Differentiate between dual beam and dual trace oscilloscope, Electrical Engineering

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Q. How do you differentiate between dual beam and dual trace oscilloscope?

Sol. There are two separate vertical input channels A, B and these use separate attenuator and preamplifier 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 a time into the vertical amplifier, via the delay line.

       There are two common operating modes for the electronic switch, called alternate and chop and these are selected from the instrument's front panel. The alternate mode is illustrated.

 In this, the electronic switch alternates between channels A and B, letting each through for one cycle of the horizontal sweep. The display is blanked during the fly back and hold-off periods, as in conventional oscilloscope provided the sweep speed is much greater than the decay time of the CRT phosphor; the screen will show a stable display of both the waveform at channels A and B. The alternate mode cannot be used for displaying very low frequency signals.

 In this 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 channels 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. If the sweep rate approaches the chopping rate then the individual segments will be visible and the alternate mode should now be used.

    Switch allows the circuit to be trigged on either the A or B channel waveforms, or on line frequency or on an external signal. The horizontal amplifier can be fed from the sweep generator or the B channel via switch S. This is the X-Y mode and the oscilloscope operates from channel A as the vertical signal and channel B as the horizontal signal, giving very accurate X-Y measurements. Several operating modes can be selected from the front panel for display, such as channel A only, channel B  only, channels A and B as two traces and signals A+B, A-B, B-A or (A+B) as a single trace.

   Dual Beam CRO: The dual trace oscilloscope can not capture two fast transient events, as it cannot switch quickly enough between traces. The dual beam oscilloscope has two separate electron beams, therefore two completely separate vertical channels, as in fig.

The two channels may have a common time base system, as shown. Or they may have independent time base circuits. An independent time base allows different sweep rates for the two channels but increases the size and weight of the oscilloscope.

Two methods are used for generating the two electron beams within the CRT; the first method uses a double gun tube. This allows the brightness and focus of each beam to be controlled separately but is bulkier than a split beam tube.

 In the second method known as split beam, a single electron gun is used. A horizontal splitter plate is placed between the last anode and the Y deflection plates. The plate is held at the same potential as the anode and it goes along the length of the tube between the two vertical deflection plates. It therefore isolates the two channels. The split beam arrangement has half the brightness of a single beam, which has disadvantages at high frequency operation. An alternative method of splitting the beam, which improves its brightness, is to have two apertures in the last anode instead of one, so that two beams emerge from it.

The disadvantage of the split beam construction is that the two displays may have noticeably different brightness, if operated at widely spaced sweep speeds. The brightness and focus controls also affect the two traces as the same time.


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