Rigol Scope – Alternate Triggering

Another Rigol triggering mode is the so called “Alternate” Triggering. This reminds me of the old analog scopes with two channels, but better. In those days the time swept by and something was going to get drawn. You had your choice of “alternate” or “chop” mode. In alternate mode, each trigger caused one of your channels to draw. The next trigger would draw the second channel. That made the traces nice and solid but you weren’t really looking at the same time on each trace. If the signals repeated together at the trigger point, it didn’t matter much. But if the signals were not exactly coordinated it could drive you crazy. Chop mode didn’t look as good, but you saw both signals at the same time because the scope would draw a little bit of channel 1 and then a little bit of channel 2 and then back up to 1 and so on.

Well with a digital scope there’s no need for chop mode at all. You just acquire both channels and display both on the screen which, after all, is really a random access device. So what’s alternate mode? Well suppose you are looking at two signals that don’t really have anything to do with one another. What you really want is two scopes, right? One to trigger on one signal and another to trigger on the other. You might not even want the same time scale for each channel. That’s what alternate triggering does. It splits the screen in half and each half is like an independent scope.

For example, see the picture at the head of this article. The top signal is another JavaScript creation. This one is a good candidate for pulse width triggering. Here’s the code:

io=createGP3();
io.openPort("/dev/ttyS0");
io.setLow(6);
while (1)
{
  io.pulseOut(6,100);
  io.pulseOut(6,50);
}
Just a long pulse and a short pulse over and over again. That's the top signal and a perfect candidate for pulse triggering.
The bottom half of the screen is just the scope's 1kHz calibration signal. No worries using edge trigger there. If you trigger on either signal alone, the other one will go crazy. Notice each half is even using its own time base (200uS/div up top and 1mS/div at the bottom).  But with alternate triggering its like getting two one channel scopes out of your dual channel scope. Of course, you lose some functions (notably delayed trace, a topic I'll cover some other time). But you gain the ability to see two unrelated signals at the same time. 

 

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