Double Beam Oscilloscope

The dual beam oscilloscope emits two electron beams that are displayed simultaneously on a single scope, which could be individually or jointly controlled. The construction and working of the dual beam oscilloscope are completely different from dual trace oscilloscope. The tubes are more complicated to build, and the whole thing is more expensive.
A special type of double beam oscilloscope can display two electrons beam by generating or deflecting beams. Now a days, double beam oscilloscope is outdated, as this function could be performed by the digital scope with greater efficiency and they do not require a dual-beam display. The digital scope captures a single beam of electron and simultaneously and splits into many channels.

Construction of Double Beam Oscilloscope

There is two individual vertical input channel for two electron beams coming from different sources. Each channel has its own attenuator and pre-amplifier. Therefore, the amplitude of each channel can be controlled eventually.
The two channels may have common or independent time base circuits which allow different sweep rates. Each beam passes through different channels for separate vertical deflection before it crosses a single set of horizontal plate. The horizontal amplifier is compiled by sweep generator to drive the plate which gives common horizontal deflection. The horizontal plates allow both the electron beams across the screen at the same time.

construction of double beam oscilloscope with common time base
Dual beam oscilloscope can generate the two electron beams within the cathode ray tube either by using double electron gun tube or by splitting beam. In this method, the brightness and focus each beam are controlled separately. But two tubes increases the size and weight of the oscilloscope and it looks bulky.

The other method is split beam tube, a single electron gun is used in this method. There is a horizontal splitter plate between the Y deflection plate and last anode. The potential of the plate is same as that of the last anode and it goes along the length of the tube between the two vertical deflection plates. Therefore, it isolates the two channels. As the single beam is split into two, its brightness of the resultant beam is half of the original. At high-frequency operation, it works as a disadvantage. The alternative way to improve the brightness of resultant beam is to have two sources in the last anode instead of one so that beams emerge from it.

Difference between Dual Beam and Dual Trace Oscilloscope

The dual beam oscilloscope has two different electron gun which passes through two completely separate vertical channels, where as dual trace oscilloscope has single electron beam which get split into two and passes through two separate channels.
Dual trace CRO cannot switch quickly between the traces so it cannot capture two fast transient events whereas dual beam CRO there is no question of switching.
The brightness of the two displayed beam has drastically different as it operated at widely spaced sweep speeds. On the other hand, dual trace brightness of the resultant display is same.
The brightness of the displayed beam of the dual trace is half of the brightness of dual beam CRO.
block diagram of dual trace oscilloscope

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