Tektronix 175, Руководство пользователя

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Circuit Description —Type 575
when the transistors have a beta of 8 or greater. When
checking diodes, you will notice that the waveform of cur ­
rent pulses is such that a curve of approximately 15 amperes
maximum is drawn.
By means of the PEAK VOLTS RANGE switch, each set of
rectifier diodes is connected in parallel for the 0-20 volt
range, or in series for the 0-200 volt range. The polarity of
the output sweeps is determined by the POLARITY switch
SW708. The DISSIPATION LIMITING RESISTOR switch SW710
connects the desired value of resistance in series with the
collector to protect the transistor.
To compensate for the stray-circuit-capacitance charging
current through the Current Sampling Resistor, a sample of
the collector sweep voltage is applied through the cathode ­
follower V733 to the top of the Current Sampling Resistor.
Capacitors C706 and C735 are used to balance the circuit
capacitances.
Step Generator
The circuit diagram of the Step Generator may be con­
sidered in two sections: the pulse-generator section (left
side) which develops rectangular pulses from the sine-wave
input, and the staircase-generator section which uses these
pulses to develop a staircase waveform. VI 71 is the “heart"
of the Step Generator and its operation will be described
first.
Staircase Generator
The staircase waveform is generated by increasing the
charge on a capacitor by equal steps and then discharging
the capacitor after the desired number of steps has been
generated. A simplified example is shown in Fig. 3-2. When
the switch is closed the voltage will rise at the normal RC
charging rate as in curve A. If the switch is closed in a series
of short, equal intervals, a staircase waveform like that of
waveform B is produced. It is a very poor staircase wave­
form because the steps become progressively smaller as the
voltage across the capacitor increases. To achieve a series
of equal-amplitude steps, the capacitor charging current, and
hence the voltage across the resistor, must be kept constant.
The diagram of Fig. 3-3 shows a method of achieving this
end. It is called the Miller integrator. With the switch in
position 1, the plate of the pentode is at +100 volts, the
quiescent output voltage, and the charge on Cl 77 is 101.5
volts.
When the switch is moved to position 3, C177 charges
through R1 and the grid of V171 tends to become more neg­
ative. But since a negative signal on the control grid reduces
the plate current, the plate voltage increases, raising the
voltage at the top of Cl77. The coupling of this positive
change at the top of C177 to the control grid almost com ­
pletely cancels the negative-going tendency of the control
grid. Since the de gain of the pentode stage is very high,
the plate-voltage change is always very large compared to
the voltage change that occurs on the grid.
When the switch is moved to position 1, the charging pro­
cess stops and the tube returns to its initial condition, dis­
charging Cl 77 to 101.5 volts.
Waveform A of Fig. 3-3 is the output waveform which re ­
sults from moving the switch from position 2 to position 3 at
a regular rate. Note that this staircase waveform has steps
which are of equal amplitude, since C177 is charged at the
same rate whenever the switch is in position 3. Waveform
B is the corresponding grid waveform.
The circuit of Figure 3-4 is a modification of the one in Fig.
3-3, the only changes being the addition of a cathode follow ­
er between the plate of the pentode and the top of Cl 77 and
an additional switch position which permits the coupling of
negative-going pulses to the bottom of Cl 77.
With the switch in position 1, the plate of the pentode is
again at +100 volts; however, the output terminal (top of
C177) will be about ground potential.
With the switch in position 4, and with no input pulses fed
into diodes V172A and V172B, the output voltage is constant
since the electrical path through C177 is incomplete. When
Fig. 3-2. Basic circuit (a) for generating a step waveform (waveform B in (b).
3-2