Special Oscilloscope Variants Part 2

TV Triggering

The study of TV signals may be required both in the field and laboratory. For the ease of operation, a TV sync separator may be built into an oscilloscope. The synchronization pulse separator (sync separator) provides two types of trigger pulses for the oscilloscope:

  1. Line trigger pulses derived from the TV line sync pulses which are present at each line from which the TV picture is built up.
  2. Field trigger pulses derived every time a completely new TV picture (field) is started.

In the smaller (service) oscilloscopes, the provision may be extremely simple. In the TV mode, the selection of a TV line or a TV field trigger pulse is automatically coupled with the TIME/DIV setting.  From 0.5 ms/div down to 200 ms/div, the TV field is chosen; from 0.2 ms/ div up to 0.5 µs/ div, TV line triggering takes place.

Depressing the ± pushbutton permits the oscilloscope to accept both positive and negative polarity of the composite video signal. The word “LINE” above “INT” and “ExT” on the front panel indicates triggering on the line or mains frequency (50 or 60 Hz).

In the larger (laboratory) oscilloscopes, the provisions and the selection criteria for TV triggering are a little more complicated. An extra position, TV, is added to the trigger coupling switch in the main time-base section. When the TV and HF push buttons are depressed, triggering of the MTB occurs at the TV line sync pulse (T YL). Similarly, the combination of TV and LF causes triggering at the TV field sync pulses (TVF). Again, the word “LINE” at the trigger source selector switch on the front panel indicates triggering at the line or mains frequency.



Details of the front panel of the Philips PM3240X Oscilloscope, showing the options for TV triggering.

Depending upon the polarity of the composite video signal, the LEVEL/SLOPE pushbutton must be in the appropriate position (IN + or OUT -). The MTB trigger level control is not in operation when the TV push­button is depressed.

The delayed time-base section has TYL facility also. If the TYL push­button is depressed, the delayed time base is triggered by the TV line sync pulses, if the MTB is operating in the TV mode. In the TYL mode of the DTB, both the trigger level control and the slope (IN + or OUT -) are not in operation.

The DTB in the TYL mode functions as the trigger in the normal mode. When the DTB is activated (the TIME/DIV switch moved from its OFF position) and the MTB is displayed, then the TV line of interest in the TV field may be selected (intensified on the screen) with the DELAY TIME multiplier, after which pushing the DEL’ D TB button results in a triggered display of the selected TV line. The upper trace shows the intensified part of a full-field TV test signal which is displayed by the DTB mode in the lower trace.

The Four-Channel Oscilloscope

There are many applications where the measuring time of a device under test can be considerably reduced by using a four-channel oscilloscope. This is particularly true in the case of digital and computer applications. Also, in education, a four-channel oscilloscope can be of great help. For example, the operation of a three-input NAND gate can be observed from the screen in one glance.

The vertical display mode is selected by the six-way pushbutton switch A, A + B, B, C, C + D, and D.  Up to four channels can be displayed by depressing one or more of the pushbuttons A, B, C, or D. Two more traces can be displayed by depressing the A + B and C + D pushbuttons, so that altogether six traces from four channels can be displayed.  Furthermore, each channel can be inverted by pulling the center knobs of the AMPL/mv switches which, in combination with the pushbuttons A + B and C + D, also provide ±A ±B and ±C±D display facilities.  The channels can be either chopped (CHOP) at a free running rate of about 1 MHz or switched to the next channel at the end of each sweep (ALT).

Trigger-source selection for both the main time base and the delayed time base can be from any channel (A, B, C, or D) or from a combined signal after the electronic switch (COMPOSITE). The main time base can also be triggered by an externally applied signal (EXT) or from a signal derived from the line voltage (UNE). The HOLDOFF control can be set to prevent the occurrence of double-picture displays on complex waveforms.




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