Frequency Modulator; Output Amplifier; Power Supplies - GE Marquette 418793-001 Service Manual

Frequency Modulator

Output Amplifier

Power Supplies

6-10
UPPER LEVEL ASSEMBLY: Theory of Operation
The frequency modulator section phase modulates the carrier signal,
from the frequency doubler, using the low-pass filtered digital data input
from the transmit data filter. This is done using mixer U1 as a phase
modulator. The input digital data acts as a switch, forward and reverse
biasing diodes internal to the mixer. This biasing action causes phase
shifts of 180 degrees in the carrier signal. The result is a binary phase-
modulated signal at the carrier frequency.
The output amplifier section amplifies the phase-modulated carrier
signal and isolates the frequency modulator section from the antenna. A
transistor amplifier is formed using Q2 and is coupled to the antenna by
matching elements at the output of Q2. The input to the amplifier is
filtered using C1, C2, C3, C7, C8, L2, and L4 (for -001/002/003 and C3,
C7, C8, L4, and L19 for -004). This attenuates the first and third
harmonics of the VCO. The diodes in D1 provide static protection.
All voltages are referenced to RETURN which is the same potential as
battery ground. The analog and digital circuitry is powered by +3.3V and
the analog circuitry utilizes a +1.25V virtual ground for operational
amplifier reference. The loop circuitry utilizes an additional +6.0V
supply.
APEX Telemetry Transmitter
401566-166
Revision D