Dual Trace Oscilloscope

 

 Figure 2.1 shows the block diagram of dual trace oscilloscope which consist of following steps

1. Single electron gun 

2. Two separate vertical input channels

3. Attenuators and pre-amplifiers

4. Electronic switch.

With the help of electronic switching the two separate input signals can be applied to single electron gun. Produces a dual trace display .Each separate vertical input channel uses separate attenuators and pre-amplifier stages, so, that the amplitude of each signal can be independently controlled. Output of the pre-amplifiers is given to the electronic switch, which passes one signal at a time into the main vertical amplifier of the oscilloscope. The time base-generator is similar to that of single input oscilloscope. By using switch S2 the circuit can be triggered on either A or B channel, waveforms, or an external signal, or on line frequency. The horizontal amplifier can be fed from sweep generator or from channel B by switching S1. When switch S, is in channel B, its oscilloscope operates in the X-Y mode in which channel A acts as the vertical input signal and channel B as the horizontal input signal. 
  From the front panel several operating modes can be selected for display, like channel B only,  channel A only, channels B and A as two traces, and signals A + B, A - B, B ~ A or - (A + B)  as a single trace. Two types of common operating mode are there for the electronic switch,  namely

 1. Alternate mode 

 2. Chop mode. 

1.  Alternate Mode       

     In case of alternate mode, electronic switch alternately connects the main vertical amplifier to channel A and B and adds a different D.C. component to each signal. Due to this D.C. component the beam alternately goes to the upper and lower half of the screen. Switching is carried out at the start of each new sweep of the time base generator. To obtain the cathode ray tube spot it traces the channel A signal on one sweep and the channel B signal on the succeeding sweep. The switching rate of the electronic switch is synchronized to the sweep rate. The alternate mode cannot be used for displaying very low frequency signals.

2. Chop Mode
     In chop mode electronic switch runs at a very high frequency of the order of 100-500 kHz. This results in connecting small segment of waveform A and waveform B, alternately to vertical amplifier to display it on the screen. The display will be a continuous line for each channel when chopping rate much faster than horizontal sweep rate. In case when the chopping rate is slow the continuity of the display is lost hence it is preferable to use alternate mode of operation.