Block Diagram; Detailed Circuit Description - Tektronix 191 Instruction Manual

Introduction
This section of the manual contains an electrical des­
cription of each circuit in the Type 1 91 . A block diagram
of the instrument is located in the Diagrams section of this
manual. The block diagram shows the relationshi ps between
the circuits of the instrument.
Schematic diagrams are also located i n the Diagrams
section. These diagrams should be referred to for electrical
val ues and relationships of components.

BLOCK DIAGRAM

Oscillator
The Osci llator output signal is suppl ied to both the Peak
To Peak Detector and the 50
frequency of the Oscil lator is controlled by the FREQUENCY
RANGE switch and the Frequency dia l. The output signal
am plitude of the Oscil lator is controlled by the DC voltage
applied to the plate of the Osci llator tube V1 0 via the
Series Regulator tube. This DC voltage is controlled by the
AMPLITUDE switch and the VARIABLE control setti ngs and
by the comparison process occuring in the Peak To Peak
Detector.
Q
50 Wide band Attenuator
n
The 50 Wideband Atten uator takes the normal
volt signal supplied from the Oscillator a nd, depending
upon the AMPLITUDE RANGE switch setti ng, either supplies
the signal through 50 ohms to the OUTPUT connector (.5-5V),
attenuates the signal l O X (50-500 mY), or attentuates the
signal lOO X (5-50 mY).
The output resista nce of the Type 1 91 is 50 ohms in all
positions of the AMPLITUDE RANGE switch.
Comparison Voltage
The DC voltage from the Comparison Voltage circuit
applied to the Peak To Peak Detector determines the Osci l­
lator output signal a m plitude applied to the 50
Atten uator.
Peak To Peak Detector
The output of the Osci llator a nd the Comparison Voltage
are compared in the Peak To Peak Detector and any neces­
sary correction voltage to correct the output signal amplitu.de
is sent to the Feedback DC Amplifier.
Feedback DC Amplifier
This amplifier recieves the correction voltage from the
Peak To Peak Detector and then supplies the a mplified
correction voltage to the grid of V94, the Series Reg u lator
Tube.
®I
SECTION 3
CIRCUIT DESCRIPTION
n
Wideband Attenuator. The
1
to 1 1
n
Wideband
Series Regulator
The correction voltage from the Feedback DC Am plifier
varies the grid pote ntial of V94, which causes the Series
Regulator tube to adjust the a mount of plate voltage avail­
able to the Oscillator tube.
Unregulated Power Supply
The U n regulated Power Supply makes available about
+450 volts of DC voltage to the plate of the Series Regula­
tor tube , V94.

DETAILED CIRCUIT DESCRIPTION

Oscillator
Oscillator tube V1 0 is connected as a grounded cathode
Class C Colpitts Osci llator. Plate current is supplied to the
grid end of switched inductors through R90, R99, R98, L98
and Zener diode D98. L98 and resistors R98, R99 and R90
isolate the Oscillator from the low output impedance of the
Series Reg ulator, thereby increasing the Q of the Oscil lator
circuit. Zener diode D98 provides a voltage drop to furnish
Q94 with sufficient col lector voltage when the Osci llator
output amplitude is minimum.
Feedba ck to the grid of V1 0 to mai ntain osci l lations is
provided from the g rid end of the switched i nductors through
C8 and R8.
The tuning capacitor (Freq uency dial) for the Oscill­
ator is composed of C1 0A, B, C and D. Sections C1 0A, B
and C of the tuning capacitor are electrically connected in
parellel in the g rid circuit while only section ClOD of the
tuning capacitor is connected to the plate circuit. This per­
mits the ratio of plate to grid AC voltage to be 3 to 1 .
The switched i n ductors which provide the various fre­
quency ranges have adjustable powdered iron slugs and
i ndividual trimmer capacitors. These adjustments permit the
frequency span of each range Jo be accommodated to the
preca librated dia l. The selection of inductor is determined
by the FREQUENCY RANGE switch, SW1 0.
The switched inductor for the 50 kHz ONLY range also has
an adjustable powdered i ron slug. Large fixed capacitors
are connected in parallel with C1 0 to reduce the necessary
inductance for 50 kHz operation. The large capacitors result
in a neg lig ible frequency difference (less tha n
50 kHz at any setti ng on the frequency dial.
The ferrite beads L1 1 and L1 2 mounted on the leads of
T1 0 mini mize high frequency currents i nduced when the
oscillator is operati ng in the 42 MHz to 1 00 MHz range.
The individually switched output coils, ind uctively coupled
to the oscillator coils, provide oscillator signal to the output
networks. The inductor and capacitor in each of the output
networks except for the 42 to 1 00 MHz range output coil,
constitute low pass filter which partially removes the higher
frequency harmonics from the output oscillator signal. The
%) from
+- 1
3-1