LC Low Pass Filter refuses to work properly - RF Cafe Forums
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Post subject: LC Low Pass Filter refuses to work properly
Unread postPosted: Thu Apr 07,
2005 2:47 pm
I'm very perplexed. I designed a 5th order LC lowpass (Chebyshev) filter to have
a cut-off frequency of 108 MHz (the upper frequency of the FM radio band) and have a minimum attenuation of 40 dB
at 216 MHz. I went through all the usual procedures like normalising the design and getting the frequency scaling
factor etc. The component values I obtained and the circuit layout is as follows: The input is connected between
C1 (38 pF) and L1 (115 nH), the other leg of C1 goes the ground. Then the second leg of L1 is connected between C2
(66 pF) and L2 (115 nH), the second leg of C2 also goes to ground. Now, the second leg of L2 is connected between
the output and C3 (38 pF), the second leg of C3 also goes to ground. Sorry I can't draw a picture here but its
just a basic layout. I chose to build an LC lowpass circuit as I thought there would be too much power dissipation
across the resistor in an RC lowpass design? The trouble is that when I built my lowpass filter it behaves the
same as a Notch filter. That is, the signal gets attenuated at (approx.) the designed cut-off frequency, but
slightly beyond this frequency the signal level shoots up to its original (unattenuated) level. I have used proper
components (trimmer capacitors and tuneable coils) and their values are very close to the ideal values listed
above. I kept my leads as short as possible, I screened the two inductors in case they couple with each other, I
etched some copper away from the ground plate under the inductors in case this might affect the coil Q. None of
these steps seem to make any improvement - my LC lowpass filter is still behaving like a notch filter! I have
simulated my design on ADS and it works almost exactly as designed. It should be so simple but I just don't know
why the thing isn't working in real life! You can probably tell that I'm quite frustrated by my filter (or lack of
filter!) so any helpful suggestions will be most welcome.
Unread postPosted: Fri Apr 08, 2005 12:03 pm
There are two parts to the operation of a
1. The filter circuit itself, including parasitics, and
2. The "context" - the circuitry surrounding
the filter. Since most filters work by reflecting the power that is to be rejected, in the stopband the input VSWR
gets really bad.
That means that you generally don't want lots of transmission line between filters and
whatever drives them - cable length can affect filter response in odd ways. So: How are you testing this filter?
Unread postPosted: Fri Apr 08, 2005 2:46 pm
Thank you for your reply. I am testing the filter on a Network Analyser. I have also looked at it
(my filter) on the spectrum analyser. That is, I input a known signal from the signal generator and sweep through
the frequency range and no matter what way I look at it or no matter what I do, it still behaves as a Notch
filter. I have built it and re-built it, I have tried different components, I have reduced the length of my leads
etc. Now I am going to try an build it one more time. I'll keep you informed.
Unread postPosted: Fri Apr 08, 2005 4:26 pm
I really don't underatand why
you use tuneable coils? Why not use fixed value wire wound coils, like those manufactured by Coilcraft.
From the description of your problem sounds like you have some sort of resonance frequency in your filter. Could
be that one of your coils is working at its SRF (Self-Resonance Frequency). Use fixed value components with high Q
as possible for the inductors (as their Q is the dominant).
What is the BW of your filter is it 20MHz
Build your filter on a board with good ground plane. Use vias to connect between the GND nodes
to the ground plane. You should also take into consideration the Er and thickness of the substrate (even tough
this is a relatively low frequency, this might affect the filter's performance), the Er of the filter will also
dictate the 50 ohm trace width.
Unread postPosted: Sat Apr
09, 2005 9:57 am
I agree with the gentlemen above. At this low frequency the simulator should be "dead
nuts" on the response. Most likely, the tuned coil is not behaving as you think it is, as suggetsed. I bet if you
replace the tunable components with fixed components, like coilcraft coils and ATC caps, your filter will behave
Unread postPosted: Tue Apr 19, 2005 12:09 pm
I'm happy to report that I eventually managed to get my LC lowpass filters working, thanks in no small
part to the suggestions made in response to my question. I took the advice of using wire wound ceramic chip
inductors rather than tuneable coils. This change, along with keeping copper/wire lengths to a minimum and
establishing very good grounds, helped to make the filter function properly. Once again, many thanks to those who
postPosted: Wed Apr 20, 2005 6:20 am
You welcome. Nothing compares to experience!