.56 seconds and .28 seconds time measurement on an analog oscilloscope

[danny davis]

.56 seconds and .28 seconds time measurement on an analog oscilloscope

I had to do these time measurements at work using an analog oscilloscope

All I could get was a Dot moving up and down , is this because the pulse waveform is to fast or to slow?

I had to use a storage analog oscilloscope and turn the moving Dot into a stored waveform

The Oscilloscope turned the moving DOT swinging from +10 to 0 into a Pulse square waveform , how did it do that?

Without using a Storage analog oscilloscope , how do you use a analog oscilloscope to measure the One time and Period of a waveform that is .28 seconds?

 

[betwixt]

What you saw is absolutely normal, the 'line' drawn on a CRT oscilloscope is actually a single dot, it is the afterglow of the phosphor and persistence of vision that make it look continuous. At slow sweep speeds, both these effects are insufficient and you see the travelling dot instead.

An analog storage scope and all digital scopes work by digitizing the analog signal at whatever rate you select but then reeatedly playing it back at higher speed so the POV and phosphor again give the apearance of a continuous waveform.

It is still usually possible to use a scope without storage but as you have seen, it isn't the best instrument for measuring long periods. The trick is to use ensure the sweep is triggered at the start of one of the graticule markers, this is best done by either ensuring the trigger level is set properly or by using single sweep mode. When the dot moves across, look closely for the graticule position when it jumps from one level to the other. The delay is the length of graticule multiiplied by the sweep rate per division.

Brian.

 

[danny davis]

So when you see signals or waveforms that are a travelling DOT, are these really SLOW waveforms or Low frequency waveforms?

How do you see a Continuous waveform when you're measuring waveforms that are a Travelling DOT?

At Slow Sweep Speeds on the O-scope, What kind of waveforms are these? are they short?, long? low frequency?

I have tried single sweep and I couldn't get the it to CATCH or lOCK onto the waveform to give a Travelling DOT or a continuous waveform

A .28 second waveform? how does a Electronic Tech know how to calculate on the SWEEP SPEED on the O-scope what the setting should be? to give a continuous waveform?

 

[betwixt]

I think you are misunderstanding. ALL analog scopes ALWAYS show a single dot. The phosphor that glows to produce the display has a decay time, it doesn't extingush imediately so if the sweep time is fast enough, it replenishes the glow before it has had time to decay and hence it looks like a line rather than a dot is being shown. An analog storage scope and all digital scopes work on a slightly different principle. Instead of displaying the waveform 'live', they repeatedly measure the signal voltage at short intervals using an ADC and store the result in a memory. The values represent the shape of the waveform, rather like the Y axis values if you plotted the waveform versus time on a graph. The X axis values are the sample number stored in the memory. By reading the samples at higher speed and moving the dot along the X asis as you do so, then moving the dot up or down according to the sample value, you redraw the digitized waveform. The big advantage of this is you can 'loop' the reading of the memory at a higher speed than you made the recording so the dot redraws the screen fast enough that the waveform looks continuous.

The kind of waveform and the sweep speed are not related, an oscilloscope display is simply a graph of voltage (your waveform) against time (the sweep speed). You adjust the sweep speed so you can observe the relevant part of the waveform you want.

In single sweep mode, you will never 'lock' the waveform. Single sweep does just that, you have to press the reset button before it will sweep again. However, it can be very useful to find the best trigger level setting. If you select single sweep mode and keep pressing reset while adjusting the trigger level, you should find a level that waits for the start of your waveform before the sweep starts. Leave the trigger level at that setting and the go back to continuous sweep mode, it should now lock the waveform to the display.

As far as setting the optimum sweep speed, consider what you want to measure. It's a time interval. The X axis is calibrated in small divisions across the graticule (graph lines on the display), how long each of these represents will depend on the sweep speed you set using the timebase control. You will see it is marked something like "Secs/Division", each 'division' is one square on the graticule. So if you selected 1 second/division, the dot would travel across one division in one second. Using that setting, your .28 second waveform would last .28 divisions. If you increased the timebase speed to say 0.1s/division, the dot would travel ten times faster and your waveform would last 2.8 divisions on the graticule. You select the sweep speed that best scales your waveform so you can count the graticule divisions easiest. The time will always be the number of divisions multiplied by the time it takes the dot to travel per division, in other words 'sweep speed x length of waveform'.

Brian.

 

[danny davis]

If you know the pulse period time , how do u calculate the sweep speed on the o-scope?

pulse period time is .28 seconds , how do u calculate the sweep speed?

 

[betwixt]

You normally use one of the pre-set sweep speeds on the timebase selector switch. You would use a setting that showed the pulse as wide as possible so you get most divisions across it on the screen and therefore most resolution.

If the sweep speed is unknown, which would be strange, given that any test instrument should be calibrated, you divide the number of divisions along one cycle of your waveform and divide it by the length of the cycle.

Brian.

 

[danny davis]

To measure .28 seconds you need a sweep speed of .20 seconds to see it Display a waveform to measure it
My 0-scope only goes to .5 seconds

500milliseconds = 2hz , What is the formula to calculate the sweep speed?

280milliseconds = 3.57hz, What is the formula to calculate the sweep speed?

I know the Period time and frequency but I don't know what sweep speed I should use , I'm just moving the sweep speed knob back and forth until i get a waveform but i'm guessing

1 millisecond = 1khz, But how do I calculate what the sweep speed should be?

Example:
The Formula should be waveform Period Time ( which is 1 millisecond) X or divided by what ? = Sweep Speed on O-scope

- - - Updated - - -

you divide the number of divisions along one cycle of your waveform and divide it by the length of the cycle.

1 millisecond Divided by 10 divisions on the 0-scope is 100uSec. micro-seconds

Divide it by the length of the cycle? what is this , the time period is the length of the cycle which is 1 millisecond

 

[adi0803199]

The timebase is measured in time/division.First of all,we have options to measure the time in Seconds,milliseconds and microseconds.This can be seen from the round dial in the oscilloscope.To determine the how much time has elapsed for the oscilloscope to move through one square unit(1cm by1cm),you change the knob on the time base.From this you can determine the time elapsed for the signal to travel one square unit along the X-axis(time axis).Suppose say you keep the knob at 1ms/division,it means that the waveform takes 1 ms to travel across 1 square unit in the horizontal direction.The best way to determine the frequency is by turning the time base knob to try to fit the whole screen of the oscilloscope display so as to get the accurate signal time period.

To measure the period,assume the timebase is kept at 1ms/div.The waveform takes 3.5 squares before repeating itself again,then the time period is ,

TimePeriod(waveform)=Timebase(1 ms/div)*Length occupied in terms of number of squares(3.5 squares).

 

[betwixt]

@danny davis, what you say makes no sense. First you say the pulse time is 0.28 seconds, then you say the length of the cycle is 1mS, that's a difference of 280 times!

A cycle, when referring to a waveform is the part of the waveform from start to finish. In a repetitive waveform, it is the part from a point on one cycle to the same point on the next cycle.

You should be using the oscilloscope like a stopwatch, the trace starts when triggered and you look for where it ends. The dot moves across at a known speed per division so if you count the number of divisions you can calculate the time. As with any distance calculation, you use an appropriate unit of measurment. That unit is set by the timebase speed selector knob.

Example: you want to measure the length of a car. It would be inappropriate to use a map of your whole country and try to figure out the length by thinking of the car as a tiny spec of ink and looking it up against the map scale. Equally, you wouldn't use a micrometer and measure out thousands of tiny lengths and add them all up. You would use something, for example a tape measure, which was just a bit longer than the car. On an oscilloscope you have an adjustable scale, the timebase speed selector, which lets you 'zoom in and out' to find the most appropriate length of measure.

It is entirely up to you to pick the best speed, you do it so the waveform you are investigating fits an appropriate number of units across the screen to take the best measurement. It's visual, not done by calculation.

Brian.

 

[jiripolivka]

From all it can be clearly seen that an analog oscilloscope is not a good instrument to measure slow pulsed signals. One can use a digital storage oscilloscope which has a memory, but the best way is to use a simple lab counter.
To be sure that the counter shows the wanted parameters of such slow pulsed signal, the digital storage scope is fine to accompany the counter.