# RL bandpass filter

1. Oct 12, 2009

### Idea04

Does an RL low pass and high pass filter need to have the same impedance in the circuit as to the source impedance to work properly. I put an inductor in series with the source with a resistor to ground as the low pass, and a resistor in series with the source with an inductor to ground as a high pass. and there is no filtering of the voltage signal.

2. Oct 12, 2009

### Staff: Mentor

It doesn't need to, but the source impedance certainly comes into play. What transfer functions do you calculate for the HPF and LPF configuratoins? How are you measuring the filter characteristic?

3. Oct 12, 2009

### Idea04

I calculated a transfer function of .002. I measured the signal by having two different multi meters, one for ground and one for output. The voltage gradually increased, but I didn't get any current amplification.

4. Oct 12, 2009

### Staff: Mentor

A transfer function is a function of frequency, not a number. You will get a polynominal function that varies with the frequency of the excitation.

http://en.wikipedia.org/wiki/Transfer_function

You don't measure the transfer function with multimeters. You can measure it with a signal generator and a dual-channel oscilloscope, though.

5. Oct 12, 2009

### Idea04

I'm a bit confused here. To measure the output voltage, frequency and current I have to use an oscilloscope. Why won't a multimeter measure the signals voltage, frequency, and current properly.

6. Oct 12, 2009

### Staff: Mentor

Multimeters are usually used to measure DC values and AC values up through AC Mains power distribution frequencies (50/60Hz). You may find some that can go a bit higher in frequency on the Vac setting, but not very far. Check the specifications for your multimeter -- what is the max frequency that it says the Vac setting can handle?

What frequency range do you want to test the filter transfer function over?

7. Oct 12, 2009

### Idea04

The frequency I am trying to filter is 60Hz, My meter can read to 1 MHz. The meter measures the signal voltage and it gradually rises, but with the components I used I should have a higher current through the output. The calculations show an output current of 1.7 amps but I don't get any output current. So the filter must not be working properly.

8. Oct 12, 2009

### Staff: Mentor

A 1MHz bandwidth -- that's an impressive multimeter!

BTW, you aren't making measurements connected directly to the AC Mains line, are you? What is your voltage source? Those are larger currents than you get with most signal generators.

Can you post a sketch of your circuit and how you are making the measurements? If you can make a PDF or JPG file, you can add it as an attachment.