Control Logic

The control logic senses which button is pushed on the front panel. Since the buttons are the momentary action type, the logic must remember what button was pushed, turn on the LED indicator within the button, and activate the appropriate configuration of the oscillator, signal, and pulse generator sections.

After a button is pushed, the unit will remain in that configuration until another selection is made or the power is secured. The HIGH LOCKOUT and PULSE GENERATOR push buttons are the only ones that require repetitive pressing to be turned on or off.

The channel relay is controlled by CHANNEL buttons A, ALT, and B. The relay is a single-pole, double-throw type and is de-energized for channel A and energized for channel B. If CHANNEL A is already selected and the CHANNEL B button is pressed, CHANNEL A will be canceled and CHANNEL B selected. When the ALT button is pressed, another control line is set, which enables an internal clock to toggle the channel relay on and off, causing the unit to alternate between channels A and B.

The internal clock is controlled by the RATE control knob on the front panel. When the ALT mode is active, the LEDs within the A and B buttons flash alternately, and the ALT LED is on continuously. Pressing either of the channel buttons will cancel the ALT mode, and the selected channel will then become active.

The FREQUENCY buttons (50/60, 400, and 2000 Hz) directly control the operation of the oscillator and the pulse generator. The RANGE buttons (LOW, MED 1, MED 2, and HIGH) control four relays in the signal section that select the appropriate terminal characteristics for each impedance range (table 5-4).

Table 5-4. - Terminal Characteristics for Impedance Ranges

You can select the four ranges manually by pressing the RANGE button, or you can scan them automatically by using the AUTO function. When AUTO is activated, the control logic will follow the sequence LOW, MED 1, MED 2, and HIGH over and over if HIGH LOCKOUT is off. The current active range is always indicated by the LED for the range selected. The AUTO mode will stay active until you select a particular range by pressing its associated button. While AUTO is active, the AUTO LED is continuously on.

The speed at which the ranges are scanned is controlled by the front-panel RATE control knob. This allows you to adjust the time each range is displayed for signature analysis on the CRT display. If AUTO and ALT (alternate) are active at the same time, the RATE control affects the speed of both functions with ALT having priority. This is done so that the two channels can be compared to each other within one range before the next range is selected (figure 5-26) .

Figure 5-26. - Range scanning sequence with AUTO and ALT active.

The HIGH LOCKOUT function disables the HIGH range and limits the maximum test signal to 20 volts peak vice 60 volts peak. When you select the manual mode (AUTO off), activating the HIGH LOCKOUT prevents the HIGH range from being selected. If the HIGH range is active when the HIGH LOCKOUT is pressed, the HIGH range is canceled and the next lower range (MED 2) will be selected and become active. When you select the AUTO mode, the RANGE sequence with the HIGH LOCKOUT active will start with LOW, and sequence through MED 1, MED 2, back to LOW, and continue until the AUTO mode is stopped.

The PULSE GENERATOR buttons (positive [+], negative [-], and PULSE/DC) toggle control lines that change the polarity and output type of the pulse generator.

Oscillator

The oscillator produces a constant amplitude, low distortion sine wave test signal. The frequency of the test signal is programmable between one variable frequency (50/60 Hz), and two fixed frequencies (400 Hz and 2000 Hz). The variable frequency depends on the input power line used for the Tracker 2000; a 50-Hz line produces a 50-Hz test signal, and a 60-Hz line produces a 60-Hz test signal. If a 400-Hz power line is used, an 80-Hz test signal is provided. This versatility is built in to ensure you will always have low, medium, and high frequencies to work with.

Signal Section

The signal section is considered to be the heart of the Tracker 2000. In this section, the signal from the oscillator is applied across two terminals of a device being tested via the front-panel jacks.

The test signal causes a current to flow through the device and a voltage drop to occur across its terminals. The current flow causes a vertical deflection of the trace on the CRT display, while the voltage drop across the device causes a horizontal deflection of the trace on the CRT display. By combining these, the current-voltage signature of the device being tested is displayed on the CRT.

The test signal appears at the front-panel test jacks as though it is being originated by a voltage source (V_s) with a series output impedance (Z_s). An equivalent circuit of the signal section is shown in figure 5-27. As you can see, the figure also shows how the terminal voltage affects the horizontal deflection plates of the CRT, and how the current through the terminals affects the vertical deflection plates through current sensing point I.

Figure 5-27. - Signal section equivalent circuit.

An open circuit has zero current flowing through the terminals and maximum voltage drop across the terminals. In all ranges, this is represented by a straight horizontal trace from left to right on the CRT display, as seen in figure 5-28 view A. When a short occurs, maximum current flows through the terminals, and the voltage drop is considered to be zero. This occurs in all ranges and is represented by a straight vertical trace from top to bottom of the CRT display, as seen in figure 5-28 view B.

Figure 5-28. - Open circuit display.

Pulse Generator

The pulse generator provides dynamic testing for certain types of devices by driving the control input of the device under test. The normal two-terminal mode of testing can be considered a static test, since devices with three or more terminals are not tested in their active mode. However, with the pulse generator, an in-circuit active test is possible.

In the PULSE mode, this circuit uses the zero crossing of the test signal to trigger the start of the pulse. When positive (+) is enabled, a positive-going zero crossing triggers a positive pulse. When negative (-) is enabled, a negative-going zero crossing triggers a negative pulse. If both are enabled, then both positive and negative pulses are produced on alternate crossings (composite pulses). Once a pulse is triggered, its duration is set by the WIDTH control knob. Figure 5-29 shows the waveforms for three pulse polarity types at various settings of the WIDTH control.

Figure 5-29. - Pulse generator waveforms.

The LEVEL control adjusts the peak of each pulse from zero to 5 volts with the polarity dependent on the pulse polarity selected. When an open circuit is present, a maximum output of 5 volts peak-to-peak is present with either positive or negative selected, and 10 volts peak-to-peak when the composite pulse is active.

Q.10 What minimum/maximum voltage level can be attained in the pulse generator section by adjusting the LEVEL control?