Auto Intensity

The purpose of the Auto Intensity circuit, shown in Diagram 6, is to keep the intensity of the trace on the crt at a constant level with changing sweep speeds and trigger signal repetition rates. In conventional oscilloscopes, as the duty cycle of the displayed trace changes, the intensity will vary. The Auto Intensity circuit compensates for this effect by increasing the Z-Axis Drive voltage for low Sweep duty factors. The elements of the Auto Intensity circuit consist of four blocks: the duty-cycle averager, the boost-factor converter, the intensity-control multiplier, and the crt triode compensation circuit. The duty-cycle averager consists of an electronic switching circuit composed of U825A, U825B, and U825C. The Swp Duty signal that is applied to U825B pin 11 causes the output voltage at pin 14 to be switched between ground and +5 V. The output voltage of U825B is averaged by R821 and C821 and applied to U835A pin 3 via U825C.

As the sweep duty factor decreases, the crt beam current must be increased to maintain a constant intensity. To accomplish the task, the boost-factor converter increases the drive in inverse proportion to the duty factor of the trace being displayed.

Amplifier U835A is a high-impedance voltage follower. For 100% duty factor, the output voltage will be approximately zero. Decreasing the duty factor to 10% results in approximately 4.5 V output, and when no sweep occurs (0% duty factor) the output will be 5 V. The output of U835A is applied to a network consisting of CR828, CR830, and resistors R827, R828, R829, R830, and R831. This network produces an output current which is a nonlinear function of the duty-factor voltage. For 10% duty factor, the output current is 10 times greater than the current at 100% duty factor. Maximum available boost limits at a factor of about 25:1.

The nonlinear current is connected to the emitters of the differential amplifier composed of 0811 and Q812. The emitters of the two amplifier transistors are held at a constant voltage by the action of 0813. AUTO INTENSITY control R807 is connected to the base of Q811 via R811. It controls the portion of the boost current that goes to the summing junction of U835B. Boost current is proportional to the true beam current required at the faceplate of the crt.

The crt triode compensation circuit is an inverting operational amplifier with nonlinear feedback. It is composed of U835B, R834, R835, C834, and CR834. Output voltage of the circuit changes in response to the input current in a manner that complements the nonlinear triode characteristics of the crt. This output voltage is applied to the Z-Axis Amplifier via the Z-Axis Gating circuit. The Intens Level signal is also applied to the Focus circuit (Diagram 9) for use in focus tracking of the intensity level changes.

The intensity of the display is allowed to reduce to zero through the action of CR809, VR809, and R809. Without this circuit, the Auto Intensity circuit would not allow the intensity to go to zero when the AUTO INTENSITY control is set to minimum intensity.

Z-Axis Amplifier

The Z-Axis Amplifier controls the crt intensity level via several input-signal sources. The effect of these input signals is either to increase or decrease trace intensity or to completely blank portions of the display. The Z-Drive signal current and the input current from the Z-AXIS INPUT connector (if in use) are summed at the emitter of common-base amplifier transistor Q841. The algebraic sum of these signals determines the collector current of Q841. Input transistor Q841 provides a low-impedance termination for the input signals and isolates the signal sources from following stages of the Z-Axis Amplifier.

Signal current from Q841 flows through CR844 and develops a signal voltage drop across R844. Increasing current through Q841 reduces the forward bias of Q844, thereby reducing the current through Q844. This action causes the collector voltage of Q844 to go more negative (toward the —8.6 V supply) and increases the forward bias on emitter-follower Q845. As emitter current of Q845 increases, negative-going voltage developed across R847 is applied to the bases of complementary-pair output transistors Q847 and Q850. Positive transistions of the Z-Axis signal are coupled to the base of Q850 via C852. The fast-rise transitions are amplified by Q850 to speed up the response time. For negative transitions of the Z-Axis signal, as well as for dc and low-frequency signal components, Q847 acts as the amplifier, with Q850 supplying the current.

Diode CR856 prevents the Z-Axis output signal from going negative, and neon lamps DS854 and DS856 provide protection to the Z-Axis Amplifier in the event of highvoltage arcing in the crt.

The amplifier gain with respect to the Z-Drive current is set to about 10 by the negative feedback supplied from the collectors of Q847 and Q850 to the base of Q845 via feedback resistor R846. The gain with respect to the external Z-Axis Input signal is held to about three by R801, R802, and R803 in series with the external input signal. Diodes CR801 and CR802 provide protection for the

Z-Axis Amplifier in case of an accidental application of excessive signal amplitude to the Z-AXIS INPUT connector.

When CHOP VERTICAL MODE is selected, the Chop Blank signal is applied to the collector of Q841 during the display switching time. Signal current is shunted away from CR844, and the forward bias of Q844 increases to the blanking level. When blanked, the output of the Z-Axis Amplifier drops to about +10 V, and the crt beam current is reduced to below viewing intensity to eliminate chop switching transients from the display.

For an X-Y display, the Sweep Z-Drive signal current is switched off. When the XY signal is LO, CR837 is forward biased and Intens Level current flows through R837 to Z-Axis Amplifier transistor Q841 to establish the display intensity.

The last input to the Z-Axis Amplifier is the Beam Find current. Normally, BEAM FIND switch S390 is closed, and —8.6 V is supplied to the base bias network of Q841 and 0844. When the BEAM FIND switch is opened, the -8.6 V is removed, and the bias voltage becomes more positive. Transistor Q841 becomes more forward biased while Q844 becomes much less forward biased. The current through 0844 is reduced, and the base bias voltage of Q845 is thereby increased. The output of Q845 then goes to a level that produces a fixed, predetermined Z-Axis output signal level. Thus neither the AUTO INTENSITY control nor the Z-Drive signal have any control over the intensity level of the crt display whenever the BEAM FIND push button is pressed in, and a bright trace (or dot if no sweep is present) will be displayed.

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