Hi All,

I need to convert a PMW signal to an analog signal. To do this, i'll prob need an LC filter. My load resistance is 4ohms and cut off frequency is 20kHz. (1) Is there anyway to find the value of L and C?

(2) Is the value of L and C dependent on the input signal?

Because when i did simulation, they would always ask for the "AC amplitude" for doing small signal analysis. (3) What is this AC amplitude for a PWM signal?

Thanks!

 

Have you tried using Elsie?
http://www.tonnesoftware.com/elsie.html
It's for LC filter design.

Try a capacitor-input low-pass Chebychev 5th order with a cutoff around 18kHz and a bandpass ripple of around .8 or so.

 

4 ohms is low, thus high current. Is you pwm output capable of the required current. What are you driving and is the load inductive by its self ( a motor for example )? This seems to be more than a signal processing aplication.

 

With a load of 4 Ohms and a cutoff of 20kHz, I'd bet money that our OP is working on a class D audio amplifier.

If that's the case, they won't really want to use a Chebyshev filter. It has a nice brick-wall-like response cutoff, but lots of ripple in the passband.

 

Yup! It's a class D audio amplifier So i should use butterworth right? Using Elsie program it has a resistor at the input. Does it matter what this value of the resistor is? Because i don't think my data sheet actually writes out the value.

 

Use 4 Ohms, as that's what your output is. The filter will not perform impedance matching. If your amp's output impedance doesn't match your speaker's input impedance, that's not the filter's problem

 

Go ahead and try a Butterworth, just to see what it looks like. 5th order.

Then try a Constant K, bandwith 24k, 5th order, still 4 Ohms.

Then try an M-derived, BW 21k.


Analysis settings:
Start freq: 1Hz
Stop: 30k
Analysis steps: 401
Sweep: linear
X intervals: 10
Transmission bottom:-21
Delay top (secs): 3m
VSWR top: 2.2
Impedance top: 1.2k
Nr of y intervals: 7
Q of inductors: 150 (your mileage will vary here)
Q of capacitors: 3k
Transmission: absolute