This is the AM Transmission Section of
The Broadcast Archive
Barry Mishkind - The Eclectic Engineer
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 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
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