DETAILED DESCRIPTION
2-23
the modulation. When this output is fed back to the power amplifier via the Amplitude Control
board, RF power and modulation depth will be held constant, and distortion will be practically
eliminated. This detected output voltage is also used for measurement purposes.
A low-pass filter is inserted between the detector diode and the coupled lines to prohibit RF
harmonics generated by the detector to couple back onto the main RF line. Coupler directivity
is significantly improved by very small capacitances connected across the lines. No adjustable
components are used on this board.
2.4.6
CD1238A Combiner Detector
General Description:
The function of CD1238A is to combine the 90 Hz modulated RF signal and the 150 Hz mod-
ulated RF signal from FD1236A to form the CSB and SBO RF signals to be transmitted. In
addition, detected and filtered CSB and SBO signals for measurement purposes are provided
to BNC test connectors on the front panel.
The 90 Hz and 150 Hz modulated RF input signals are of equal amplitude in phase quadra-
ture, with the 90 Hz signal lagging. A 90 degree 3 dB hybrid is used as a combiner. When two
RF signals of equal frequency and amplitude but with 90 degrees phase difference are
applied to the two input ports of such a hybrid, the signals will add in phase at one output port
but will be in reverse phase and cancel each other at the other port. This is the situation for the
carrier frequency, resulting in twice the input power at the CSB port and no power at the SBO
port. However, the sideband frequencies at the two inputs are not identical. The power of each
sideband frequency will therefore be split equally between the two outputs. The resulting
depth of modulation at the CSB output will be one half relative to the input value. At the SBO
output, the two sidebands will be in reverse phase at the instant when the LF components are
in phase. Thus true CSB and SBO signals are generated at the outputs.
A pair of directional couplers sample the CSB and SBO RF output signals for measurement
purposes. The RF samples are detected by matched diodes that are slightly forward biased.
Detector filters have short time constant to preserve the LF envelopes. A low-pass filter is
inserted between each detector diode and the coupled lines to prohibit RF harmonics gener-
ated by the detector to couple back onto the main RF line. Coupler directivity is much
improved by very small capacitors connected across the lines.
2.5
COA1207
C
Change-Over Assembly
General Description:
The COA1207C consists of one Change-Over Relay Assembly CRA1228A for the Course sig-
nals and one Changeover Relay Unit CR1222A for the Clearance signals.
The Change-Over Assembly for Course utilizes two double-throw coaxial relays to connect
the CSB and SBO output signals from either the main or standby transmitter (GPA1231A) to
the antenna system or to a dummy load.
The assembly includes attenuators and phase shifters required to obtain the correct CSB/
SBO relationship.
For the CLR Signal: Only one relay is used as the GP CLR only consists of CSB.One
CRA1228A is shown below. CR1228A contain the CSB circuitry only.
Block Diagram:
