Class C and AM - Are they compatible?

[jamesinnewcastle]

OK - But the output isn't exactly a classic 'modulated' AM signal, but it's amplitude does change in accordance with the modulation.

Now if that signal is applied to a tuned load would that load not 'ring' in such a way that the classic AM signal is restored?

There is a condition. As the Class C transfer curve does not let the modulation drop to zero the peak to peak of the modulating signal must not cause 100% AM modulation - I think that there is a term for this but I can't remember. (You can see some parts of the envelope that have been clipped in the example).

How does that stack up? I think I've argued myself in a circle!!

James

[jamesinnewcastle]

In looking for any guidance on this whole subject what I find is lots of people talking about how 'kicking' the tuned circuit brings about reformed sine waves and how the harmonics can be reduced. But no-one addresses or shows how the AM envelope is maintained.

Equally, discussions about Class B talk about 180 degree conduction and Class C being about less - BUT in the circuits shown I don't see any active bias present in the Class C examples (well just one, and that was by way of explanation!). Class C amps just seem to use the 'natural' bias that the amplifying devices have, ie transistors 0.6V needing to be applied before conduction starts. Sort of pseudo Class C or Class C by default.

James

[wr8y]

You aren't far from the truth! If you REALLY want to understand all of this, you have some reading to do.

I suggest going to eBay and picking up a copy of "The ARRL Radio Amateur's Handbook" or Bill Orr's Radio Handbook (a better choice, IMO - but out of print as he's gone!) Don't worry about getting THIS YEAR's edition. You will save a lot of money if you buy one a few years old.

[bertus]

Hello,

The 1959 edition of the radio handbook can be downloaded free:
http://www.pmillett.com/Books/orr_radio.pdf

This is a link from this page:
http://www.pmillett.com/tecnical_books_online.htm

Greetings,
Bertus

[KL7AJ]

Originally Posted by bertus

Hello,

The 1959 edition of the radio handbook can be downloaded free:
http://www.pmillett.com/Books/orr_radio.pdf

This is a link from this page:
http://www.pmillett.com/tecnical_books_online.htm

Greetings,
Bertus

Yep, those are all classics. Also, R..L. Shrader's Electronic Comminucations is very good.


I might add that in conventional high level A.M., you don't even consider any possible linear function of the final amplifier. The amplifier is always operated in a fully saturated state...that is, the r.f. drive signal is high enough that any INCREASE in drive causes no increase in output power. Any "identity"of the R.F. driving signal is totally lost....it is merely a "clock".

A good high-efficiency A.M. broadcast transmitter runs close to 90% efficiency in the final stage, which means the grid is only conducting for 10% of the cycle...but during that 10% it's driven HARD...approaching a true switching mode.

In a properly operating A.M. transmitter, the R.F. output power is always the exact square of the applied plate voltage.


Eric

[jamesinnewcastle]

Originally Posted by KL7AJ

I might add that in conventional high level A.M., you don't even consider any possible linear function of the final amplifier. The amplifier is always operated in a fully saturated state...that is, the r.f. drive signal is high enough that any INCREASE in drive causes no increase in output power. Any "identity"of the R.F. driving signal is totally lost....it is merely a "clock".


Eric

Hmmm, where does the AM part come into play? How does the carrier level rise and fall in order to achieve amplitude modulation? Your description gives the impression of a screaming transmitter which has forgotten that it is supposed to modulate - This would be great for morse-code, but it doesn't seem to be amplitude controlled in any way?


I've downloaded Orrs books, thanks guys, and there is a lot of stuff in there to read, perhaps the bits that answer my questions - lots on AM and some other great stuff as well.


James

[KL7AJ]

Originally Posted by jamesinnewcastle

Hmmm, where does the AM part come into play? How does the carrier level rise and fall in order to achieve amplitude modulation? Your description gives the impression of a screaming transmitter which has forgotten that it is supposed to modulate - This would be great for morse-code, but it doesn't seem to be amplitude controlled in any way?


I've downloaded Orrs books, thanks guys, and there is a lot of stuff in there to read, perhaps the bits that answer my questions - lots on AM and some other great stuff as well.


James

Hi James:

You modulate the D.C. supply voltage to the final amplifier. This is what gives it the square law multiplication.

By the way, all the modulation power is put into the sidebands. If you have a very narrow passband filter centered precisely at the carrier frequency, the power level stays the same. (The FCC has very stringent rules on "carrier shift", which require the average carrier power to remain within a couple of percent of licensed power with or without modulation)

eric