Explanation of Dual Trace CRO

Dual Trace CRO

Block diagram of dual trace CRO is shown in the fig. there are two separate vertical input channels A and B. These use two separate attenuators and preamplifiers stages. Therefore the amplitude of each input as viewed on the oscilloscope can be individually controlled. After pre-amplification the two channels meet at an electronic switch. This has the ability to pass one channel at time into the vertical amplifier, via the delay line.

Pic 1

The dual trace oscilloscope is basically operated in alternate and chop modes. The operating modes are selected form the instrument front panel. In alternate mode the electronic switch alternates between channels A and B, letting each through one cycles of the horizontal sweep display is balanced during the fly back and hold off periods. Provided the sweep speed is much greater than the delay time of the CRT phosphor, the screen will show a stable display of both the waveforms at channel A and B.

In chopped mode the electronic switch free runs at a high frequency of the order of 100 KHz to 500 KHz. The result is that small segments from channel A and B are connected alternately to the vertical amplifier and displayed on the screen. Provided the chopping rate is much faster than the horizontal sweep rate, the display will show a continuous line for each channel.

The time base circuit of dual trace CRO to similar to that of single trace CRO. Switch S₂ allows the circuit to be triggered on either the A or B channel waveforms or an external signal. The horizontal amplifier can be fed from the sweep generator or the external horizontal input through channel B via switch S₁. This is the X-Y mode with channel A as the vertical signal and channel B as the horizontal signal. Several other operating modes of dual trace CRO are channel A only channel B only, channel A and B as two traces and signals A + B, A – B, B – A or         – (A + B) as a single trace.