Oscilloscope triggering circuits can be adapted to other applications, as precision synchronizers. Both analog and digital ‘scope triggering has been enhanced by various auto-trigger schemes. This tutorial overview of ‘scope triggering circuits and schemes presents the basics of trigger generators and also some enhancements.
Oscilloscope triggering systems appear at the front-panel as two basic controls: trigger level and slope. Implementation can be envisioned in its simplest form as a comparator and slope polarity selector, as shown below.
The comparator outputs an edge when the voltage waveform from the trigger source, which can be the internal trigger signal from the vertical amplifier, an external BNC connector, or the 50/60 Hz power line, crosses the voltage level set by the TRIGGER LEVEL control. Then an XOR gate either inverts or not the comparator edge, effectively selecting which polarity of edge will be the active (positive-going) edge as the trigger event.
The above scheme is not generally practical, for it lacks the ability to select which of the sequence of active trigger edges will start the scope sweep, or in a DSO, start waveform digitization and storage. In analog scopes, the sweep must not be restarted until it has completed, retraced back to its starting position on the left side of the screen and settled. During this time, holdoff timing keeps the sweep from running. Holdoff is added using a D flop, and the trigger circuit grows as shown below. The flop also eliminates comparator output bounce from slow inputs.
The D flop trigger output at Q is kept low by the assertion of holdoff at the reset (R) input. While held off, the trigger is low. When holdoff releases, the next trigger from the XOR gate causes Q to become high, thus generating the sweep gating function for analog scopes.
Adding the flop to the above trigger generator might seem all that is needed, though one more refinement is required. What if the holdoff releases at the moment a new trigger asserts the clock input of the flop? With insufficient setup time, a race condition can occur causing the Q output transition to be delayed by an indeterminate amount of time. It becomes high after some delay, starting the sweep late, and causing trigger jitter on the screen as the sweep starts late relative to the triggering event; or it might generate a glitch or “runt” pulse. To avoid this, an additional flop is triggered after a delay, as shown in the next addition to the trigger circuit, shown below.
The delay line lets the second flop be triggered somewhat later than the first, giving the output of the first flop time to settle to a valid logic level before the second flop is clocked. The delay time is chosen so that an indeterminate level at the D input of the second flop occurs for a statistically-insignificant fraction of the input triggers. This scheme is a basic practical triggering circuit. It is essentially what is found in most triggering oscilloscopes in some particular implementation. And it is what is referred to on the front-panel by the third trigger control - the mode control - as the normal triggering mode.
A mode is a particular structural configuration of an electronic system. Modes are selected by electromechanical or electronic switches. Added to the basic triggering scheme in some scopes is the auto triggering mode, a mode where the triggering system triggers itself. Why would we ever want that to happen? In normal mode, if no input waveform is present, no trace will occur on the scope screen. There is nothing to see. Consequently, we do not know where the trace is set by the vertical position control and do not know where 0 V, or ground, is either. On analog scopes, the trace might not even be on the screen were it to run. Some means of forcing a trigger is needed.
This problem is solved with the auto triggering mode. The auto circuit inputs the output trigger, firing a MMV (one-shot) with a timeout that exceeds the period of a 50 Hz waveform. The reasoning is that the line frequency is the lowest frequency we would want to trigger on without interference from auto triggering. The MMV is retriggerable and does not change output state until no trigger occurs for its timeout duration. When it times out, it generates an auto-trigger and forces the sweep to run. With no actual trigger input, auto-trigger runs the sweep at a somewhat less than line-frequency rate. Then when a source trigger comes in, the auto MMV is shut off again and disappears.
The next advance in triggering convenience beyond the auto triggering mode is peak-to-peak auto. In this mode, attention is turned to the trigger level control. For a given waveform the range of the trigger level will be wasted beyond the voltage range of the waveform for no triggering will occur at the range extremes. To make the full range of this control useful, positive and negative peak detectors acquire the maximum and minimum voltages of the waveform and output them to the two ends of the trigger-level pot, shown below.
There is no setting of the pot outside the range of the trigger-source waveform. Tektronix 7000 series scopes have featured this triggering mode. One disadvantage of this scheme is that the peak detectors must function at the full bandwidth of the scope. At high frequencies, the detectors do not respond fast enough to output the exact peaks and the trigger range becomes less than the peak-to-peak range of the source waveform.