Circuit Description; Functional Block Diagram Analysis - Wavetek 182A Instruction Manual

4.1

FUNCTIONAL BLOCK DIAGRAM ANALYSIS

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 sums voltage inputs
from the frequency dial and the VCG IN connector.
This sum voltage controls the magnitude of a comple­
mentary current source and current sink. This current
varies linearily from approximately 2 mA to 2 pA. and
over the 1000:1 (4.0 to .004) range of each frequency
multiplier. The VCG also controls the trigger baseline
compensation circuit, which consists of another cur­
rent 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
+
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
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
ranges.
The TIL square from one side of the comparator is
buffered and sent to the SYNC OUT TIL 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
The buffered triangle is applied to the 'V FUNCTION
switch and to the sine converter input. The sine con­
verter, using the nonlinear characteristics of its
diodes, converts the triangle into a sinusoidal current
for the
The selected function is sent to the preamplifier,
where it is inverted and buffered. The preamplifier out­
put
AMPLITUDE control where it is summed with offset
voltage from the DC OFFSET control. Here, waveform
and offset are inverted and amplified to a 10V peak
signal which can drive a 500 termination from a 500
source impedance. The output amplifier drives the
son OUT HI connector and a resistor divider produc­
ing the son 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 flip-flop output is low, each of the two trig­
ger diodes conduct a current I, sourcing 21 to the
+ 2V, the triangle is baseline compensation circuit. This removes the cur­
rent I from the VCG current source and forces a OV
baseline at the triangle amplifier input.
When the CONT switch is released, trigger logic is in­
hibited from passing any trigger signals and the trig­
ger flip-flop output is held high. This prevents the trig­
ger diodes from conducting and the generator loop
operates continuously.
When the CONT switch is pressed, the generator loop
is held at the OV baseline. Pressing the TRIG/GATE
TRIG/GATE switch puts the instrument in triggered
mode and any external or manual trigger signals at
the trigger logic input will be transformed into a nar­
row pulse corresponding to the low-to-high transition
of the trigger input. This pulse sets the trigger flip-flop
high and allows the generator loop to run. When the
triangle negative peak is reached, the comparator
low-ta-high transition clocks the trigger flip-flop low
SECTION

CIRCUIT DESCRIPTION

rtJ
rv
F UNCTION switch.
goes to the output amplifier
4
3 multiplier
FUNCTION switch.
through the
4-1