This is the AM Transmission Section of
The Broadcast Archive

Maintained by:
Barry Mishkind - The Eclectic Engineer
4/28/07

AM Antennas

AM Transmission

AM is not "ancient modulation" ... it refers to Amplitude Modulation, where the carrier has modulation impressed on it by increasing or decreasing the carrier and sideband according to the audio.

HEISING MODULATION

Heising modulation operates in the following manner: The modulator tube, like any tube, acts like a time-varying resistance. The anode-to-cathode resistance pulls the voltage up and down at an audio rate. The large inductance of L1 allows this to happen. The collapsing or expanding field in L1 actually allows the voltage to go above the B+ rail. (But not as cleanly or far as it would with a transformer and push-pull modulator, where the voltage can be forced up.)

Another way to view this is the modulator and PA tube share current through the power supply ‘constant-current’ choke. A basic property of inductors is they try to adjust voltage to maintain constant current. While capacitors try to maintain voltage across terminals with varying current, opposite behaving inductors try to maintain constant current through terminals while allowing voltage to vary!

If the modulator current decreases, the collapsing field in the choke causes a increase in voltage and the PA tube has higher operating voltage. Current that formerly went through the modulator tube now goes through the PA tube, along with the higher voltage.

When the modulator tube draws increased current, the choke tries to oppose the change and generates a counter EMF opposing the supply voltage. In this case PA and modulator voltage is reduced, and current that was in the PA is now diverted to the modulator tube. 

So the net affect is always RF output power envelope is always equal to E x I and if one changes, the other will do so as the 1/change and thus negate any change in over all power use.

CLASS B MODULATION

In 1930 Loy Barton from the Univ of AR was the first to use a modulation transformer. Barton later went to work at RCA and championed the push- pull class B audio amplifier/ modulator to replace the class A Heising modulation system in high power transmitters.

HIGH LEVEL MODULATION

In the 1940s, .... 

In the 1970s, high speed ICs allowed the development of Pulse Width Modulation (PWM), also known as Pulse Duration Modulation (PDM). With a 70 kHz switching rate, these transmitters had a high modulation capability with clean audio.