In the design
of an AM transmitter there are two ways to go:
(1)
The low
level generation of AM (DSB + Carrier) and the progressive amplification
of that RF signal with the final stage being a Linear RF amplifier--Class
AB.
In the case of
the low level modulation approach, one could use either a 2 quadrant or
4 quadrant multiplier as the modulator.
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The
difference being: with the 2 quadrant multiplier, negative modulation of
greater than 100% causes severe distortion as well as interference on adjacent
bands. This is due to the carrier being cut off when the 2 quadrant multiplier
cannot furnish any output for negative values of the modulating signals,
hence the RF output becomes a pulsed spectrum. |
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If,
however, a 4 quadrant multiplier is used, negative modulation starts to
appear as a double sideband suppressed carrier--or in this case, reduced
carrier. |
(2) The
second method is the progressive amplification of only the Carrier Wave
with the output stage being, a more efficient, Class C (non-linear) RF
amplifier; the modulation is introduced as a very high level audio signal
at the final stage --more precisely, the positive plate supply of the RF
"Final" Amplifier is made to vary as the modulation audio input signal.
The High Level
modulation cannot handle negative modulation of greater than 100%. As with
the 2 quadrant multiplier in the first approach, the carrier is cut off
during negative peaks that exceed 100% negative modulation.
Most commercial
AM and FM transmitter output stages--called "Finals"--use Class "C" amplifiers.
Other transmitters,
like Television (visual), SSB, etc., use "Linear Amplifiers," Class AB1
or AB2, which are a combination of Class A and Class B (both
being much less efficient than the Class C amplifier).
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