# coils, pulse trains and resonance

Discussion in 'Electronic Design' started by Matt, Jan 4, 2004.

1. ### MattGuest

I have been doing some testing with finding out what the resonant
frequency of a parallel connected L-C circuit is.

I connected the L-C circuit to a 1k resistor which one end was
connected to ground.

I then supplied a sine wave accross the whole circuit.

I then connected one trace from an oscilliscope to the top of the L-C
circuit and the another trace from the top of the 1k resistor

Sine wave input >---------------------------------> To Trace 1
| |
| ---
Capacitor --- --- Coil
--- ---
| ---
| |
-------------------------> To Trace 2
|
|
---
| | 1K Resistor
| |
---
|
|
-------------------- GND
I compared the two traces on the oscilloscope, starting from a low
frequency
I started increasing the input frequency. Trace two bacame smaller and
smaller until the point of resonance where the trace stated to
increase in size.
This is all normal and what I would expect to see.
The voltage drop accross 1k decreases because less current is flowing
through the resister due to the resonant circuit regecting the current
at resonance.

Now when I change the input wave form from a sine to a pulse of say
20% duty at the same frequency both trace one and trace two look
pretty identical. So this would seem that no current reduction through
the 1k resister is seen. Which in turn means that the L-C circuit is
not rejecting any current.

So heres the question
Does the resonant frequency change in an L-C circuit based on the
input wave form?

2. ### valentin tihomirovGuest

Talking about frequency in AC we mean sine waveform as opposed to rectangle
in digital electronics. A pulse consists of multiple sine+cos signals (cos
is a shifted sin, see how to convert any signal into this form in Fourier
chapter of math.) of different amplitudes. This means, there is no specific
frequency you can tune your filter on.

3. ### Kevin AylwardGuest

No.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

That which is mostly observed, is that which replicates the most.
http://www.anasoft.co.uk/replicators/index.html

4. ### MattGuest

So in theroy if the circuit was in resonance with a sine wave it should be
in resonance at the same frequency with a pulse train then?

Is this what you are suggesting?

5. ### James MeyerGuest

Dear Kevin,

It is seldom necessary to quote an entire posted question simply to add

Jim

6. ### James MeyerGuest

Dear Matt,

It is seldom necessary to quote a single word reply and the question
that prompted it in order to make a reply of two sentences.

Jim

7. ### Fred BloggsGuest

A pulse train is actually the superposition of an infinity of sinusoidal
voltages at all integer harmonics of the pulse repetition frequency.
Resonance of linear circuits relates to the response to sinusoidal input
functions.

8. ### MattGuest

Dear Jim,
It is obtuse to make such remarks in the context of this discussion.

Thanks for you input!

9. ### Kevin AylwardGuest

Resonance is a property of the system, not of the input signal.
Technically, if it is non-linear the circuit resonance can change, e.g.
if the capacitance or inductance changes with the input signal
instantaneous voltage, but by default it is usually assumed that one is
discussing a linear circuit.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

That which is mostly observed, is that which replicates the most.
http://www.anasoft.co.uk/replicators/index.html

10. ### Paul BurridgeGuest

Followed by a dozen lines of sig. :-(

11. ### Reg EdwardsGuest

No. Why should it?

12. ### Peter BennettGuest

Yes - However, you must remember that a square wave is the sum of many
components - the fundamental frequency (where your circuit is
resonant) and all odd harmonics (where it isn't).

13. ### Wim LewisGuest

And a pulse train with a 20% duty cycle will have less of its energy at
the pulse repetition frequency than a 50-50 square wave would. Still,
I would expect to see some distortion as the input signal is tuned
through the resonance frequency. (Or as any of the harmonics pass
through the resonance.)  