Operating instructions

TECHNICAL DESCRIPTION
4-10 46882-311D
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, in physical
contact with the 20 dB pad on the ‘N type’ connector, triggers the overload circuit in the event of
excessive power being applied to that connector for a period long enough to cause an excessive
temperature rise
Switched attenuator
A switched attenuator with one 40 dB pad, one 20 dB pad or 0 dB attenuation, selectable in any
combination and controlled by the instrument software, is situated between the input switching
board A11/1 and the first frequency changer/mixer board A2.
IF circuits
First frequency changer/mixer
The RF signal from the switched attenuator, enters the first frequency changer/mixer board A2 and
passes through an overload prevention circuit, which limits any input signal to approximately 0.7V
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 first 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.
Where the required frequency of the monitor receiver circuits is to be established by the ‘Auto
Tune’ function, the variable oscillator is made to sweep through its operating range by the
instrument's software. When a signal is detected at the demodulators, the sweep rate is slowed and
locked.
Filtering in the mixer output removes the upper frequency components of the mixing process.
Second frequency changer/mixer
The signal from the first mixer is further reduced in frequency by a second frequency changing
mixer which has an output of 79.3 MHz. This is on the second and third mixer board A3. The
signal arriving at board A3 is first passed through a 1.3593 GHz band-pass filter and is then
applied to the input of the second mixer.
Second local oscillator
The local oscillator signal for the second mixer has a frequency of 1.280 GHz and is generated on
the second and third oscillator board A9/1. This oscillator is a phase locked loop device, locked to
the instrument's 10 MHz reference oscillator.
The output from the second mixer is passed through a 79.3 MHz band-pass filter, to remove the
unwanted products of the mixing process, before being given 20 dB amplification.
The 79.3 MHz 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 third 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 second and third mixer board A3.