DETAILED DESCRIPTION
2-20
ules
The GPA1232A contains the RF blocks in the GP Clearance transmitter system. It receives
RF signals and LF signals from the OS1221B oscillator and LF1223A low frequency generator
respectively. The output of the GPA1232A is a carrier sideband (CSB) signal. Detected sam-
ple of the CSB signal is available on the front panel. Other system parameters are transmitted
to the RMA1215A.
Block Diagram:
See transmitter block diagram.
Block Description:
The PC1225B Phase Control receives the RF signal from the oscillator OS1221B. The RF sig-
nal is fed into the 90Hz branch and further through buffers and amplifiers. The 150Hz branch
is inactive.
The PA1234A Power Amplifier modulates the incoming RF signal from PC1225B with a CBS
LF signal to obtain an AM-signal and amplifies it.
The AC1226B Amplitude Control generates the required CSB LF modulation signal to the
power amplifier, to keep the output RF level and amplitude modulation constant.
The FD1224A Feedback Detector provides a feedback signal for amplitude correction of the
CSB modulated RF signal.
2.4.2
PA1234A Power Amplifier
General Description :
Power amplifier PA1234A is a three stage single-ended amplifier.
The first stage works as an AM-modulator. The modulation tone (90Hz or 150Hz) is fed to the
collector of a bipolar transistor. This modulation stage is operating in class C and has imped-
ance matching network at both input and output. Nominal RF input level is approximate
20dBm.
Second stage consists of a 5W MOSFET-transistor operating in class B. The input impedance
network is matched to 50
Ω
, while the output impedance network is matched to the complex
conjugated impedance of the third stage transistor. This stage has a collector-to-gate feed-
back and a input shunt loading resistor at the transistor gate, to prevent unwanted oscillations
and keep the transistor unconditionally stable.
Third stage is a 30W MOSFET-transistor operating in class AB. Its output impedance network
consists of microstrip transmission lines and a variable capacitor to achieve maximum power
and efficiency. This stage has also feedback and shunt resistors to prevent unwanted oscilla-
tions. As gain control, both the second and third stage have potentiometers to set the operat-
ing point. A positive voltage regulator is used to keep a fixed input voltage to these gain
control circuits.
1
Power Amplifier
PA1234A
1
Amplitude Control
AC1226B
1
Phase Control
PC1225B
1
Feedback Detector
FD1224
