Operating instructions

OPERATING MANUAL 2945A
46882-311D 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 enters the monitor at either the BNC antenna input or the
‘N Type’ connector on the input/output switching board. A detailed block diagram of the input
switching 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, 20 dB between it and the input/output switching circuits.
The RF signal, from the transmitter under test, can be within the frequency range of 100 kHz to
1.05 GHz. A 10 dB attenuator pad can be switched into the transmitter test RF path by the
instrument software.
Broad band power meter
A proportion of the 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. As the integration is carried out using the inverting and
non-inverting inputs of an operational amplifier, 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 equalling the voltage of the
RF input to the detector. This voltage is taken to the voltmeter ranging circuits on the AF Analyzer
Board B1 and then to the AF voltmeter circuit on the microprocessor board.