LC Low Pass Filter Refuses to Work Properly - RF Cafe Forums
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
Post subject: Filter
Unread postPosted: Fri Apr 08, 2005 12:03 pm
There are two parts to the operation of a filter:
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?
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
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 (88-108MHz)?
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.
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 as expected.
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 contributed.
Unread postPosted: Wed Apr 20, 2005 6:20 am
You welcome. Nothing compares to experience!