Operating instructions

TECHNICAL DESCRIPTION
4-9
Receiver circuits
Input switching
➤
➤
OVERLOAD
DETECTOR
0/10dB
ATTEN.
RF
DETECTOR
LIMITER
25kHz
DETECTOR
AGC
AMPLIFIER
OVERLOAD
DETECTOR
0/10dB
ATTEN.
TO
OVERLOAD
WARNING
CIRCUIT
FROM
RF GENERATOR
VIA ATTENUATOR
TO FIRST
FREQUENCY
CHANGER
VIA ATTENUATOR
TO
OVERLOAD
WARNING
CIRCUIT
TO OVERLOAD
WARNING CIRCUIT
BNC
RF OUT
’N’ TYPE
RF IN/OUT
25kHz
1V RMS
25kHz POWER LEVEL
9mV FOR +7dBm
AT ’N TYPE’ INPUT
BNC
ANTENNA
RF IN
20dB
ATTEN.
INTEGRATOR
+
-
C1411
Fig. 4-4 Input switching and broad band power meter block diagram
The RF signal from the transmitter under test enters the Service Monitor at either the BNC antenna
input or the ‘N type’ connector on the input/output switching board A11/1. A detailed block
diagram of this board is shown in Fig. 4-4, Input switching and broad band power meter block
diagram.
The ‘N type’ connector, which is also used as an output for receiver testing, has a 20 dB power
attenuator between it and the input/output switching circuits. A 10 dB attenuator pad can be
switched into the transmitter test RF path by the Service Monitor software.
Broad band power meter
A proportion of the input signal is fed directly to the broad band power meter circuits. The RF
signal is passed through a diode detector, and the resultant DC signal integrated with a similar
signal derived from a 25 kHz signal from board B2/1. As the integration is carried out using the
inverting and non-inverting inputs of an op-amp, a stable condition will result when the two inputs
are of the same value.
The output from the integrator is used as a control signal for an AGC amplifier acting on the
25 kHz signal, with the output from the AGC amplifier supplying the integrator. The loop will
therefore remain stable, with the AGC amplifier output voltage exactly equaling the voltage of the
RF input to the detector. This voltage is taken to the voltmeter ranging circuits on the audio
processor board (B1/1 or B1/2,) and then to the AF voltmeter circuit on the microprocessor board
B2/1.