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4.5.1.4
Symmetry Control
When the function switch is set to a symmetrical
waveform: sine, triangle or square, the symmetry con-
trol R19 is shorted out. This allows the current to flow
directly from the positive to the negative current
source.
But if the ramp or pulse is selected, SW2-B connects
the wiper of the SYMMETRY potentiometer to circuit
ground, and SW2-C opens the short across
R19.
This
increases the total resistance in the string
R17,
R19,
R21 by ten times, thus reducing the current and fre-
quency to 1/1Oth.As the SYMMETRYpotentiometer is
rotated from one end to the other, one current
4.5.1.3
Negative Current Source
The negative source is similar to the positive source,
except it supplies the current that discharges the
modulation generator loop's timing capacitor. The
negative current source receives its input at U1 pin 9
from the input buffer U1 pin 7. The amplifier output U1
pin 8 drives the two transistor bases U2 pins 5 and 8
via the dc bias shifter CR2and R23.As with the positive
current source, the emitter resistors R24 and R26
are matched and biased to the - 15V supply;
therefore, the collector currents U2 pins 6 and 9 are
equal. U2 pin 9 drives the current switch in the modu-
lation generator loop (refer to paragraph 4.5.2). The
RC network, C2 and R22, across U1 pins 9 and 10 pro-
vide loop
stabilization.
4.5.1.2
Positive Current Source
U1 pin 1 is an amplifier with a gain of -1 that drives
the positive current source U1 pin
13.
The positive current source produces a current that
charges the modulation generator
loop timing
capacitor.
The positive current source receives its
input at U1 pin
13.
The output U1 pin 14 is dc coupled
by a zener diode CR1 and resistor R13, which shifts
the dc bias, to the two bases U2 pins 11 and
14.
The
base bias U1 pins 11 and 14 controls the emitter cur-
rents U1 pins 10 and 13. Emitter resistors R14and R16
are matched and supplied by the same
+
15V supply,
thus providingequalemitter currents
and,consequently,
equal collector currents. The collector current U2 pin
12 flows through resistor R17 and the function switch,
to the negative current source. The collector U2 pin 12
connects to U1 pin 12, this provides the feedback to
stabilize the positive current
source.
The collector cur-
rent at U2 pin 15,which equals the collector current U2
pin 12, flows to the current switch in the modulation
generator loop(seeparagraph 4.5.2.1).The RCnetwork
R12 and C1, across U1 pins 12 and
13,
provide loop
stabiIization.
4-10
4.5.1.1
Input Buffer
The input buffer U1 pin 7 sums inputs from the AUX
VCG (J16) and FREO VERNIER. The buffer drives
both positive current source inverter U1 pin 2 and the
negative current source U1 pin 9. With FREO
VERNIER full cw and no AUX VCG input, the buffer
supplies - 0.546V, and - 0.014Vwith FREQVERNIER
full ccw.
4.5.1
Modulation Generator VCG System
Refer to Sweep/Mod board schematic sheet 1. The
modulation generator VCGsystem consists of an input
buffer, the positive current source, and negative cur-
rent source.
4.5
SWEEP MODULATION BOARD DETAILED
CIRCUIT DESCRIPTION
4.4.14'
Output Attenuator
Also refer to sheet 7. Each attenuator button selects
an independent voltage divider, which has
son
input
and output impedances to correctly load the amplifier
and to provide a constant
son
impedance at the
FUNC OUT terminal.
The 10dB attenuator has a 3.16/1 voltage division
ratio. The 20dB attenuator has a 10/1 voltage division
ratio,
and the 40dB stage has a 100/1 ratio. These
ratios multiply in voltage. For example if the 20dB and
40dB buttons are depressed, the voltage division ratio
is 1000/1. The attenuators add algebraically in dB,
therefore any attenuation from 10 to 70dB may be
selected in 10dB steps.
the positive portion of the signal, and the bottom half
amplifies the negative part. 030 and 031 form an ac
gain stage. An emitter follower stage is formed by 032
and 033, to provide a low
impedance
drive to the
second voltage gain stage 036 and 039. This stage
drives the parallel output emitter followers 037 and
038 on the positive
side,
and 040 and 041 on the
negative. Diodes CR23 and CR26 set thermally stable
bias for the output transistors. Networks R211, R212,
C94 and C93 bypass emitter
resistor
R208, while
R245, R246, C107 and C106 bypass R244. As fre-
quency is increased, these components decrease the
local negative feedback in the driver stage, increasing
the high frequency
gain.
Voltage regulators VR5 and
VR6 have external current limiting circuitry set to limit
at about 220 mA to prevent damage in the event of a
shorted transistor. When the offset button is depressed,
offset current is injected directly into both nodes in
'
proportion to the feedback resistor values. The ampli-
fier responds exactly as described above for a dc input.
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