Output Amplifier
Q21 and Q22 are complementary emitter followers and buffer the output of IC19.
The DC Offset control is buffered by IC23A and IC23C and applied via R120 and R175.
Amplitude Modulator
When the AM button is depressed, the amplitude modulator is inserted between the preamplifier
and the output amplifier. IC21 is a balanced modulator. IC24 is a differential amplifier which
amplifies the difference between the open collector outputs of IC21. The circuit around IC37
forms a 400Hz oscillator the output is passed through a Sallen and Key low pass filter IC20B.
IC20A buffers the signal on the modulation depth control VR33 and also sets the dc bias for IC21.
Sweep – Ramp Generator
The rate control VR31 determines the magnitude of the current source IC7A and Q12. This
current is steered into and out of C25 by the operational transconductance amplifier (OTA) IC8.
The voltage on C25 is buffered by IC7B to drive the stop control VR35 and the window
comparator IC10A and IC10B. The thresholds of the window comparator determine the limits of
the ramp which are 0V and +4V. Q13 and Q14 are level shifters to drive flip flop IC12A. The
output of IC12A is level shifted by R74 and R57 and clamped by D5 and D4 to be symmetrical
about 0V. When the input to IC8 is high the OTA sources current into C25; when the input is low
the OTA sinks current from C25. This loop is very similar to a function generator. If either the set
start or set stop buttons are depressed the loop is closed around the ramp generator forcing the
ramp to be held precisely at its lower or upper limit. IC10D buffers the signal on VR21 to drive the
main board directly or via the log converter.
Sweep – Log Converter
IC3D sums the output of the ramp generator with an offset current to drive the logging transistors
IC13A and IC13B. Temperature compensation of the log converter is by TDR1 which has a
temperature coefficient of +0·3%/°C. The log converter itself simply uses the exponential
relationship between Vbe and collector current. IC13 is a matched transistor array. The
right-hand transistor is the logging transistor and its emitter is driven by the input voltage via the
left-hand transistor which compensates for the logging transistor Vbe. The left hand transistor’s
collector current is defined by R16 and is held constant by IC3A; its Vbe is therefore constant and
so the input voltage at the junction of R19 and TDR1 is impressed across the Vbe of the logging
transistor. The logarithmic change in collector current of the logging transistor is converted into a
voltage by IC3B. The log converter is actually an antilog converter.
Digital Display
IC30 is a microcontroller that provides frequency and voltage measurement and displays the
results on the LCD. In frequency measurement mode IC29-A outputs the GATE signal, which is
controlled via the J (START) and K (STOP) inputs. The GATE is synchronised to the positive
edges of the input signal. Whilst GATE is high IC30 counts the internal reference clock and
IC29-B outputs the input signal divided by two, which is also counted by IC30. When GATE goes
low the frequency is calculated and displayed.
Voltage measurements are made via the A-D inputs RA0 (pin 5), RA1 (pin 6) and RA3 (pin 9).
RA0 input measures the amplitude voltage (VA). RA1 input measures the Frequency Dial voltage
(VF) which is used to display low frequency measurements. RA3 input measures the offset
voltage (VOFT).
IC31 is a DAC that outputs a correction voltage to IC3-C when frequency lock is selected. This
correction voltage is adjusted by IC30 to maintain frequency lock.
IC35 and IC38 are multiplexers that are used by IC30 to input the instrument status; IC30
updates the display with the correct settings.
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