# 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
Bench Equipment have different input and output impedance and Max RMS voltage

50 ohms = Max 5 volts RMS
75 ohms = Max 10 volts RMS
1K ohm = Max 15 volts RMS
1 Meg ohm = Max 20 volts RMS

How did you get the RMS voltage? without knowing the current?

What happens when you put a higher AC voltage RMS into what is rated max RMS?

Is it easier to overload the input of a bench equipment that is 50ohms compared with bench equipment that is 1K ohms or 1 meg ohms?

example:

Universal Counter: 50ohms , max 5 volts RMS

If you put 10 volts RMS or 15 volts RMS into the input , what will happen? i know it will overload it

It will cause the Universal counter to do what?

Does Universal Counter track frequency better using a square waveform compared to a sine waveform?

2. ### davennModerator

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Sep 5, 2009
you don't need to know current
Vrms = Vp / 0.707 (Vp = Peak voltage)

cheers
Dave

3. ### Davewalker5

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Sep 20, 2014
50 ohms = Max 5 volts RMS

But why does 50 ohms have a max 5 volt RMS? how did they get this

75 ohms = Max 10 volts RMS

Why does 75ohms have a 10 volt RMS?

1K ohm = Max 15 volts RMS

Why does 1K ohms have a max 15 volt RMS?

1 Meg ohm = Max 20 volts RMS

Why does 1 meg ohm have a max 20 volt RMS?

4. ### davennModerator

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Sep 5, 2009
not knowing how the circuit was designed, I cannot answer those questions
Other than saying .... that was their design specifications

Dave

5. ### Davewalker5

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Sep 20, 2014
So the input resistance and the max voltage RMS has nothing in relationship?

Isn't all bench equipment that is 50ohms always 5 volts RMS? and 75 ohms is 10 volts RMS?

Its standardized to this for all bench equipment

6. ### davennModerator

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Sep 5, 2009
They may do ... if so I don't know what the relationship would be ?

no, its whatever the designer decides what he wants as a maximum input voltage

I have one freq counter that can handle only 10mW, I have another that can handle 100 W

Dave

7. ### Davewalker5

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Sep 20, 2014
When you overdrive the input on a frequency counter, does the display not stable, display is random?

Also when using banana red and black banana wires from a function generator to a frequency counter, can this pickup 60 cycle hum and other radio interferences causes the counters display to not be stable and displaying random?

What is the best wires to used that is shielded? but has to have a banana plugs

8. ### davennModerator

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Sep 5, 2009
that's possible, but I would be more worried about doing physical damage to the sensitive input of the counter

yes it can, if the other sources were similar or greater in strength than the signal you are trying to measure

either coax or good screened audio lead ( coax of correct impedance if dealing with higher freq's and you don't want to upset impedances too much)
keep the open length of wire from the end of the screened lead as short as possible where it goes to the banana plugs

Dave

9. ### Davewalker5

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Sep 20, 2014
Thanks I will try the Coax cable instead of the red and black banana cables

Since my frequency counter is 50ohms , Max 5 volts RMS

If you're circuit under test needs a 10 volt peak, how do you get 10 volts peak ( 20 volts peak to peak ) on the O scope but also have the frequency counter hooked up at the same time without damaging it?

Do i need an external attenuator box just for the frequency counter to not damage it when I need to have a higher voltage ?

10. ### davennModerator

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Sep 5, 2009
there are banana plug to BNC connector adaptors for that job

yes

11. ### Davewalker5

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Sep 20, 2014
Are they called BNC attenuation boxes? or what are they called for electronic bench equipment to do attenuation for inputs like this?

Also does frequency counters track low frequency better using a square waveform compared to sine waveform?

Also, what is the advantage of using a frequency counter to measure time duration compared to a O-scope?

I think a Universal counter is just faster compared using an Oscope to measure short time duration or time periods for single shot event signals

Because for Low frequency 1hz to 5hz to 10hz , to measuring on the Oscope is hard to make the waveforms stable on digital Oscope, the triggering is set to auto mode

I have a hard time getting low frequencies to display stable on a Digital o scope , they are free running, why is that?

Low frequency single shot event signals is better to use a frequency counter?

12. ### Harald KappModeratorModerator

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Nov 17, 2011
You can use a simple series resistor to attenuate the input signal to the counter. If the counter has 50Ω input impedance, putting another 50Ω in series with the input will attenuate the signal by 2 (the series resistor and the input impedance form a resistive voltage divider). For more or less attenuation chose another series resistor.

Any counter will operate more reliable when provides with a square waveform. Internally any waveform is converted to a digital signal (square wave) anyway for processing by the counter's logic. This is done by a comparator circuit (usually with adjustable threshold). A sine waveform crosses the threshold of the counter comparatively slowly. Add some noise and you'll have multiple pulses near the threshold instead of one single pulse (by "add some noise" i do not mean that you have to add noise intentionally, any noise, hum etc. picked up by your setup will have this effect).
At higher frequencies this effect is less seen because the sine signal crosses the threshold faster and there is less time for noise to create additional pulses.
Your counter may (or may not) have a filter that can be switched into the signal chain to reduce high frequency noise somewhat.

A counter is typically more accurate than an oscilloscope. You can read the period of a signal from an analog oscilloscope. The accuracy is limited by the accuracy of the timebase which in an anlog scope may be 3% or so. A digital oscilloscope may offer higher accuracy and often can display the frequency directly, but is limited to teh resolution of the sampling circuit. A counter is an instrument dedicated to counting (or measuring frequency) and is therefore optimized to minimize these problems.

For low frequencies I'd recommend not to measure the frequency at all. Measuring a frequency means counting the number of signal changes within a fixed period of time (e.g. 1 second). A low frequency signal doesn't change very often within a second. As a worst case example consider a 2Hz signal and acounter counting for 1second. Depending on the moment in time you start counting, you may see 1 or 2 pulses within that one second. Your display will be highly unstable. You'd have to measure much longer for good resolution.
Typically you measure the period of a low frequency signal instead. A counter normally has a period measurement setting. The roles of signal and gating (gating sets the time for measurement, imagine a gate that lets the signal pass only while it is open) are reversed: The counter generates an internal, precise timing signal at high frequency. The signal is used to control the gate for this high frequency reference clock. The number of pulses counted within one period of the input signal is a direct measure of the time of that period. You get frequency from f=1/T.

A single shot does not have a frequency. The frequency is a measure of the number of times a signal repeats within one second (that is why the unit of frequency is 1Hz=1/s). A signle shot doesn't repeat and therefore has no frequency. You can, however, measure the duration (not period, becasu a signle shot is not periodic) of a single shot event.

P.S.: This seems to be about the 3rd thread you opened concerning counters. Please stay within one thread. This is easier for others to follow and give advice as well as it is easier for you than to match replies from different threads.

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13. ### Davewalker5

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Sep 20, 2014
True, but a single shot waveform time duration is measured in TIME

I thought the inverse of time was frequency , so it must have a frequency

What kind of time duration are we measuring?

I think you mean to put a 50ohm resistor across the Red and black terminals of the universal counter

I can put a 50 ohm resistor across the red & black banana to coax adapter

If you put it in series , how would u do this?

Yes the counter has a Gate

O-scope don't have a Gate? or is the counters gate the same things was a trigger input? or something different?

There is also a Gate delay or hold knob, on my counter, when is it good to set it for what kind of waveforms?

I have a counter that has a 50 ohms , 5 volts RMS input
I have another counter that has a 1 meg ohm, 20 volts RMS input

Does the RMS input voltage rating mean it's sets the triggering or gate input voltage?

Because the counters Gate or triggering input is a peak detector circuit or a comparator circuit

14. ### davennModerator

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Sep 5, 2009
Dave

I don't mind helping you out BUT you really have to start learning to help yourself at the same time
having us guys spoonfeed you isn't going to help you much in the end

frequency describes a regular periodic change in waveform .... a single shot pulse doesn't count

15. ### Harald KappModeratorModerator

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Nov 17, 2011
YOu have the countes. Do you have the manuals? Read the instructions, they tell you how the device operates and which adjustments you can make.
If you don't have the manuals, find them on the internet or read at least a tutorial, e.g. this one.

16. ### Davewalker5

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Sep 20, 2014
This is what the manual says HP 5313B

Take a look if u want:
http://exodus.poly.edu/~kurt/manuals/manuals/HP Agilent/HP 5315A, B Operation.pdf

Gate Time- is the time for which the counter is counting the number of pulses or transition crossings

Gate Time- controls the resolution of the measurement

A --> B :
Measure the Time interval between a START signal at INPUT A and a STOP signal at INPUT B

What I don't understand is why would you want to do measurements from a Rising Edge to a Falling Edge?

Mostly I have done time duration measurements from Rising Edge to the Next Rising Edge, not to the next falling edge

Take a look at the 4 different combinations of measuring Time durations , but when all they used

I have just done Rising Edge to the Next Rising edge

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17. ### davennModerator

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Sep 5, 2009
If its a square wave the is the ON time length of the pulse and in many circumstances that is what the tech wants to know. He may also measure the off time as well so he can determine the Mark/space ratio

18. ### Davewalker5

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Sep 20, 2014
The Square waveform ON TIME Length is from Rising edge to the next pulse rising edge?

Or the Rising edge to the next pulse falling edge?

What you mean the the Mark/space ratio? the duty cycle you mean?

Not sure why a tech would want to know the mark/space ratio

The off time is the low state time from falling edge to the next pulse rising edge

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

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Jan 21, 2010
ON = High = 1, so ON time is from the rising edge to the falling edge (technically from the point of the rising edge where the voltage reaches 90% of the final voltage to the point on the falling edge where it falls below 90%)

If you were to read wikipedia, then yes.

However duty cycle has a more nuanced meaning.

It's often a critical thing to know!

True. Again, technically often a 10% threshold applies.

For practical computations the time spent between 10% and 90% are usually ignored in any calculation of mark/space ratio.

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20. ### davennModerator

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yes
one of the main applications being in PWM ( Pulse Width Modulation ) control

cheers
Dave