Table of Contents
Advertisement
quencies. Figure 4-3 is the simplified diagram showing cur-
rent -divider· operation. By reducing integration current
precisely. by a factor of 10 while holding triangle wave am-
plitude constant, it is possible to extend the lower frequency
range by a factor of 10 with fixed capacitance C. Since
points A and 8 are at the equipotential points, constant
current output I can be divided by resistance ratio of R and
9R. Then, integration current of capacitor C is reduced to
0.1 I. The lower current extends the frequency range of the
function generator by a factor of 1
O~
The same theory is
applied to extend the frequency range by a factor of 100.
4.3
WAVEFORM OUTPUT
The inverted output of the hysteresis switch is fed to the
TTL buffer amplifier and also the square wave shaper (fig-
ure 4-1 ). The square wave shaper consists. of a shaping cir-
cuit which limits the output swing to ±1.25 volts.
The output signal from the triangle buffer amplifier is applied
to the sine converter, which uses a diode-resistor network
with nonlinear sections to shape a sine wave.
The sine, triangle or square waveform isfed to the summing
amplifier through the waveform selector switch. The output
of summing amplifier is fed through the amplitude control
to the output amplifier. The output amplifier is an inverting
amplifier whose output is capable of driving 10V p-p into
selected load impedance.
SQUARE
WAVE
+I
-I
Figure 4-3. Current Divider
TRIANGLE
B
WAVE
4.4
OUTPUT ATTENUATOR AND PROTECTION
CIRCUIT
The signal from the output amplifier is routed through an
attenuation network for step attenuation.
If
the optional
protection circuit is installed, the instrument output circuits
are protected from input voltages by a zener and fuse net-
work protecting both the signal and ground sides of the
output BNC's. Two source impedances, 50 and 600 ohms,
are provided at the outputs.
4-2
4.5
PHASE LOCK LOOP
A phase lock loop is shown in figure 4-4. The phase detector
output is proportional to the difference in frequency between
the two inputs. This difference signal is used to control a
voltage controlled oscillator (VCO). The VCO output signal
is fed back to the phase detector, where
th~
VCO frequency
is compared to the reference input to the detector. This ref-
erence input is, for example, a constant 1.0000 kHz signal.
The loop becomes stable when the VCO frequency equals
the reference frequency, at which time the detector differ-
ence signal becomes zero.
REFERENCE
PHASE
DETECTOR
~----
&FILTER
Figure
4-4. Phase lock loop
4.6
SYNTHESIZER LOOP
VCO
The reference frequency for the synthesizer is provided by
an internal 4 MHz crystal or by an external 1 MHz signal
(figure 4-5). Either source provides a 1 MHz REF OUT signal
which is also reduced to 1.0000 kHz for the reference input
to the phase detector.
The phase detector difference signal is a pulse train whose
spacing and magnitude reflect the phase difference. The low
pass filter ( LPF) shown in the synthesizer loop converts this
pulse train to a voltage inversely proportional to the phase
difference. Noise, particularly 1 kHz, is filtered by the 400 Hz
cutoff filter. The de voltage controls the VCO frequency
using a varactor diode as the tunable element.
The programmable divider in the loop
(7
P) is controlled by
the front panel digital switch; for any division of frequency
programmed, the loop rapidly drives the phase detector's
other input to 1.0000 kHz. For example, if divide by 16000
is programmed, an imbalance is initially set up and the VCO
frequency is increased such that f vco-=- 16000
=
1.0000 kHz,
which returns the loop to a stable condition. (The VCO fre-
quency (fvco) was 16 MHz.)
The VCO has a tunable range of only
3:
1, hence the VCO
range has been limited to 8 to 20 MHz. When below 8 MHz
from the VCO would
be
required,a prescaler is placed in the
loop, which is also controlled by the digital switch, to keep
the division factor large, which in turn keeps VCO frequency
greater than 8 MHz. The prescaler output is a 1 M to 20 MHz
signal. The output of the prescaler is divided by 10 before
the frequency rangingcircuitto give the frequency equivalent
to the digital switch programmed value. The resulting fre-
Hide quick links:
Advertisement
Table of Contents
