Amplitude Modulation Receiver - Wavetek 193 Instruction Manual

4-14
Figure 4·6. Amplitude Modulation Receiver
AMPLITUDE MODULATOR
U7 PIN 14
R103
U8 PIN 8
U8 PIN 7
U7 PIN 2
REF
VOLTAGE
4.5.10 Amplitude Modulation Receiver
Refer to sweep/mod board schematic sheet 3 and
figure 4-6. The amplitude modulation receiver con-
verts the differential current from the amplitude
modulator to a single-ended output signal, which
drives the output amplifier of the main generator. The
receiver consists of two amplifiers (a bridge amplifier
and output amplifier) and a resistor bridge.
To best understand this circuit refer to the simplified
schematic figure 4-6. Basically, resistors R92, R93,
R101, and R103 form a resistor bridge. When the cur-
rents 16and 17from the modulator outputs U7 pins 14
and 2 are equal, the bridge is balanced and the output
E2 is OV.
The amplitude modulator differentially varies the cur-
rents 16and 17.For example, when 17increases and 16
decreases, both an equal but opposite amount, cur-
rent 12increases, while current 13remains unchanged.
This increases the voltage E1. As a result, current 14
increases, but the modulator demands less current 16,
therefore, the current 15flowing through R103 equals
the difference between 16 and 17. Thus, E2 swings
negative.
amplitude modulator U7 is a wide-band four-quadrant
multiplier that is configured as an amplitude
modulator. The amplitude modulator produces an out-
put current that is the linear product of the CARRIER
SIGNAL U7 pin 9 and MODULATOR DRIVE U7 pin 8.
The CARRIER SIGNAL, originating at the main genera-
tor preamplifier (FU NC:AM only), drives the non-
inverting X input U7 pin 9 with a fixed ± 0.2V sine wave.
Resistors R73, R74, R75, and R76 bias the inverting X
input U7 pin 12 at OVdc, this provides the reference
voltage for the CARRIER SIGNAL.
The MODULATION DRIVE signal from the amplitude
modulation level shifter, controls the inverting
Y
input.
This signal level depends upon the type of AM
selected: double-sideband modulation - 2.5 to
- 5.0V (max) and suppressed carrier modulation 0 to
- 5.0V (max). The MODULATION DRIVE signal is
referenced to - 2.5V at the non-inverting Y input U7
pin 4.
The modulator outputs U7 pins 2 and 14 each receive
4 ma current from the amplitude modulation receiver.
The modulator differentially varies these currents up
to ±2
mao