Chop Mode used for measuring logic signals

When using a multi-channel oscilloscope to measure logic time relationships or verifying a timing diagram it's best to use the chop mode. The Alt mode causes timing issues like positive edge time offsets. Why does using the chop mode locks and sync in the timing relationships on multi-channels of logic signal compared to using the alt mode which doesn't lock and sync multi channels of logic signals. It might have something with the scanning of the positive edge triggers in chop mode compared to using alt most scans.

Chop? Alt? How old is your scope???? Jeez, buy a new digital scope, they're cheap enough. Alt mode is definitely not the way to do any timing measurements.

barry said:

Alt mode is definitely not the way to do any timing measurements.

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Rubbish - alt mode is perfectly satisfactory and I've been doing it for 40 years. Clearly you don't know how such scopes actually work. You do need stable trigger and a repetitive signal, of course.

Chop mode, OTOH, is only suitable for signals where the time difference is much greater than the chop frequency.

I believe that the A+B mode is the best. The chop rate is normally .5-2 MHZ so its difficult measuring time differences of nanoseconds. Like wise the Alt mode leaves leaves you with the uncertainty of the trace flyback time.
Frank

So use the Chop mode when measuring the time differences for logic signals that are high frequency in the MHZ. When do you use the alt mode to measure time difference in logic or analog circuits?

johnburford said:

So use the Chop mode when measuring the time differences for logic signals that are high frequency in the MHZ.

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No. If the chop frequency is 10kHz (period 100ns), then you cannot see time differences less than 100ns. Simply fire up the scope and see to see why!

When do you use the alt mode to measure time difference in logic or analog circuits?

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Providing you have a stable repetitive signal (and therefore trigger), the alt mode is normally the best mode.

Chop mode is design to lock to not sync to the incoming repetitive signal but lock to an offset with at least 100 Hz difference frequency to create a smooth trace.

Alt mode synchronizes to the selected polarity from the selected source and with retrace blanking , may or may not be synchronous to the incoming signal. THis problem of an injection locked loop or pleiochronous syncing causes potential correct image capturing of all channels.

SunnySkyguy said:

Chop mode is design to lock to not sync to the incoming repetitive signal but lock to an offset with at least 100 Hz difference frequency to create a smooth trace.
Alt mode synchronizes to the selected polarity from the selected source and with retrace blanking , may or may not be synchronous to the incoming signal. THis problem of an injection locked loop or pleiochronous syncing causes potential correct image capturing of all channels.

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I'm sorry, I really don't understand what you are trying to say there.

A scope's operation is really quite simple...

There is a trigger, which is derived from channel A or channel B or ext trig. This is completely independent of chop/alt mode.

When the trigger occurs in chop mode, both traces are alternately "sampled" at the chop frequency, and displayed one above the other during the same sweep. Yes, that is a perversion of the word "sampling", but it serves to highlight that time differences less than the sampling/chopping frequency are invisible (only one channel is displayed at any given time).

When the trigger first occurs in alt mode, channel A is displayed for the complete sweep. On the next occurrence of the trigger, channel B is displayed for the complete sweep. Provided that the signals on channel A and B are repetitive, the signals on channel A and B therefore shown in the correct relative positions on the display. That means that any time differences within the scope's front-end analogue bandwidth will be visible.

Note that alt mode is even useful when the signals are not repetitive. Providing the trigger is stable, the "eye diagram" is displayed. Note also that the eye diagram is a key specification and test method for high-speed signals, where signal integrity need to be assured.

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chuckey said:

I believe that the A+B mode is the best.

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That entirely depends on the signals being measured. A universal statement has to be wrong.

Like wise the Alt mode leaves leaves you with the uncertainty of the trace flyback time.

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No. You misunderstand how scopes work.

Measuring logic signals are occur simultaneously using a Dual Trace Oscilloscope in chop mode will have different time intervals and time alignment measurements between the two signals will be different between the two signals when switching to alternative mode. The dual traces will be misaligned using alternative mode and the logic signal the occur simultaneously will be misaligned.

In Alternate Mode: The top trace is triggered and the scan begins when the scan finishes then the next positive edge of the top trace triggers the lower trace and then that scan begins.The Reason is that the scope scans the traces alternately. This will cause a nanosecond delay between dual traces so the positive edge will be misaligned and won't occur together.

Both ch1 & ch2 are selected as the trigger to measure timing relationships between two signals with respect to each other. Switching modes from chop to alternative you will see how the positive edges will be misaligned. Digital signals are aperiodic because that have no repeating pattern so you have to use chop mode.

Chop electronic switching rate is from 100K to 500k. The electronic switch is "independent" from the sweep generator frequency. In Alternate the electronic switch is synchronized to the sweep generators frequency. There is a change over and flyback period of the sweep. The Time relationships of the dual traces is different when using chop mode compared to using alternate mode because of the time relationships of the dual traces.

Sorry to confuse you but Tektronix had the best chopper stabilized trigger system in the business that never locked on the incoming frequency no matter what sweep rate was used. That's because it locked to an offset frequency to create intentional blur or an alias frequency that prevented flicker in the chop rate.

Weaker designed scopes in chop mode would occasionally lock onto the incoming signal sync'd to the sweep rate to give a square wave between signals.

Is it true to use chop mode when analog or digital signals/waveforms are aperiodic not repeating a pattern?

The Alternate mode triggers on the positive edges of the incoming signal. The Chop mode is free running it doesn't trigger to any signals edge. The scanning is alternate mode has a delay time in nanoseconds from ch1 to ch2, using chop mode the scanning has no delay time between ch1 to ch2.

If you measure a logic buffer input signal compared to it's output signal using alternate mode you will have a delay time in nanoseconds from input to output. Switch to Chop mode and the delay time in nanoseconds is not there from the buffers input signal to the buffers output signal. The oscilloscope in alternate mode adds a delay propagation from the alternate scanning and where it's triggering edge point is referenced.