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LC Low Pass Filter refuses to work properly - RF Cafe Forums

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RF Burns
Post subject: LC Low Pass Filter refuses to work properly
Unread postPosted: Thu Apr 07, 2005 2:47 pm

Dear all,
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.


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Guest
Post subject: Filter respons
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?

Good Luck


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RF Burns
Post subject:
Unread postPosted: Fri Apr 08, 2005 2:46 pm

Hi,
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.


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Guest
Post subject:
Unread postPosted: Fri Apr 08, 2005 4:26 pm

Hello,

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 (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.


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Guest
Post subject:
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 as expected.


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RF Burns
Post subject:
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.

Respectully yours

RF Burns


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Guest
Post subject:
Unread postPosted: Wed Apr 20, 2005 6:20 am

You welcome. Nothing compares to experience!





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