrier envelope is overmodulated?

The distortion,both harmonic & intermodulation,will increase,but strangely enough,on speech and/or program material on a normal AM system,it is not immediately obvious.

On one occasion,we had a Modulation Monitor sent in for repair & recalibration,because it was reading too high.
We sent off an exchange unit,& the local guys complained that it was faulty,too.

The Boss bumped the routine visit forward on the schedule,& on arrival there,we stuck an Oscilloscope on the RF probe,& were
confronted by massive overmodulation,with carrier cutting.
We quickly wound the input attenuator down,roughly setting it on time pips,& waited till closedown.

We found that the local Telephone Techs,who were used to +8 dBm lineup levels,had set the Transmitter input attenuators for 90% mod at this level.
Program material hits+16dBm!!
The funny thing was that this gross overmodulation was not noticeable on casual listening.

 

The funny thing was that this gross overmodulation was not noticeable on casual listening.

Let me guess. Heavy metal music station.

 

Let me guess. Heavy metal music station.

Nope! Easy listening!

 

Nope! Easy listening!

Isn't that elevator music?

 

no....easy listening is not elevator music, more middle of the road.

 

umm what about the sine wave that comes out from demodulator? amplitude of a sine wave clips?

You may or may not observe any distortion - depends on the demodulation method.

 

Over-modulation may cause higher harmonics to appear in the RF spectrum. That would be largely rejected by the bandpass filter in the receiver but would cause "splatter" noise in any adjacent channels. See this for a demo.

 

Over-modulation may cause higher harmonics to appear in the RF spectrum. That would be largely rejected by the bandpass filter in the receiver but would cause "splatter" noise in any adjacent channels. See this for a demo.

On a similar subject,Newbie "Hams" sometimes buy cheap CB-type "linear" RF amps on eBay.

More knowledgeable Amateur Operators advise them:-

"Dont use them,as they are horribly non linear!"

Their answer is usually:-

"But I used it already on 10m & got good reception reports!"

Other spectrum users are far less happy!

Of course,this isn't anything to do with overmodulation,but it demonstrates how difficult it is for a Receiver operator to discern quite severe distortion by ear.