Dual Trace CRO Working and Block Diagram


The single trace oscilloscope can be modified to offer dual triple and four traces using electron switching. Such CROs can be called multi-trace scopes. Dual beam CROs can be constructed with two guns that give two electron beams. Storage oscilloscopes, Sampling oscilloscopes, and Digital read out oscilloscopes are the other special types.

Dual Trace CRO Working and Block Diagram: 

A block diagram illustrating the principle of the dual trace oscilloscope is given in Figure. It consists of a CR Tube, the usual deflecting system consisting of the sweep generator, vertical amplifier, horizontal amplifier. There are two vertical amplifier stages. An additional stage is an "eIectron switch” in this CRO. As the vertical amplifiers are two in number the delay lines are also two.
With the above arrangement the dual trace oscilloscope is nothing but a single beam oscilloscope, with the input switched alternately to give the impression of two patterns on the screen. The beam produced by the cathode ray follows the signals of channel A, once and channel B, the other time. The sweep is common for both the channels. The following display modes are possible with the dual trace oscilloscope.
Block Diagram of Dual Trace CRO

1. A only.
2. B only.
3. A and B chopped.
4. A and B alternate.
5. A and B added.
6. A vs B (X-Y mode).

A mode selector on the front panel to select the required mode of operation. In the alternate mode the electron switch alternately connects the main vertical amplifier to channel A and channel B alternately. Each of the vertical preamplifiers have a calibrated attenuator, and position control. Therefore the individual positions of the pattern and also the amplitude can be individually controlled. The electron switch switches the two channels in synchronization with the sweep. One channel is covered in one sweep cycle. This results in the two images being stable on the screen. This mode is suited for fast sweep rates, when the two images appear as one simultaneous and stable display.

The sweep trigger signal is obtained from either of the channels before the electron switch. This gives the correct phase relation between the A and B channels.

In the chopped mode the sweep generator is made free running at the rate of 100 to 500 kHz. If the choping rate is faster than the sweep rate the individual little segments fed to the main vertical amplifier form original channel A and B waveforms without the interruptions. If the sweep rate is close to the chopping rate the continuity in the waveform will be lost.

In the mode 'add' the signals of A and B channels are added algebraically. The sum signal is displayed on the screen. Using the polarity inversion switches in both channels we can have the following displays. A + B, A — B, B — A and A — B.

In the X-Y mode the 13 channel is connected to the horizontal amplifier, disconnecting the sweep signal. As the calibration of the two preamplifiers is the same and they have the same delay time accurate measurements can be made.

In addition to the above modes of operation this CRO can be used as single trace one using either of the channels individually.

When cost is the problem in acquiring a dual beam CRO this dual trace oscilloscope is advantageous.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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