Author Topic: Need help with RC circut waveform  (Read 394 times)

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Offline akintos

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Need help with RC circut waveform
« on: April 10, 2019, 02:04:11 pm »
I simulated the circuit described in first image with PSpice and got second image as result, which is expected.

(Value of resistor is 0.1k ohm and input signal is 5kHz square wave, -1 ~ 1V)

The problem is when I build same circuit in real life and measure input and output voltage(third image), I get almost same waveform.


I also attached 5kHz input signal images.

When I changed the resistor to 1k or 10k, the input signal was almost square (not perfect)


What can cause this?
« Last Edit: April 10, 2019, 02:11:03 pm by akintos »
 

Offline rstofer

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Re: Need help with RC circut waveform
« Reply #1 on: April 10, 2019, 03:41:51 pm »
Tau = R * C => 100 us
Full charge/discharge occurs at, say, 6 Tau or 600 us
Max frequency to see full cycle => 1 / 0.0006 = 1.67 kHz.
As you use higher frequencies, you won't get full charge or discharge waveforms.

Hit the circuit with 1kHz and change the time/div on the scope to get just one charge and one discharge.  The exponential nature will be more apparent when time is spread out.

Using 10k will change things, you would want a maximum frequency of just 167 Hz to allow enough time for full charge and discharge cycles.

There's nothing magic about 6 Tau, 4 or 5 Tau may be workable.  It depends on what you want to see.


As to the last trace, I'm not sure what is going on there.  What is the yellow trace?  Is that the input?
 

Offline rstofer

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Re: Need help with RC circut waveform
« Reply #2 on: April 10, 2019, 05:58:28 pm »
In the last figure, it the cyan trace is the input and the yellow trace is the output then it looks like the signal source has fairly high impedance.

When you simulate a signal source, you usually assume 0 Ohms output impedance and the only resistance in the circuit is the 1k.  But that isn't reality, everything has some kind of impedance.  It could be that your source has a pretty high impedance (600 Ohms?) and it is about equivalent to the 1k resistor.  Some signal sources have selectable output impedance of 600 Ohms and 50 Ohms.
 

Offline mikerj

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Re: Need help with RC circut waveform
« Reply #3 on: April 10, 2019, 07:59:31 pm »
Have you compensated your scope probes correctly?
 

Offline bson

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Re: Need help with RC circut waveform
« Reply #4 on: April 11, 2019, 03:44:21 am »
The linear ramping instead of 1-e^-kt charging indicates the signal source is current limited.
« Last Edit: April 11, 2019, 04:05:47 am by bson »
 


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