Tektronix 502 series, Инструкция по эксплуатации

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The Upper Beam and the Lower Beam Vertical
Deflection Amplifiers are identical, so the de­
scription that follows applies to both.
General Description
The push-pull Vertical Amplifier consists of
three stages of amplication, the Input Amplifier,
the Second Amplifier and the Output Amplifier,
and a cathode-follower stage to drive the Out­
put Amplifier. The overall gain of the Amplifier
is controlled by two feedback networks, one pro­
viding negative feedback and the other positive
feedback (see Fig. 4-1). The Input Amplifier is a
cathode-coupled paraphase amplifier (it may
also be operated differentially) whose gain is
controlled by negative feedback from the cath­
odes of the Driver C.F. stage. The Second Ampli­
fier has a positive feedback network that extends
from the plate circuit on one side to the grid
circuit on the other; this configuration makes this
stage an almost “ infinite-gain” amplifier. The
result of both feedback networks is an amplifier
having a sensitivity of 200 microvolts/centimeter.
The Input Circuit
The Input Selector switch SW403 determines
the mode of operation for the Amplifier. When
in any of the three positions marked AC the
signal is ac-coupled through C400 (for Input A)
and/or C401 (for Input B). When in any of the
three positions marked DC the input capacitor
(C400 or C401) is bypassed and the signal is
dc-coupled to the Input stages.
The sensitivity of the Vertical Amplifier, as
mentioned previously, is 200 microvolts per centi­
meter. However, by means of attenuation and
degeneration networks, the vertical deflection
factor can be increased to 20 volts per centi­
meter.
Either of two attenuation networks can be con­
nected in series with the Input connectors of the
Vertical Amplifiers, one attenuates the signal by
a factor of 10, the other by 100. For dc and low
frequency signals, these networks are resistance
dividers, and the degree of attenuation is pro­
portional to the ratio of the resistance values.
The reason for this is that the impedance of the
capacitors, in this range of frequencies, is so
high that their effect in the circuit is negligible.
For higher-frequency signals, however, the im­
pedance of the capacitors is less and their effect
in the circuit is more pronounced. Near the up­
per-frequency range of the Amplifier the imped­
ance of the capacitors becomes so low, com­
pared to the resistance of the circuit, that the
attenuators become capacitance dividers. For
these frequencies the degree of attenuation is
inversely proportional to the capacitance ratio.
In addition to providing the proper degree of
attenuation, the resistance values of the attenua­
tors are chosen so as to provide the same input
resistance, regardless of the setting of the SENSI­
TIVITY switch. For example, in the “ straight
through” positions of the Input Selector switch
(200juv/cm to .2-v/cm), the 1-megohm grid resis­
tors. . . R410 for Input A and R440 for Input B. . .
constitute the input resistance of the Vertical Am­
plifiers. In the range from .5 v/cm to 2 v/cm,
the X 10 Attenuator is connected into the input
circuit. The resistor in the low end of the divider
. . . R406E for Input A and R407E for Input B. . .
shunts the grid resistor to create an equivalent
resistance of 100K ohms. This 100-K equivalent
resistance is then in series with the resistor in the
high side of the divider (900 K ohms) to produce
a total input resistance of 1 megohm. The X I 00
attenuator works in the same manner. The 10.1 -K
resistor at the lower end of the divider shunts the
1-meg grid resistor to form an equivalent resis­
tance of 10 K ohms. This equivalent resistance is
then in series with the 990-K
resistor in the upper
side of the divider to create a total input resis­
tance of 1 megohm.
The capacitance values in the attenuators are
also selected to provide a constant input capaci­
tance. . .4 7
jLijUif . . .regardless of the setting of
the SENSITIVITY control. In the “ straight
through” positions of the switch, the total input
capacitance is equal to the capacitance of C410
(or C440 for Input B) plus the tube and stray
capacitance. C410 is then adjusted so that the
total input capacitance is 47
jujuf.
When the X I 00 attenuator is connected into
the circuit, C406L (or C407L for Input B) shunts
the 47
jocjuf capacitance. This value of capaci­
tance is then reduced to a very small value by
series capacitor C406J (or C407J). The capacitor
at the input to the attenuator then shunts this
small capacitance, and is adjusted to make the
total capacitance 47 jujuf. The X10 attenuator is
designed in the same manner, except that stray
capacity forms the lower branch of the divider.
Since the attenuator networks are frequency-
compensated voltage dividers, a constant atten­
uation ratio is maintained from DC to the upper-
frequency limits of the Vertical Amplifiers.
The Input Amplifier
When the Input Selector switch is in the A
position (either AC- or DC-coupled) the grid of
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4-2
CIRCUIT DESCRIPTION — TYPE 502