Sometimes we require to compare two different signals simultaneously. In that case, we may use two different oscilloscopes for the purpose. But it becomes difficult to trigger two different sweep generators at the same instant. We can resolve this problem by using a dual-trace oscilloscope.
There are two separate input channels in a dual-trace oscilloscope. Each of the input channels has its attenuator and pre-amplifier. The pre-amplifier of both the channels feeds a common vertical amplifier through an electronic switch. Then this vertical amplifier causes two separate traces on the screen for two input signals with the help of a single electron beam.
That means the dual-trace oscilloscope uses a single electron beam to create to separate traces on the screen. It is possible only due to the functioning of the special electronic switch. The entire logic for the operation of a dual-trace oscilloscope is in that electronic switch.
This electronic switch has two modes. One is alternate mode and another is chop mode.
Alternate Mode in the Dual Trace Oscilloscope
When the switch is on the alternate mode, the vertical amplifier receives its input from both pre-amplifiers alternatively. The frequency of this alteration should match the frequency of the sweep signal. Therefore one cycle of the one input signal appears at the vertical amplifier for one sweep cycle and the next cycle of the second input signal appears at the vertical amplifier for the next sweep signal.
After appearing the trace takes some time to disappear on the screen. This is due to the decay time of the phosphor coating. If the sweep frequency is sufficiently high, the speed of the sweep will also be high. For this high sweep speed, the trace of the waveform appears on the screen before disappearing. As a result, there will be a continuous trace of both waveforms on the screen. Hence we can be able to see both the waveforms simultaneously on the dual trace oscilloscope screen.
Chop Mode in the Dual Trace Oscilloscope
In the chop mode, the electronic switch alternatingly chops the input signals at a very high frequency. That means the switch feeds the input signals to the vertical amplifier alternatively at a very high speed. The speed of the alteration of feeding the signals to the vertical amplifier is independent of the sweep speed. The frequency of this alteration is typically in the range of 100 kHz to 300 kHz.
The sweep frequency is much less than the said frequency ranges (100 kHz to 300 kHz). Because of this high chopping frequency or in other words for the high speed of the input signal alteration compared to sweep speed a small segment of each signal appears on the screen after each alteration. Because of the high frequency, the segments of a trace become so closed that it creates a continuous line. This is how a dual-trace oscilloscope draws the waveform of two input signals simultaneously on the screen in the chop mode.
Selection of Modes in a Dual Trace Oscilloscope
For the proper trace of a waveform on the screen, the sweep frequency should be equal to or an integer multiple of the frequency of the input signal. Hence for high-frequency input signals, the sweep frequency of the oscilloscope will be equally high. So for higher frequency input signals, the sweep frequency may become equal to the chopping frequency of the electronic switch. In that condition, the individual segments of the waveform appear on the screen. Hence for such high-frequency input signals, we need to select the alternate mode of the electronic switch. Otherwise, at normal frequencies, we generally select the chop mode of the switch.
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