Tektronix Type 82 PLUG-IN Instruction Manual page 19

Circuit Description—Type 82
At low frequencies the dividers are resistive because the
impedance of the capacitors is high and their effect in the
circuit is negligible. As the frequency of the input signal
increases, however, the impedance of the capacitors de-
creases and their effect in the circuit becomes more pro-
For high-frequency signals the impedance of the capacitors
is low, in comparison to the resistance of the circuit, and the
attenuators become capacitive voltage dividers. For these
frequencies, the attenuation factor is similar to the resistance
case, except that the capacitive reactances are the domi-
nant factors. A variable capacitor in each attenuator, such
as Clil in the X50 attenuator (see Fig. 3-1}, provides a
method for adjusting the capacitance inverse ratios equal to
the resistance ratios.
The variable capacitor across each attenuator, such as
C111B in the X50 attenuator (see Fig. 3-1) provides a
means of adjusting the input capacitance of the attenuator
to a "standard" value of about 15 picofards by using a
15-pf Input Time Constant Standardizer. A similar method
is provided to standardize the input capacitance when the
V/CM switch is in the .1 or "straight-thru" position.
In addition to providing the same input capacitance, the
resistance values in the attenuators are chosen to provide an
input resistance of 1 megohm for each setting of the V/CM
switch. Thus, when the P6008 Probe is connected to the In-
put connector, the probe will work into the same rc time
constant regardless of the setting of the V/CM switch.
A few attenuator dividers contain spiking networks to aid
in preserving the risetime of the system when these attenua-
tors are switched into the circuit.
Input Cathode Follower
The Input CF (Cathode Follower) stage V133 employ: a
nuvistor which is a sub-miniature triode. This stage presents
a high-impedance, low-capacitance load at the input con-
nector and isolates the input from the succeeding stages.
A protective network consisting of C126 and R126 in the
grid circuit limits the grid current to about 1 ma if up to
600 vde overload is applied to the input.
The Input CF stage draws about 8ma at 96 cathode to-
plate volts and has a gain of about 0.9. The load consists
of R137 and the base impedance of Q164 which is about
3k. The cathode of V133 is "long-tailed""; that is, cathode
resistor R137 returns to a voltage which is well below the
grid level. Long-tailing the cathode improves the gain sta-
bility of the stage. At the junction of R137 and R237,
the cathode resistor is Zenered at —25 volts by D236. This
diode provides a means of obtaining the desired operating
potential from the—150-volt power supply.
Channel A GRID CURRENT ZERO adjustment R121 applies
a dc voltage to the junction of RI17 and R118. When this
control is set properly, the voltage developed across R118
offsets any grid-current-developed voltage across R117.
The offset voltage minimizes any display shift due to
the grid-current-developed voltage assuring accurate dc
measurements.
3-2
Diodes D138 and D139, normally reverse biased, protect
Input Amplifier transistors Q164 and Q174 from cathode-
follower overloads. In addition, the diodes protect the
transistors in case the nuvistor filament fails or the nuvistor is
withdrawn from its socket during operation.
SN 101-2999
Diode D134, connected to the junction of R132 and R133,
is Q protection diode which is normally forward biased.
During normal operation the diode clamps the junction
at +100 volts. In the event that the grid is suddenly driven
too far positive, the diode unclamps (reverse biases). R133
drops the plate voltage of V133 to protect the tube against
excessive plate dissipation and cathode current.
Input Amplifier
The signal from Input CF V133 is applied to base of
the Input Amplifier stage Q164. This stage consists of
transistors Q164 and QI174 with associated circuitry. Both
transistors are connected as an emitter-coupled paraphase
amplifier. In addition to converting the single-ended signal
to push-pull, this stage drives the 93-ohm coaxial lines which
constitute a 186-ohm balanced line, terminated at each end.
The single-ended to push-pull gain of this stage is about
one. This is determined by the ratio of collector impedance
to emitter impedance. SN101-2999 Channel B emitter im-
pedance is variable by means of GAIN BAL. control R277
to permit setting the gain of channel B to equal the gain of
channel A. SN 3000-up Channel A emitter impedance is
variable by means of the GAIN BAL Control R177 to permit
setting the gain of Channel A to equal the gain of Chan-
BALANCED BRIDGED-T ATTENUATOR —— VARIABLE
TY VOLTS/CM
WA ze /,
/ /
/
/
/
WW *
/ 7
/ /
/
1862, 1862,
/
/
R197 F
L
R194 / R195
/
/
Fig. 3-2. VARIABLE VOLTS/CM Contro! R180 shown in the CAL.
(calibrated) position.