# Help in understanding Single pole filter in a STM Setup

1. Oct 16, 2007

### Tanja

Hi,

I'm working on a UHV STM. Recently I opened the multigain switch to set higher gains and I also changed the bandwidth.
In the manual I read that the bandfilter is a so called 'single pole filter' and has a 6dB/Octave roll off.

Can someone help me to understand what a single pole filter actually does?

(I studied physics and my knowledge in electronics and filters is very limited ... )

Tanja

2. Oct 16, 2007

### Staff: Mentor

Welcome to the PF. A single pole filter is the simplest filter. It will be either lowpass (it passes low frequencies but attenuates higher frequencies), or highpass (it passes higher frequencies and attenuates lower frequencies. Here is a general background piece on filters, with links to more information:

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

The term "6dB/octave rolloff" refers to the attenuation characteristic with respect to frequency. If you draw a plot of the attenuation of a lowpass filter versus frequency, it will look like the graph on this page:

http://en.wikipedia.org/wiki/Low-pass_filter

For every doubling of frequency above the cutoff frequency, you get double the attenuation of the AC voltage waveform (you get half the signal), which is where the 6dB per octave (and octave is doubling of the frequency) comes from.

Hope that helps.

3. Oct 17, 2007

### Tanja

Unfortunatelly the manual didn't provide more information on the single pole filter and I don't know if it is a low-pass or a high-pass or both. I guess both because it is possible to change the bandwidth by moving a jumper. Or is there any other use of the word bandwidth, e.g. like a synonyme for cut off frequency?

Do you know a good reference with details about the transfer function of a single pole filter?

I had a look at fremotares.org. and I'm kind of impressed. Citicens of Fremont can be very happy. It must be a good and save felling to know that there are people who have the knowledge and the skills in communications and are not only interested in earning money and working for big companies only.

4. Oct 17, 2007

### Staff: Mentor

The filter would be a lowpass, and the bandwidth is the width of the passband, from DC up to the cutoff frequency shown in the link.