14-6
Block Diagram Theory of Operation
Block Diagram 1
Tomeasure this signal it is necessary to “tee” the connection so that the dc control voltage
isalways available to the A11 Receiver Mixer assembly from the A16 Receiver assembly.
It is then possible to measure the dc voltages with an external voltmeter or using a
blocking capacitor, a spectrum analyzer can be connected to view the RF signal from the
mixer.Failure to use a blocking capacitor will cause the switch in the A11 Receiver Mixer
assembly to be indeterminate and accurate measurements cannot be made. Although the
A11 Receiver Mixer assembly does not contain any diagnostic test points, it is used
extensively during diagnostics to route RF test signals into the A16 Receiver and A18
Spectrum Analyzer assemblies.
A16 Receiver
The A16 Receiver assembly input signal from the A11 Receiver Mixer assembly is either
114.3MHz or 614.3 MHz. If the signal is 614.3 MHzit is immediately downconverted to
an IF of 114.3 MHz by a 500 MHz reference signal from A15 Reference assembly.
This signal at 114.3 MHz is then filtered and split. It is routed to the A18 Spectrum
Analyzer assembly and to another mixer where it is further downconverted for
demodulation.The LO for the next downconversion is 125 MHz which is derived from the
same500 MHz reference signal that was used earlier. The signal is now at 10.7 MHz ±50
kHz.This signal is a duplicate of the input signal except the frequency has been translated.
It still contains the pulse and modulation information. The primary signal path for this
signalis to the A9 Global Test and Demod assembly where frequency and phase accuracy
are measured.
The 10.7 MHz signal also drives an FM discriminator and pulse detector that demodulate
the signal. The demodulated waveforms are then routed to the front panel, A3 Audio
Analyzer 1 assembly, or A9 Global Test and Demod assembly depending on the switch
settings. These connections are detailed in Block Diagram 1.
The A16 Receiver assembly has extensive diagnostics which test the internal filters,
switches,and demodulators. The most critical signals from the A16 Receiver assembly are
the 114.3 MHz signal for the A18 Spectrum Analyzer assembly and the 10.7 MHz to the
A9 Global Test and Demod assembly. The signal to the A9 Global Test and Demod
assembly should be near +3 dBm for good signal to noise ratio (and no compression) in
the A9 Global Test and Demod assembly.
A9 Global Test and Demod
The first function of the A9 Global Test and Demod assembly is to downconvert the 10.7
MHzsignal from the receiver to 700 kHz ±50 kHz. This signal is then routed to a counter
on the A19 Measurement assembly. The frequency of this signal is measured and is used
to calculate the RF input signal frequency. Measuring an RF input signal frequency is a
good method of determining if the frequency translation hardware up to the A9 Global
Test and Demod assembly is functioning properly.