# Square wave through low-pass filter

Discussion in 'Electronic Basics' started by Amanda Robin, Jun 17, 2004.

1. ### Amanda RobinGuest

Hello,

I am sending different kinds of wave shapes through a couple of
amplifier circuits that include low-pass filters (LTC 1164 8th order
linear phase). The amplification is down with op amps and a Burr Brown
INA 118 instrumentation amp. My sample frequency is 1000 Hz and cutoff
frequency is about 500 Hz. When I input a sinusoid at 125 Hz, the
output wave shape is the same. When I input a square wave, the shape of
the output wave is smoothed so it resembles a sinusoid.

I can give more details if necessary, or e-mail a JPEG of the plots. I
am just wondering whether this is typical characteristic of either
low-pass filters or the amplification.

Thanks in advance for any help. Book titles or links to relevant web
info about signal shape in general would be welcome as well.

Thanks,
Amanda

2. ### CaptainGuest

Amanda,

When you say sample frequency, are you refering to the clock frequency of
the LTC 1164. This should be 50 or 100 times the frequency of the signal to
be filtered, so if you are trying to use 1 KHz, you will be filtering out
everything above either 10 Hz or 20 Hz, depending on how you have set up the
filter.

If on the other hand you made a typo and meant 10 KHz, this would reduce the
amplitude of anything above either 100 Hz or 200 Hz. Assuming you have the
filter set up to attenuate anything above 200 Hz, then a 125 Hz sinusoid
will pass unaffected. However, a square wave consists of the base frequency
sinusoid and all odd harmonic sinusoids with amplitudes decreasing as their
frequency increases.

The third harmonic of 125 Hz is 425 Hz, which is above the 200 Hz filter
pole. However, no filter is perfect so some of this harmonic may remain to
be added to the basic sinusoid at the output, which would explain why you
have a shape which resembles a sinusoid but isn't an exact match.

If you need to investigate this further, e-mail me with more details and
I'll see what I can do to help

Cap

3. ### Bob MastaGuest

The 3rd harmonic of your 125 Hz sine wave is right near the 500 Hz
cutoff frequency, so it will be attenuated about 3 dB... or maybe
6 dB, since some people use a 6 dB criterion for Bessel and
linear phase filters. Higher harmonics will be attenuated by about
6 dB for every doubling of frequency above 500 Hz. So yes, your
square wave will look pretty smooth.

You mention "sample rate"... is this filter to be used ahead of
an A/D converter? Many folks read about Nyquist criterion and
are scared by textbook authors into using anti-aliasing filters,
Then they realize that the filter is going to screw up the
waveshape ulness they use Bessel or some sort of linear
phase, which have absolutely terrible cutoff behavior. The
upshot is that these are almost never suitable for the job
unless you have lots of poles and place the cutoff well
below Nyquist. If you are in fact using this for data
acquisition, please write back and tell us the details
of the application. There is almost always a better way.

Bob Masta