Tektronix 502A, Instruction Manual

Page: 19 / 110

SECTION 3
CIRCUIT DESCRIPTION
Introduction
The Type 502A is a dual-beam, high-gain, low frequency
oscilloscope employing a T5021 type dual-gun cathode-ray
tube. The instrument has identical Vertical Deflection Am­
plifiers, one for the UPPER BEAM and one for the LOWER
BEAM. Simultaneous horizontal deflection of both beams is
provided by a single Time-Base Generator and Horizontal
Sweep Amplifier circuit.
The Type 502A circuitry is arranged so that the instru­
ment can be used in any of several configurations. It may
be used as a conventional single-beam oscilloscope by apply­
ing an input signal to either of the Vertical Deflection
Amplifiers. It may be used to
examine two waveforms
simultaneously by applying input signals to both Vertical
Amplifiers. Both deflection amplifiers may be used in a
differential mode, to examine the difference between, or the
algebraic sum of, two signals. The UPPER BEAM Deflection
Amplifier can be connected to the horizontal deflection
plates, so that the instrument may be employed as a single­
beam X-Y oscilloscope. And, by means of the EXTERNAL
horizontal-input connector, the instrument may be used as
a dual-beam X-Y oscilloscope, with both traces plotted on
the same X scale.
The UPPER BEAM Vertical and the LOWER BEAM Vertical
Deflection
Amplifiers are identical, so the description that
follows applies to both.
The push-pull Vertical Amplifier consists of three stages
of amplification, the Input Amplifier, the Second Amplifier
and the Output Amplifier. A cathode-follower stage
drives the Output Amplifier. The overall gain of the Amplifier
is controlled by three feedback networks, two providing
negative feedback and the third positive feedback (see Fig.
3-1). The Input Amplifier is a cathode-coupled paraphase
amplifier (it may also be operated differentially) whose gain
is controlled by negative feedback from the cathodes of the
Driver C.F. stage and the Output Amplifier stage. The
Second Amplifier 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 net­
works is an amplifier having a sensitivity of 100 microvolts
per centimeter.
The Input Circuit
The Input Selector switch SW403 determines the mode of
operation for the Amplifier. When in any of the three posi­
tions 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 and C401) is
shorted and the signal is dc-coupled to the Input stages.
The sensitivity of the Vertical Amplifier, as mentioned
previously, is 100 microvolts per centimeter. However, by
means of attenuation and degeneration networks, the vertical
deflection factor can be increased to 20 volts per centimeter.
It is possible, through the use of the VARIABLE SENSITIVITY
Control, to introduce enough cathode degeneration into the
cathodes of the Output Stage to increase the deflection
factor to approximately 50 volts per centimeter.
Either of two attenuation networks can be connected 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, how­
ever, the impedance of the capacitors is less and their
effect in the circuit is more pronounced. Near the upper­
frequency range of the Amplifier the impedance of the capa­
citors becomes so low, compared 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 attenuators are chosen so as
to provide the same input resistance, regardless of the
setting of the SENSITIVITY switch. For example, in the
"straight through” positions of the SENSITIVITY switch
(.1 mVOLTS PER CM to .2 VOLTS PER CM), the 1-megohm
grid resistors . . . R410 for Input A and R440 for Input B
. . . constitute the input resistance of the Vertical Amplifiers.
In the range from .5 VOLTS PER CM to 2 VOLTS PER CM,
the X10 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 100 K ohms. This 100 K equiv­
alent 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 X100 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
resistance of 10 K ohms. This equivalent resistance is then
is series with the 990 K resistor in the upper side of the
divider to create a total input resistance of 1 megohm.
The capacitance values in the attenuators are also selected
to provide a constant input capacitance ... 47 pf .. .
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
3-1