RC timing measurements using a DVM frequency mode doesn't work why?

[danny davis]

There is RC networks in logic circuits

I put my Fluke 87 DVM meter and set it to Frequency MODE and put my probe on the junction/via between the RC network

Why doesn't it tell me the frequency of the RC network?

Isn't between it's an exponential curve , charging and discharging curves it can't convert the RC into frequency on the meter?

You can calculate the RC time which you can convert to frequency , so why doesn't the fluke 87 DVM in frequency mode measure anything on the Via's of RC networks?

I tried measuring a clock output signal at 0.5hz and another one at 0.25hz, Both of them didn't get picked up by the Fluke 87 DVM meter why is that? the waveform is to low for a DVM meter?

 

[Audioguru]

The frequency counter in a Fluke 87 has its specifications in its owner's manual online:
150mV RMS minimum sinewave level from 0.5Hz-200kHz. If the signal has pulses then they must be >0.2us. It measures the frequency of an oscillator, not an RC network.

Why don't you download its owner's manual and study it??

 

[danny davis]

It measures the frequency of an oscillator, not an RC network.

Yes it does fine measuring Clock output but it has a problem measuring clock output signals at are 0.5hz and 0.25hz , why is that?

Why can't it not measure a RC network? i don't get it

RC is frequency , the fluke 87 is a frequency counter , so whats the problem? i don't get it?

Or what can measure the RC network? since a frequency counter doesn't work, what will please?

 

[Audioguru]

A frequency counter measures the frequency of a signal. An RC does not produce a signal unless it is in an oscillator then the output frequency of the oscillator can be measured by a frequency counter.

I do not know why you want to measure the timing of an RC network. Maybe it is used for a delay. The values of the resistor and capacitor should not change unless they are obviously damaged.

 

[danny davis]

I do not know why you want to measure the timing of an RC network.

What will measure the Time or frequency of an RC network?

Because the delay time or pulse duration it "out of tolerance" , and I have to change the parts
plus i need to fine tune the delay time of an RC network

A frequency counter measures the frequency of a signal.

Yes an RC network is a frequency and time, so it should work right? i don't get why a frequency counter can't work

The values of the resistor and capacitor should not change unless they are obviously damaged.

They drift out of tolerance
There are 10% resistors and Capacitors and 5%


Why won't the Fluke 87 frequency counter measure 0.5hz or 0.25hz?

 

[Audioguru]

An RC network IS NOT a frequency so a frequency counter cannot measure it. A frequency counter measures the frequency of an oscillator.
I have not seen a 10% resistor for about 52 years. If it drifts more then replace it.

Your meter will measure the 0.5Hz frequency of the output of a clock oscillator, not an RC network.

 

[danny davis]

Your meter will measure the 0.5Hz frequency of the output of a clock oscillator

I tried it and it doesn't work, it won't measure a 0.5 hz or a 0.25hz output of a clock or oscillator

An RC network IS NOT a frequency so a frequency counter cannot measure it.

RC= TIME

R is 1K
C is .01uf
Time = 10uSeconds , 10 micro seconds
Frequency = 100Khz

The Fluke 87 frequency counter should measure the 100Khz on the VIA of the RC network

- - - Updated - - -

Supply voltage is 5 volts
VC output voltage is 2 volt
R is 1K
C is .01uf

Time Delay is 5 milliseconds for the RC to reach 2 volts

5 milliseconds = 200hz

The Fluke 87 frequency counter should measure the 200hz of the RC network to reach 2 volts right?

Why can't a DVM meter measure this, i don't get it

 

[Audioguru]

I told you a few times why a frequency counter cannot measure the frequency of a resistor connected to a capacitor.
I told you a few times that a frequency counter measures the AC (alternating) frequency of the output of an oscillator.
Please GO AWAY!

 

[danny davis]

I told you a few times why a frequency counter cannot measure the frequency of a resistor connected to a capacitor.

No, you didn't please explain why? it's because why?

I can CYCLE/switch back and forth the output "state" which causes the capacitor to charge and and discharge

So why can't the frequency counter measuring the frequency of an RC network? i don't get it

RC= TIME

R is 1K
C is .01uf
Time = 10uSeconds , 10 micro seconds
Frequency = 100Khz

The Fluke 87 frequency counter should measure the 100Khz on the VIA of the RC network right?

I told you a few times that a frequency counter measures the AC (alternating) frequency of the output of an oscillator.

Why Can't a frequency counter measure the charging or discharge frequency of an RC network? i don't get it

What would I have to do to the frequency counter for it to measure an RC networks frequency? or time?

Some DVM meters measure Duty Cycle, do I have to use this setting to measure the RC network frequency or time?

 

[WimRFP]

Danny, maybe you can tell us the actual problem you encounter (without your own interpretation) so that others can/may provide you a solution.

Audioguru is very right, an R and C don't have a frequency. If it takes me 2s to go from A to B, it doesn't mean that my frequency is 0.5 Hz.


R and C can only act as the timing components in an oscillator, however the actual frequency depends not only on R and C, but also on the oscillator type and dimensioning (such as relative hysteresis).

It is not going to work the way you are acting now.

 

[danny davis]

an R and C don't have a frequency.

How can an RC not have frequency when RC=TIME , TIME = Frequency

RC= TIME

R is 1K
C is .01uf
Time = 10uSeconds , 10 micro seconds
Frequency = 100Khz

The Fluke 87 frequency counter should measure the 100Khz on the VIA of the RC network right?

 

[Audioguru]

TIME is how long it takes for a capacitor to charge from a powered resistor or how long it takes to be discharged by a resistor. The voltage either goes up one time or it goes down one time. It is not a frequency.

A FREQUENCY is a continuing alternation where the voltage goes up then down then up then down over and over so many times per second. It is not a time.

A frequency counter measures a frequency, not a time. A stopwatch measure time, not a frequency.

A 0.01uF capacitor charges to 63.2% of the voltage (one time-constant) through a 1k resistor in 10us only one time so it is not a frequency.

A 1k resistor feeding a 0.01uF capacitor to ground can be a lowpass filter. Its -3dB frequency is 16kHz. Higher frequencies are reduced -6dB per octave.
A 0.01uF capacitor feeding a 1k resistor to ground is a highpass filter. Its -3dB frequency is also 16kHz. Lower frequencies are reduced -6dB per octave.

Why did you never learn about electronics? Why are you not learning electronics on these forums?

 

[danny davis]

TIME is how long it takes for a capacitor to charge from a powered resistor or how long it takes to be discharged by a resistor. The voltage either goes up one time or it goes down one time. It is not a frequency.

A FREQUENCY is a continuing alternation where the voltage goes up then down then up then down over and over so many times per second. It is not a time.

Ok I was confused the TIME = Frequency , I always thought you can convert a TIME to frequency , or you can convert a frequency to TIME, that's what they taught me at electronic school

A frequency counter measures a frequency, not a time. A stopwatch measure time, not a frequency.

I always thought you can convert a TIME to frequency , or you can convert a frequency to TIME, that's what they taught me at electronic school

The Charging time and discharging time is not a frequency , but it's a time

What they taught me at electronic school was you can always convert a TIME to frequency , or you can convert a frequency to TIME

 

[WimRFP]

You can do that, but whether it is usefull or not depends on the physical phenomenon. In the time of travel example, frequency has no usefull meaning.

It can have a usefull meaning when you keep travelling back and forth.

If you make a low pass RC filter, then one can say the the frequency where the attenuation is 3 dB equals 1/(2*pi*R*C). Your DVM can determine the frequency of a signal (AC voltage), but not the -3 dB frequency of an RC filter.

To determine the -3 dB frequency of the RC filter, one needs to measure its tranfer function, draw the graph (attenuation versus frequency) and read the frequency where the attenuation is 3 dB.

 

[danny davis]

Some DVM meter can measure the TIME, like for cars, they have DVM meters for cars , and they have a duty cycle mode and TIME mode

But also fluke meters can measure duty cycle and time measurements

Why can't a DVM meter measure the frequency or time of an RC filter or network? it's because there is no AC waveform or AC voltage on the input of the RC network

What would the DVM meter have to do in order to measure the frequency or time of an RC network? The DVM meter would have to inject an TEST SIGNAL : AC waveform or AC voltage TEST VOLTAGE into the VIA of the RC network?

 

[godfreyl]

Yes, that's right.

 

[WimRFP]

You are right, you need to inject some signal to see the behavior of an RC combination in the time or frequency domain.

There are various ways to determine the product of RC (tau). For frequency domain method, see my posting #14, you need a (variable) frequency source and a means to measure the amplitude. One may use fixed or switchable frequency source and have a phase sensitive detector as one can also determine RC product based on phase difference between input and output voltage.

For time domain method charge the RC combination to 1V and measure the time it takes to discharge to 0.368V (to be more exact: 1/e). This would require a voltage source, a switch to connect and disconnect the voltage source and a comparator to measure when the voltage decays to 0.368V.

Other method is to put the RC combination in an hysteresis (Schmitt Trigger) oscillator and measure the frequency of that oscillator. Based on the hysteresis and output voltage swing, one can calculate the RC product. Search for schmitt trigger oscillator.

 

[danny davis]

You are right, you need to inject some signal to see the behavior of an RC combination in the time or frequency domain.

What can I inject?

I have Tricked and tripped the logic combination on the input of the gate or op amp so the output switches states going to the RC network , this changes the RC network to charge and discharge , I put my Fluke 87 meter on frequency counter mode and place my probe on the VIA of the RC network, shouldn't it measure a frequency? or how can this be done please? without using an O-scope?

 

[Audioguru]

You still do not understand about an RC network and a frequency counter. I give up.

 

[danny davis]

You still do not understand about an RC network and a frequency counter.

No, I don't understand why they don't work together , I'm trying to inject a signal or an external power supply or something to make it work

What I'm trying to do is to measure the frequency of the RC network using the frequency counter on my Fluke 87 meter, how can this be done?

THe problem is my meter is not injecting an AC test signal into the RC network to measure the frequency of the RC network , so i can measure it on the VIA of the RC network

What can I do to make this work?

The O-scope will display the charge time and the discharge time, but I want to measure the frequency of the RC network

Once I have the frequency of the RC network , i can then convert the frequency into TIME, then I will know the RC networks TIME