Circuit Description - Wavetek 188 Manual

This section describes the functions of major circuit
elements and their relationships to one another as
shown in figure 4-1, functional block diagram. The fol-
lowing sections in this manual provide more detailed
information for maintaining the instrument.
As shown in figure 4-1, the VCG (Voltage Control of
Generator) sums voltage inputs from the frequency
dial and the VCG IN connector. This sum voltage con-
trols the magnitude of a complementary current
source and current sink. This current varies linearily
from approximately 2 mA to 2 yA over a 1000:1 (4.0 to
.004) range or logarithmically from approximately
2 ma to 0.2 ua over a 10,000:1 (4.0 to .0004) range of
each frequency multiplier. The VCG also controls the
trigger baseline compensation circuit, which consists
of another current sink at twice the current
magnitude.
The diode gate, controlled by the comparator output,
connects either the current source or the current sink
to the timing capacitor selected by the frequency
multiplier. When the current source is switched in, the
charge on the timing capacitor will rise linearily, pro-
ducing the positive-going triangle slope. Likewise, the
current sink produces the negative-going triangle
Slope.
The triangle amplifier is a unity gain amplifier whose
output is fed to the comparator and to the output cir-
cuits. The comparator operates as a window detector
with limit points set to the triangle peaks. The +2V
output is sent back to the diode gate and to the output
circuits. When the output is +2V, the triangle is
positive-going until the + 1.25V limit is reached and
the comparator output switches to — 2V. When the
output is — 2V, the triangle is negative-going until the
— 1.25V limit is reached and the comparator output
switches back to + 2V, repeating the process. In this
manner, the basic function generator loop, the bold
path in figure 4-1, produces simultaneous generation
of triangle and square waves at the same frequency.
The output frequency is determined by the magnitude
SECTION 4
CIRCUIT DESCRIPTION
of the timing capacitor selected by the frequency
multiplier switches and by the magnitude of the cur-
rents supplied to and removed from it. Since the cur-
rents are linearily proportional to the sum of the VCG
inputs, so will be the output frequency.
To extend the lower frequency capability of the
generator, a capacitance multiplier circuit divides
VCG currents by 10 (effectively multiplying the timing
capacitor by 10) for each of the lower 3 multiplier
ranges.
The TTL square from the comparator is buffered and
sent to the SYNC OUT TTL connector. The other side
is sent to the trigger flip-flop and to a level shifter to
produce the +2V bipolar square for the diode gate
and the square shaper circuits. The square shaper
converts the square into a current signal and applies it
to the |, FUNCTION switch. The buffered triangle is
applied to the ^; FUNCTION switch and to the sine
converter input. The sine converter, using the
nonlinear characteristics of its diodes, converts the
triangle into a sinusoidal current for the Лу FUNC-
TION switch.
The selected function is sent to the preamplifier,
where it is inverted and buffered. The preamplifier out-
put goes to the output amplifier through the
AMPLITUDE control where it is summed with offset
voltage from the DC OFFSET control. Here, waveform
and offset are inverted and amplified to а + 10V peak
signal which can drive a 500 termination from a 500
source impedance. The output amplifier drives the
500 OUT HI connector and a resistor divider produc-
ing the 500 OUT LO output.
Noncontinuous modes of operation (trigger and gate)
result from allowing or preventing the VCG current
source from charging the timing capacitor. Whenever
the trigger Пір-Нор output is low, each of the two trig-
ger diodes conduct a current I, sourcing 2I to the
baseline compensation circuit. This removes the cur-
rent I from the VCG current source and forces a OV
baseline at the triangle amplifier input.
4-1