Operating instructions

TECHNICAL DESCRIPTION
4-10
Overload detection
An overload detection circuit is activated if power in excess of 1.0 W is applied to the BNC
antenna connector or to the BNC RF output connector. The overload detect lines are activated and
trigger the overload warning message from the microprocessor. A temperature sensor is in
physical contact with the 20 dB pad on the ‘N type’ connector. This triggers the overload circuit
in the event of excessive power being applied to that connector.
Switched attenuator
A switched attenuator, with one 40 dB pad and one 20 dB pad, is situated between the input
switching board A11/1 and the 1st frequency changer/mixer board A2. This provides attenuator
values of 0 dB, 20 dB, 40 dB and 60 dB.
IF circuits
First frequency changer/mixer
The RF signal from the switched attenuator enters the 1st frequency changer/mixer board A2 and
passes through an overload prevention circuit. This limits any input signal to approximately 0.7 V
peak. The signal then passes either directly, or through a 20 dB amplifier, to the 1st mixer.
First local oscillator
The mixer is also supplied with a signal from the 1st local oscillator board A8/1. This oscillator is
a fractional ‘N’ controlled oscillator, which can be made to run at any frequency between 1.36 and
2.36 GHz, in increments of 1 Hz. The actual frequency is determined by the control system of the
monitor. If the transmitter test frequency is known, and has been entered into the monitor as a ‘Tx
FREQ’ using the data input keys, the oscillator will be made to run at a frequency 1.3593 GHz
above this value.
By mixing this signal with that from the input circuits, the output from the mixer will contain a
signal of 1.3593 GHz.
When the ‘Auto Tune’ function is active, the variable oscillator is made to sweep through its
operating range. The sweep rate is slowed when a signal is detected at the demodulators, and the
oscillator brought into lock.
Filtering in the mixer output removes the upper frequency components of the mixing process.
Second frequency changer/mixer
The signal from the 1st mixer is further reduced in frequency by a 2nd frequency changing mixer
to produce an output of 79.3 MHz. This is on the 2nd and 3rd mixer board A3. The signal
arriving at board A3 is 1st passed through a 1.3593 GHz band-pass filter and applied to the input
of the 2nd mixer.
Second local oscillator
The local oscillator for the 2nd mixer produces a 1.280 GHz signal on the 2nd and 3rd oscillator
board A9/1. This oscillator is a phase locked loop device, locked to the 10 MHz reference
oscillator.
The output from the 2nd mixer is passed through a 79.3 MHz band-pass filter to remove the
unwanted products of the mixing process, then given 20 dB amplification.
The 2nd IF signal is split and fed to two independent mixers.
The secondary feed supplies the 90 MHz swept local oscillator board A10. This operates as a
swept oscillator when the spectrum analyzer
Look and Listen function is active, as described later.
It provides a locked 90 MHz signal for the 3rd mixer at other times.
The primary path is through a band-pass filter with 5 MHz bandwidth, to the main 90 MHz 3rd
mixer. This mixer is on the 2nd and 3rd mixer board A3.