# lt spice low pass rc filter

#### Neil1990

Joined Nov 13, 2020
9
Hi all,

I hope you can help me. I've recently started studying a class called electronic instrumentation and it's very new to me. I'm a complete beginner in this field. For the following circuit, I need to:
A. Measure the signal (amplitude and frequency) with a multimeter and oscilloscope
B. measure the output amplitude using an oscilloscope and the gain for different frequencies (50Hz, 100Hz, 500Hz etc.)
C. Measure the phase shift at 50Hz, 1.6kHz and 50kHz
D. Report these measurements on semi-long paper in order to draw the bode diagram in amplitude (real) and in phase (asymptomatic)

I'm sure this is really quite simple, but I really don't know what I'm doing for this class.

#### Papabravo

Joined Feb 24, 2006
16,086
So this is single stage RC filter.
The schematic is from the simulation package LTspice.
Are you asking about how to measure things in a simulation or how to measure them in a real circuit?

#### Neil1990

Joined Nov 13, 2020
9
So this is single stage RC filter.
The schematic is from the simulation package LTspice.
Are you asking about how to measure things in a simulation or how to measure them in a real circuit?
Yeah in a simluation.

#### Papabravo

Joined Feb 24, 2006
16,086
Yeah in a simluation.
So when you run the simulation a waveform viewer will show up. Click on the node between the resistor and the capacitor in the schematic and you will get a plot of the log magnitude versus log frequency. there will also be a plot of phase versus log frequency. Using the cursor in the waveform viewer you can read the value of any waveform attaching the cursor to the waveform and dragging it along.

#### Audioguru again

Joined Oct 21, 2019
3,171

#### crutschow

Joined Mar 14, 2008
27,170
Yeah in a simluation.
Well you can't use an oscilloscope for that simulation, so it sounds like you are supposed to do this with a real circuit. (?)

#### Papabravo

Joined Feb 24, 2006
16,086
This whole thread is "strange" beyond belief.
1. "Amplitude and frequency with a multi-meter"??
2. "measure phase" with a multi-meter??
3. "semi-log" paper?? (used my last piece of that 50 years ago)

#### Audioguru again

Joined Oct 21, 2019
3,171
My Fluke multimeter can accurately measure the True RMS amplitude and the Frequency of a signal.

#### Papabravo

Joined Feb 24, 2006
16,086
My Fluke multimeter can accurately measure the True RMS amplitude and the Frequency of a signal.
Good to know. My two Fluke Mutimeters (77 & 117) have worked so well for so long that I've not been motivated to upgrade.

#### Bordodynov

Joined May 20, 2015
2,865
Neil1990
I will not show you a simple way. As it was said in the Russian children's film: "Real heroes always go around". I will show you an alternative way.
Consider the diagrams shown as oscillograms. I did it in two ways. It is Fourier analysis and post-processor processing of oscillograms with Meas commands. For the process to be established, I calculated 100 signal periods and output the last three periods. The first period of the signal is not the same as in the established mode. I found a delay on the first front =T1 and T2. Then I calculated the phase and frequency, equal to the reverse period.
Received data from the output file, I put on the circuit as a comment (blue font).

#### Attachments

• 3 KB Views: 9

#### Neil1990

Joined Nov 13, 2020
9
This whole thread is "strange" beyond belief.
1. "Amplitude and frequency with a multi-meter"??
2. "measure phase" with a multi-meter??
3. "semi-log" paper?? (used my last piece of that 50 years ago)
I should probably mention that I'm studying at a French university and I'm translating the questions into English. I'm anglophone but because I'm not so familiar with the subject it's possible that my translations arent 100% correct.

#### Alec_t

Joined Sep 17, 2013
12,060
it's possible that my translations arent 100% correct.
They look fine, except that 'asymptomatic' should be 'imaginary'.

#### Papabravo

Joined Feb 24, 2006
16,086
You appear to be in a difficult position that most of the rest of us never encountered. We learned how circuits worked by building them, measuring them, and rebuilding them over and over and over again. When simulation came along we had an enormous amount of intuition about how to use the simulator as a tool. In your case you seem to be trying to learn circuits and the simulator at the same time. This has led you to a situation where you are overwhelmed by the need learn both simultaneously. You may not have the luxury of taking the time to learn these things separately. The best advice I can give you is to divide the problem into smaller pieces and attack the pieces one at a time until you build a foundation for continued progress.

#### crutschow

Joined Mar 14, 2008
27,170
Run the simulation and then click on the connection between the resistor and capacitor, and tell us what you see.
[If you label that junction (e.g. Out) then the graph will also have that label].

#### Neil1990

Joined Nov 13, 2020
9
Run the simulation and then click on the connection between the resistor and capacitor, and tell us what you see.
[If you label that junction (e.g. Out) then the graph will also have that label].
I managed to run the simulation and got my Bode plot for Vout, here are the results I got for the questions shown above. Does this look like I'm on the right track?
 50Hz​ 100Hz​ 500Hz​ 1kHz​ 1,6kHz​ 2kHz​ 5kHz​ 10kHz​ 50kHz​ -4.3mdB​ -17.1mdB​ -410.6mdB​ -1.5dB​ -3.0dB​ -4.15dB​ -10.4dB​ -16.1dB​ -29.9dB​ -1.8°​ ​ ​ ​ -45.2°​ ​ ​ ​ -88.2°​

#### Neil1990

Joined Nov 13, 2020
9
You appear to be in a difficult position that most of the rest of us never encountered. We learned how circuits worked by building them, measuring them, and rebuilding them over and over and over again. When simulation came along we had an enormous amount of intuition about how to use the simulator as a tool. In your case you seem to be trying to learn circuits and the simulator at the same time. This has led you to a situation where you are overwhelmed by the need learn both simultaneously. You may not have the luxury of taking the time to learn these things separately. The best advice I can give you is to divide the problem into smaller pieces and attack the pieces one at a time until you build a foundation for continued progress.
Yes, and learning it in a different language is adding another level of difficulty. I would really like to learn how the circuits work by building them, I do have an Arduino but I don't know if it's useful for this particular class. If you know of any good tutorials online, please let me know, and thanks for all your help so far!

#### Papabravo

Joined Feb 24, 2006
16,086
Yes, and learning it in a different language is adding another level of difficulty. I would really like to learn how the circuits work by building them, I do have an Arduino but I don't know if it's useful for this particular class. If you know of any good tutorials online, please let me know, and thanks for all your help so far!
There is an electronics textbook associated with this site. It might be a good place to start.
Are you familiar with the relationship between voltage and current in a resistor?
How about that same relationship in a capacitor, as a function of frequency?

#### crutschow

Joined Mar 14, 2008
27,170
Does this look like I'm on the right track?
Yes.
Those component values give you a 1st order, low pass filter with a -3dB corner frequency (1/2piRC) of 1.59kHz and a -6dB/octave rolloff above that frequency, which is what you have measured.

#### Neil1990

Joined Nov 13, 2020
9
Yes.
Those component values give you a 1st order, low pass filter with a -3dB corner frequency (1/2piRC) of 1.59kHz and a -6dB/octave rolloff above that frequency, which is what you have measured.
Great, thanks!

I'm having some difficulty with the next question as well. This has again been translated from French so it may not be 100% accurate:

Step response,
Inject a square signal of very low frequency, for example 10Hz (again using the LT spice schematic above)
- observe the output signal at the oscilloscope. Measure the response time at 5%. Compare with the theoretical value

If I understand correctly, this question is asking me to comment on the time taken to charge the capacitor? My response was that at 4 time constants the capacitor has reached the transient period and as such, is almost fully charged. At 95%, it's just below the transient period.

Is this correct?

#### Neil1990

Joined Nov 13, 2020
9
There is an electronics textbook associated with this site. It might be a good place to start.
Are you familiar with the relationship between voltage and current in a resistor?
How about that same relationship in a capacitor, as a function of frequency?
I'll check that out. I also noticed that on another thread you recommended a book called Analog filter design? I'm assuming this could be useful for me as well. Do you have any other recommendations?

As for those relationships you mentioned, are you talking about Ohms law?