Tektronix PG 508, Инструкция по эксплуатации

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TM 11-6625-2980-14
variations and dc offset currents are summed at the
collectors of Q945 and 0954, the output pulse high level
changes to maximum voltage and the low level remains
at OV.
It follows that any positive increase in either the high
or low level control voltages causes an increase in the
dc offset current. A decrease in either control voltage
causes a decrease in offset current. However, signal
current variations respond only to differences between
these control voltages. The larger the voltage
difference, the larger the signal current amplitude into
the output amplifier
Since the dynamic range of the pulse generator output
is 4 or - 20 V, and the maximum amplitude is only 20 V
peak to peak, clamping circuits are provided to prevent
the difference between the high and low level control
voltages from exc2.6 V, which corresponds to
maximum output. A clamping circuit also prevents the
high level from becoming less than the low level control
voltage.
The level control unity gain amplifiers U780A and
U780B contain a precision diode clamp composed of
CR782, CR790, R780, and R782. Since the feedback
for U780A is taken from the cathode of CR790, the
voltage at the junction of CR782 and CR790 is equal to
the low level control voltage plus one diode drop
(CR790). If the high level attempts to decrease below
the low level or attempts to increase above the high
level control voltage, CR782 conducts clamping the high
level at the low level voltage. Current limiting for
U780B, during clamping, is provided by R780.
U800A is also configured as a precision clamping
circuit. The output at pin 1 is equal to the low level
control voltage plus 2.6 V plus one diode drop.
Therefore, if the high level voltage attempts to exceed
the low level plus 2.6 volts, or the low level is decreased
more than 2.6V below the high level, CR805 conducts
and the high level is clamped at the low level control
voltage plus 2.6 volts. The low level control always
overrides the high level control. Current limiting for
U780B, during clamping, is provided by R780.
Output Amplifier
The positive and negative dc voltages for this
amplifier are provided by dual power supplies which
track the high and low level control voltages. These
tracking supplies ensure that the positive potential is at
least 6 V above the output pulse high level and the
negative potential is at least 6 volts below the output
pulse low level. This arrangement enables the pulse
generator to provide a 40 V dynamic range, with a 20 V
peak to peak maximum output pulse, while maintaining
minimum power dissipation and voltage requirements
for the amplifier transistors. The range of the positive
tracking supply is +6 to +26 V, with a -6 to -26 V range
for the negative supply.
The output circuit functions as a linear current driven
operational amplifier with a closed loop transresistance
gain of 2 V/mA. Negative feedback through R1042 to
the input node (collectors of Q945 and Q954) causes a
virtual ground at the input. A 10 mA peak to peak input
signal creates a 20 V peak to peak output pulse (open
circuit).
Since the output amplifier is a complementary circuit,
only the operation of one side will be explained.
For a positive-going output pulse, current is driven
into the input node. This action causes an increase in
base current to Q975 which decreases the base drive to
Q980. A cascode circuit is formed by 0980 and Q990,
with R985 and L980 serving as a constant current
source to the common collector-emitter connection.
Therefore, when the base drive to Q980 is decreased,
the current in Q990 increases. (The constant current
source supplies approximately 60 mA which, with no
signal, is equally divided between 0980 and 0990.)
Another constant current source, Q1000, causes a 2.0 V
drop across R1002. The emitter follower, Q995, buffers
this potential and provides a voltage source for the base
of the cascode transistor, Q990. An increasing current
through Q990 increases the base current from the
output parallel emitter followers, 01010 and Q1015. The
emitters of these transistors drive the output positive
through a network of resistors and capacitors.
Components R1020, R1024, and C1024, in this network,
are adjustable to provide an internal resistive 50 n
termination for the output. Network C1048 and R1048,
with potentiometer R1050, provides transient response
peaking for the amplifier.
Equivalent biasing for the complementary output
emitter followers is provided by CR990, R990, and
CR992. With zero output, the quiescent current in each
output follower is approximately 20 mA. The output
potential is available at the rear interface connector, pin
25A, for monitoring the output amplitude.
Power Supply
The +11.5 Vdc from the mainframe provides the raw
supply voltage for the series reg5.2 Vdc supply.
The precision voltage regulator, U1210, includes a
temperature compensated voltage reference supply at
pin 6. The +5.2 V potentiometer, R1210, with the
voltage divider R1209 and R1211 provides the
reference input to the non-inverting input, pin 5. The
output voltage is applied to the inverting input, pin 4,
through R1217. Sensing differences in the two input
potentials, the , regulator amplifier provides base current
drive from pin 10
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