The AM Forum

Unless I have misread what I have been running into, I believe I have come to understand that:
o  an RF carrier, amplitude modulated, will produce an AM signal
o  an RF carrier, frequency modulated, will produce an FM signal
o  an AM transmitter, unmodulated, will produce a simple RF carrier

What I would like to know then is, does it stand to reason that:
o  an FM transmitter, unmodulated, will produce a simple RF carrier

If this is true, is that carrier simple/pure enough to be used as an exciter for an AM modulator (example, using an unmodulated Yaesu FT-8900 to generate a signal to amplitude modulate on 10/6/2m)?

Should work with no problem - any RF source has the same output, until you start adding audio or something else to it.

73  Don, W2DL

Whoa there good buddies.
I ran across a nasty situation by using my TS-50 as an EXCITER for a great big Class C amplifier. I had it in the AM mode and even though there was no microphone connected and the gain control all the way down, it made weird things happen in the PA grid readings and there was some audible crap in the RF carrier of this amplifier. The slightest amount of hum or noise in the carrier was not appreciated by this amplifier. It was a 4CX1500A with the bias set for class C operation.
It would be best to use the CW position of the 'EXCITER'
The same problem might happen from an "exciter" using a transceiver in the FM mode. The CW position of the exciter removes any possiblility of modulation to the RF input of the class C amplifier.
 
Tom Vu or some of the other techies might fill in more input here.
G'day
Fred MOP radio

I use my Ten-Tec Paragon II as the exciter for my Class E  AM transmitter all the time.  It provides about 20W of RF.  No problem.  Works great.    A matching network inside the Class E presents 1:1 to the Paragon II.  A relay sequencer brings things up and down in a sensible order.    I run the Paragon in AM so I can listen through it too, but the mic gain is all the way down. 

Jon

 

The problem with using an 'FM' exciter for AM operation is that FM components of the signal are not stripped off in a Class C amplifier, they are passed right thru.  That may or may not be a problem - if the exciter has good phase noise performance and very low residual FM it shouldn't be.  Because most of these rigs have bandpass audio filters, you don't hear any low frequency FM that gets thru - and that may be noticible with an AM receiver with good audio response.

If the rig has a 'CW' mode I'd use that instead of the FM mode.  Either way, try it out and see.

73 John

If there is residual FM hum on the carrier, it will show up in an AM receiver on the receive side as hum that varies with selective fading.  When the signal peaks, the hum is at a minimum; when the signal fades, the hum is heard.

You generally don't hear this hum at the transmit side, because your receiver is too wide to slope-detect it.  But on the receive side, selective fading produces narrow peaks and valleys in the passband, and this turns slight FM into slight AM, making the hum audible.