# Input impedance and Max RMS ac voltage

Discussion in 'General Electronics Discussion' started by Davewalker5, Sep 26, 2014.

1. ### Davewalker5

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Sep 20, 2014
Measuring PWM waveforms, why would a tech want to know the Mark to Space ratio?

Mark to Space Ratio = High time / Low Time

I have never measured the Time Interval from Leading Edge of Pulse#1 to the Falling Edge of Pulse#2

I have never measured the Time Interval from Falling Edge of Pulse#1 to the Falling Edge of Pulse#2

Any Applications or when a Tech would measure the time interval like this?

2. ### davennModerator

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Sep 5, 2009
aka On time vs OFF time

well that gives you the duty cycle and from that you can work out power requirements etc

nor have I, as that isn't the mark space ratio

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010

Assuming that mark = high, then yes.

I'm not surprised.

The first one seems to measure two marks and one space. I can't think why you'd want to.

The second measures the period of a single cycle. This is a very common thing to do.

4. ### Davewalker5

125
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Sep 20, 2014

So can you please give me examples of when a tech or when you have done this in your experience so I can learn from

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
FFS

• You measure from the rising edge to the falling edge to find out how long the signal is high.
• You measure from the falling edge to the rising edge to determine the low time.
• you measure between two rising (or two falling) edges to determine the period.

6. ### Davewalker5

125
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Sep 20, 2014
I'm not talking about measuring the pulse width of a Pulse

Rising Edge of Pulse#1
TO the next
Falling Edge of Pulse#2

This is measured for what type of application? or why would a tech do this?

True, but in my experience i have only measured the period of two rises

I have never measured the period from Two falling edges , when would a tech do this? for what type of application?

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
You're an idiot.

NOBODY would measure from the rising edge of one pulse to the falling edge of the next one. (OK, maybe someone would, but there would have to be some specific reason that's peculiar to a particular signal. It's normally a totally useless thing to measure.)

Re-read what you said. Then imagine that the signal is inverted.

8. ### Davewalker5

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Sep 20, 2014
I agree with you

But hold on , and look at the chart again

Look at Time Interval A+ To B-
Look at Time interval A- To B-

Why would any tech measure these types of time intervals?

This is what i'm saying it doesn't make sense

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9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
That's 2 separate signals (not two pulses). If they're controlling some edge sensitive device any of the 4 could be applicable.

10. ### Davewalker5

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Sep 20, 2014
Are you saying because some logic TTL or CMOS chips have an enable input or trigger input that can be either on the leading edge or falling edge , but have to check the datasheets to tell the tech if the TTL or CMOS chip is a leading edge or falling edge

Or What are you saying, explain more please?

Applicable to what?

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Devices that are "edge triggered" are edge triggered.

The thing being controlled.

Let's say that you have a counter which has a "start" and a "stop" input. Both of these are edge triggered.

Now imagine that both are triggered on the rising edge, then imagine if both are triggered on the falling edge.

Then consider if one is triggered on the rising edge and the other on the falling edge. Now swap them over.

Do you see the 4 combinations?

12. ### Davewalker5

125
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Sep 20, 2014
Yes , it's ment of devices that have different edge triggered

If one TTL is edge triggered on the rising edge and another TTL is edge triggered on the falling edge

This is the only example i can think of , is when to use the 4 combinations

13. ### Merlin3189

250
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Aug 4, 2011
A light-hearted explanation in response to DaveWalker5

It might be more correct to say that, "the inverse of the dimension of time is the dimension of frequency".

It takes me half an hour to get to work, but the frequency of my going to work is not twice an hour or 24 times a day.

My frequency of going to work is 5x a week or 1x a day (which of course look like different frequencies), but my time at work is not a fifth of a week, nor 1/1 day.

Frequency has the dimension "per second" but that is not its full title! Before they decided honour Heinrich we called it "cycles per second" and since "cycles" is a dimensionless number it does not appear in the unit dimensions.

A single shot event is not cyclical and does not have a frequency. If there were a cycle - say, you trigger the event, make some measurements, jot them down, then try again - then the time for that cycle would be the inverse of the frequency of your running the test.

I have been trying to think of another unit like this, but the only ones I could find are the Bequerel which is also "per second" and wave number which is "per metre" -neither of which seems helpful to the intuitive understanding of this idea.

You are however in good company in making this leap of imagination. In digital signal processing one has to take a finite sample of a signal, which is not even going to be an exact multiple of cycles of any cyclic content. Mathematically you imagine that this sample is repeated infinitely in both senses of time and when this is processed, not surprisingly you find frequencies which are not really there and you distort the relative amounts of those that are there. But underlying all that jiggery-pokery is the assumption that a single finite sample, such as you get from a one-shot event, COULD represent a meaningful slice out of an infinite repeating signal.

Also, if you fed your one-shot square pulse signal into a spectrum analyser, the biggest peak would be the frequency corresponding to to the reciprocal of twice the duration of the pulse. (Maybe the second biggest peak, if you include zero frequency? I'm not sure how real spectrum analysers handle this.)

All that aside, You are, as people have said, much better off measuring the time for slow signals and calculating a frequency if this is appropriate.
As for the 5V into 50 Ohm, 10V into 75 Ohm, etc, I'm as mystified as you. I can see the need for a MINIMUM signal to ensure accurate triggering. But the only reason I can see for a MAXIMUM signal, is to protect the components (mainly semiconductors from excess voltage and resistors from excess current.) In which case the answer would be, if you exceed these the circuit will be damaged and simply not work any more!
In many situations an excessive input will cause distortion of the waveform, but for a counter, as far as I understand it, all that matters is the the zero crossings of the signal. Typical compression caused by overload would not affect this, I think.

Last edited by a moderator: Sep 29, 2014