Retro Gadgets: Make Your Oscilloscope Dual Channel
We live in a time where having an oscilloscope is a small luxury. But for many decades a good scope was a big expense, and hardly any hobbyist had a brand new one unless it was of very poor quality. Oscilloscopes were big and heavy, and at the price most people were willing to pay, they had only one channel. Of course, having a channel is better than nothing. But if the relative advantage of having a single channel reach is 10 points, the advantage of having two channels is easily at least 100 points. So what was a poor hacker to do when a double-trace or better scope was too expensive? Why, hack, of course. Many designs converted a single-trace oscilloscope into a poor quality multi-channel oscilloscope. Heathkit has made several over the years, such as the ID-22, ID-101 and ID-4101. They called them "electronic switches". The S-2 and S-3 were even older models, but the idea was not unique to Heathkit and had been around for some time.
For $25 you can change your scope to dual-trace!There were two common approaches. With alternate or alt mode, you can trigger a sync pulse and draw a trace. Then trigger again and draw the second trace with a fixed voltage offset. If you do this fast enough, it looks like there are two traces on the screen at the same time. The other method is to quickly switch between voltages while sweeping and use the Z input of the oscilloscope to erase the trace when it is between signals. This requires a Z input, of course, and a fast switching clock. This is sometimes called "chopper mode" or, simply, chopper. It wasn't just the realm of adapters, though. Even "true" two-channel analog oscilloscopes used the same methods, although usually with the advantage of being integrated into the oscilloscope electronics.
InteriorsThe old ID-22 was tube-based and fairly simple. The internal design of the S-3 was almost exactly the same.
The ID-22 used 11 tubes and a rectifierHea...
We live in a time where having an oscilloscope is a small luxury. But for many decades a good scope was a big expense, and hardly any hobbyist had a brand new one unless it was of very poor quality. Oscilloscopes were big and heavy, and at the price most people were willing to pay, they had only one channel. Of course, having a channel is better than nothing. But if the relative advantage of having a single channel reach is 10 points, the advantage of having two channels is easily at least 100 points. So what was a poor hacker to do when a double-trace or better scope was too expensive? Why, hack, of course. Many designs converted a single-trace oscilloscope into a poor quality multi-channel oscilloscope. Heathkit has made several over the years, such as the ID-22, ID-101 and ID-4101. They called them "electronic switches". The S-2 and S-3 were even older models, but the idea was not unique to Heathkit and had been around for some time.
For $25 you can change your scope to dual-trace!There were two common approaches. With alternate or alt mode, you can trigger a sync pulse and draw a trace. Then trigger again and draw the second trace with a fixed voltage offset. If you do this fast enough, it looks like there are two traces on the screen at the same time. The other method is to quickly switch between voltages while sweeping and use the Z input of the oscilloscope to erase the trace when it is between signals. This requires a Z input, of course, and a fast switching clock. This is sometimes called "chopper mode" or, simply, chopper. It wasn't just the realm of adapters, though. Even "true" two-channel analog oscilloscopes used the same methods, although usually with the advantage of being integrated into the oscilloscope electronics.
InteriorsThe old ID-22 was tube-based and fairly simple. The internal design of the S-3 was almost exactly the same.
The ID-22 used 11 tubes and a rectifierHea...
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