9
Super Sweep Function Generator
A function generator having a wide tuning range is shown in
Figure 10. The 1,000,000/1 adjustment range is
accomplished by a single variable potentiometer or by an
auxiliary sweeping signal. The CA3140 functions as a non-
inverting readout amplifier of the triangular signal developed
across the integrating capacitor network connected to the
output of the CA3080A current source.
Buffered triangular output signals are then applied to a
second CA3080 functioning as a high speed hysteresis
switch. Output from the switch is returned directly back to the
input of the CA3080A current source, thereby, completing
the positive feedback loop
The triangular output level is determined by the four 1N914
level limiting diodes of the second CA3080 and the resistor
divider network connected to Terminal No. 2 (input) of the
CA3080. These diodes establish the input trip level to this
switching stage and, therefore, indirectly determine the
amplitude of the output triangle.
Compensation for propagation delays around the entire loop
is provided by one adjustment on the input of the CA3080.
This adjustment, which provides for a constant generator
amplitude output, is most easily made while the generator is
sweeping. High frequency ramp linearity is adjusted by the
single 7pF to 60pF capacitor in the output of the CA3080A.
It must be emphasized that only the CA3080A is
characterized for maximum output linearity in the current
generator function.
Meter Driver and Buffer Amplifier
Figure 11 shows the CA3140 connected as a meter driver
and buffer amplifier. Low driving impedance is required of
the CA3080A current source to assure smooth operation of
the Frequency Adjustment Control. This low-driving
impedance requirement is easily met by using a CA3140
connected as a voltage follower. Moreover, a meter may be
placed across the input to the CA3080A to give a logarithmic
analog indication of the function generator’s frequency.
Analog frequency readout is readily accomplished by the
means described above because the output current of the
CA3080A varies approximately one decade for each 60mV
change in the applied voltage, V
ABC
(voltage between
Terminals 5 and 4 of the CA3080A of the function generator).
Therefore, six decades represent 360mV change in V
ABC
.
Now, only the reference voltage must be established to set
the lower limit on the meter. The three remaining transistors
from the CA3086 Array used in the sweep generator are
used for this reference voltage. In addition, this reference
generator arrangement tends to track ambient temperature
variations, and thus compensates for the effects of the
normal negative temperature coefficient of the CA3080A
V
ABC
terminal voltage.
Another output voltage from the reference generator is used
to insure temperature tracking of the lower end of the
Frequency Adjustment Potentiometer. A large series
resistance simulates a current source, assuring similar
temperature coefficients at both ends of the Frequency
Adjustment Control.
To calibrate this circuit, set the Frequency Adjustment
Potentiometer at its low end. Then adjust the Minimum
Frequency Calibration Control for the lowest frequency. To
establish the upper frequency limit, set the Frequency
Adjustment Potentiometer to its upper end and then adjust
the Maximum Frequency Calibration Control for the
maximum frequency. Because there is interaction among
these controls, repetition of the adjustment procedure may
be necessary. Two adjustments are used for the meter. The
meter sensitivity control sets the meter scale width of each
decade, while the meter position control adjusts the pointer
on the scale with negligible effect on the sensitivity
adjustment. Thus, the meter sensitivity adjustment control
calibrates the meter so that it deflects
1
/
6
of full scale for
each decade change in frequency.
Sine Wave Shaper
The circuit shown in Figure 12 uses a CA3140 as a voltage
follower in combination with diodes from the CA3019 Array
to convert the triangular signal from the function generator to
a sine-wave output signal having typically less than 2% THD.
The basic zero crossing slope is established by the 10kΩ
potentiometer connected between Terminals 2 and 6 of the
CA3140 and the 9.1kΩ resistor and 10kΩ potentiometer
from Terminal 2 to ground. Two break points are established
by diodes D
1
through D
4
. Positive feedback via D
5
and D
6
establishes the zero slope at the maximum and minimum
levels of the sine wave. This technique is necessary because
the voltage follower configuration approaches unity gain
rather than the zero gain required to shape the sine wave at
the two extremes.
7
6
5
4
3
2
0
OFFSET VOLTAGE SHIFT (mV)
0 500 1000 1500 2000 2500 3000 3500 4000 4500
TIME (HOURS)
1
DIFFERENTIAL DC VOLTAGE
(ACROSS TERMINALS 2 AND 3) = 0V
OUTPUT VOLTAGE = V+ / 2
T
A
= 125
o
C
FOR METAL CAN PACKAGES
DIFFERENTIAL DC VOLTAGE
(ACROSS TERMINALS 2 AND 3) = 2V
OUTPUT STAGE TOGGLED
FIGURE 9. TYPICAL INCREMENTAL OFFSET VOLTAGE
SHIFT vs OPERATING LIFE
CA3140, CA3140A
