High-level AM transmitters (Q)

I recently had difference in opinion to what this term "High-level" means.

I believe that the term refer to the high-level of modulation power required to achieve 100% modulation because modulation occur in the final stage where a power class C amp is in place. Modulation power required may equal RF power in some cases.

The other side of the argument was that the person said that it has nothing to do with the modulation power level but only refer to the point where modulation occur, hence in the final stage.

Books I looked at are also not very clear on this.

Which is correct?

Probably you are both right.

Most systems apply modulation at the final RF amplifier stage because it is most economical in terms of RF power efficiency. Technically, there is no reason why the modulation shouldn't be applied at earlier stages as long as subsequent amplifiers have linear characteristics.

I have always understood the term to mean 'at the final stage' though and this of course is also the place where modulation power is highest.

Brian.

Back in WWII the 100 W HF transmitters used by the US military had modulation transformers with two secondaries. One was in series with the plate power and the other was in series with the screen grid power.

The other alternative of low level is to AM one of the previous stages and use linear amplifiers after. This produces lower efficiency, but lower cost.

The term for this is Low or meduim-level AM.

This is what one reference book say about high-level AM

"With high-level transmitters, the power of the modulating signal must be considerably higher than is neccessary with low-level transmitters...the carrier is at full power where the modulation occur and.. requires a high-amplitude modulating signal to produce 100% modulation"