Post subject: LC Low Pass Filters
Sat Mar 19, 2005 3:32 pm
Joined: Sat Mar 19, 2005 3:20 pm
have constructed a 5th order LC low-pass filter to cut-off at 108
MHz (the upper frequency of the FM radio band) with a minimum attenuation
of 40 dB at 216 MHz. I connected my antenna to the input of this
filter and connected the output to the spectrum analyser. I found
that I am still able to receive substancial frequency components
well above 108 MHz. Should these not have been attenuated?
postPosted: Sun Mar 20, 2005 9:45 am
Check the filter by
itself ona network analyzer. I fyou do not have acces to a network
analyzer then sweep the input of the filter using a sweeg generator
or analyzer and see the response. If the filter has the rejection
as designed then the filter passband may be getting "pulled" by
the mismatch of the antenna.
Post subject: Low-pass Filter
Unread postPosted: Sun Mar
20, 2005 10:27 pm
Well, it might be that things are
working as designed - even if not as intended.
filter has 3 bands: the passband, the transition band, and the stopband.
The edge of the passband is generally defined as the 3 dB point
for Butterworth filters, and the frequency where the attenuation
first exceeds the passband ripple for Chebyshev and Elliptic-function
(Those last two would have either a parallel tuned circuit
in the forward path, or a series tuned circuit as a shunt path.)
So it sounds like you've designed a filter with the top edge
of the passband at 108 MHz, and the bottom edge of the stopband
at 216 MHz. In between, you'll have something that varies between
3 and 40 dB of attenuation, depending on the design and construction.
You'll still see signals at those frequencies on a spectrum analyzer!
Do also note that circuitry at theese frequencies can act as
antennas, especially inductors, so you might have to shield the
filter from the environment to get the desired performance.