How does a "Dual channel CRO" work?

[R.Vinoth Jeba Kumar]

Hi
Mine is a simple qn. Please put up with my ignorance.
How are some "CRO"s able to display signals from two channels at the same time..? Are the signals time multiplexed? If so (or otherwise) how?
Thanx to anybody who replies
R.Vinoth

 

[Nigel Goodwin]

Hi
Mine is a simple qn. Please put up with my ignorance.
How are some "CRO"s able to display signals from two channels at the same time..? Are the signals time multiplexed? If so (or otherwise) how?
Thanx to anybody who replies
R.Vinoth

Very, very, old oscilloscopes used special dual beam tubes, but for many years they have simply multiplexed the display.

There are two basic modes, 'chop' and 'alternate'.

'Alternate' shows each beam in turn, so does a sweep of beam one, then a sweep of beam 2 - this only works at fairly high timebase speeds, otherwise you can see the individual sweeps.

'Chop' switches from beam to beam at a fairly high frequency, so as the single beam travels across the CRT it's rapidly switched from beam 1 to beam 2, beam1, beam 2 etc. This is done at a fairly high frequency, and as such is too high to be seen on the scope at low timebase speeds. If you try and use 'chop' at high timebase speeds you start to see the switching waveform.

Depending on the scope, you may have manual 'chop' or 'alternate' selection, but some scopes do this automatically via the timebase speed control. Usually the timebase control will be marked accordingly, showing the point where it changes from 'chop' to 'alternate'.

Other the years there have been a great many dual-beam addon units for scopes, they were always popular magazine articles. I'm sure a googel search will find plenty on the net.

 

[Dean Huster]

Folks often mistakenly call an oscilloscope a "dual beam" scope when it's really a "dual trace" scope. As Nigel mentioned, a true dual beam oscilloscope has a special cathode ray tube (CRT) that's actually several CRTs in one. The simpler ones (the old Tektronix 551 is a good example) had one set of horizontal deflection plates and two sets of vertical deflection plates. Two vertical amplifiers fed the two sets of plates while a single timebase had to be shared by both channels.

More sophisticated dual beam CRTs had two sets of vertical plates and two sets of horizontal plates (e.g., the Tektronix 555, 556 and 7844) so that each vertical channel could have its own timebase -- essentially two complete oscilloscopes in one package. Dual beam scopes aren't necessarily OLD. The Tek 7844 was still being manufactured in the early 1980s as I recall.

I've actually seen a 5-beam CRT (one set of horizontal plates) that was part of a horrid military modification of a standard Tektronix 545.

It's that dual beam CRT that makes dual beam scopes expensive. That's why they went to an electronic system to simulate a dual beam scope, more properly called a dual trace scope. As Nigel said, they time-share the CRT with two (or more) vertical amplifiers either by chopping the signal or alternating the signal for each sweep. Some Tektronix lab scopes could be made to have as many as eight channels of vertical information, which could get a little crowded on the screen.

There was one advantage of a dual beam scope over a dual trace scope and that was when viewing the timing relationships between high-speed signals and especially when doing high-speed single sweeps, usually in conjunction with a scope camera. The dual beam scope could "see" both events simultaneously while a dual trace scope would show one event and by the time it switched channels to view the other event, that other event was long over.

Dean