Tektronix Timebase Secrets - Delayed Sweep and Variable Holdoff

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When buying an oscilloscope, it is important to know if you want one with delayed sweep and variable holdoff. This guide will explain some basics of these two Tektronix features. This will be basic stuff, no attempt will be made to go into technical details of operation or design.

Delayed Sweep

The first of these two sweep features applies to Tektronix vacuum-tube oscilloscopes with so-called "dual timebases" or "A and B sweep." These would be certain models of the 530 /540 / 550 series, the 585A and the 561 and 647. The purpose of delayed sweep is to provide an ability to view certain parts of an overall waveform in much greater detail. Why would you want to do this? Well, for instance you may wish to examine the leading or trailing edge of a pulse (the rise or fall). The sweep triggers on the leading edge at a certain point, perhaps too late to just turn up the sweep speed. Or you may not want to see the pulse that is doing the triggering, but a subsequent one. The delay acts like a variable window on the sweep, magnifying whatever potion you wish to examine in detail. Because the delay is calibrated, you can use it for very precise measurements of time within the waveform. This provides a much more accurate measurement than the oscilloscope graticule does.

I found it interesting in studying radar that the delayed sweep was used in enhanced sets developed for WW 2. Two types of radar, "A" sweep and "B" sweep, were used in different ways to provide both a long-range view, and a more detailed close-range view on the same scope. From what I have been able to gather, this worked just like our A and B sweep, and was likely its forefather.Here's how it works. A voltage comparator is driven by the "B" sweep. This compares the rising "B" sawtooth with a fixed voltage passed through a 10-turn precision potentiometer. When the voltage from the "B" sweep equals the voltage set on the potentiometer (the delay), it starts the "A" sweep. This is called "runs after delay," and is usually marked on the front panel "A delayed by B." As the "A" sweep is independant of "B" from that point on, a much faster sweep speeed can be set on "A", giving a detailed picture from the point of triggering until "A" sweep finishes.(Note: Here Im am assumoing that "B" is the "main" sweep, on some scopes it is reversed.)

Now this is good stuff, but how can we tell where on the waveform the "A" sweep is displaying? Pretty simple in retrospect, they basically fed the "A" sweep unblanking pulse to the crt grid while the "B" sweep pulse was fed to the cathode. This had the effect of making the "B" sweep trace brighter when the "A" sweep was running. Thus we get "B intensified by A." We can switch back and forth between "B" sweep and "A delayed" and see both views of the signal. By turning the 10-turn pot, we can move the "A" sweep start relative to the start of "B" and locate any portion we want to view with "A." The pot dial reads out directly in multiples of the "A" sweep speed.

A smalll variation of the above is triggered delayed sweep. In the description so far, the delayed sweep is started by the end of the delay period. In triggered delayed sweep, the delayed sweep is not started, but enabled, by the delay circuits. Then, when the next trigger comes in, it will start the delayed sweep.This is called "trigger after delay" as opposed to "runs after delay." Not all delayed-sweep scopes have both.

Where delayed sweep really shines is on a dual beam scope. A dual beam oscilloscope can display both traces at once, so we can see the overall "B" view and the faster window of "A" right below it. But with most dual-beam scopes, this requires both channels be fed from the signal source, increasing the loading and physical complexity of the measurement. The 556 allows both sets of vertical deflection plates to be fed from the same plugin. That means that we only need one probe to see both "A" and "B" sweep views on the same display, reducing circuit loading by half. With a mult-trace plugin, many different studies of the incoming signal are made possible.

Variable Holdoff

.First, let me say that variable holdoff is not related in any way to delayed sweep. It is just one of the other timebase mysteries that I am attempting to shed a few candlepower on. The variable holdoff control does not appear on any of the big vacuum tube scopes, but well it should. The need really didn't arise until oscilloscopes started to be used to view pulse trains and bit streams from computers. Starting with the 460-series, it seems to be pretty much standard.

What is "holdoff" anyway? When a sweep generator triggers, it runs until it is finished, and then stops. While the sweep is running, the trigger circuits are disabled so that another trigger cannot restart the sweep. The trigger circuits cannot accept another trigger until they are reset, which is the job of the holdoff circuit. When the sweep finishes, it doesn't reset the trigger circuits directly, instead it starts the holdoff circuit running. This is done because the sweep generator itself needs a chance to return to its initial state, ready to run again.The holdoff circuit waits for a few ticks after the sweep finishes, and then resets the trigger. How does it know how long to wait? It uses an RC circuit made up of a precision resistor and a capacitor, usually one of the timing capacitors used to generate the sweep. It waits a fixed interval, and then resets the trigger.

Now where this causes a problem is if the pulses you are viewing are not all the same, and you want to hold a certain one steady. Because the sweep will be triggered by the next pulse after the holdoff ends, the trace will jump and jitter and not display the pulse you are interested in. The variable holdoff control allows you to extend this holdoff time until the pulse you are interested in is the next one up. Then the sweep will be free to trigger just as that one steps up to the plate, and the trace will remain nice and steady.

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