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LC filter (Read 7684 times)
frank23
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LC filter
Oct 29th, 2010, 8:56am
 
Hello,

I have designed an LC bandpass filter and now in the middle of testing it. I attached the circuit in multisim with simulation. Filter resonant frequency is 38.2 MHz, 3db BW is 2 MHz with 50 ohm load. I have a question about testing a filter.

I built a resonant tank of 220nh and 79pf with 100k for coupling. I tried to test the Q of the coil, which should be more then 150, but I got very small (about 4-8). As the BW was quite high about 12 MHz instead of 1 MHz.

Any suggestions where I went wrong? The inductor I was using is

http://www.bec.co.uk/PDF/5K%20Series.pdf

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LC_bandpass.JPG
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grumpy_bear
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Re: LC filter
Reply #1 - May 5th, 2011, 6:41am
 
Hello,

how was your design approach?
You state that you have 100k for coupling, but there is nowhere in the schematic a 100kOhm resistor? Did you find this in a textbook or design it by an algorithm or found the schematic in a book and calculated the rest on your own?

How did you try to test the Q of the inductor? With the schematic and your simulation setup, you simulate the Quality factor of the filter, which is a fundamentally different from the Q of your inductor!

More information is required if you want help here

Cheers
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loose-electron
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Re: LC filter
Reply #2 - May 8th, 2011, 3:14pm
 
It looks like you have two LC tanks that are then AC coupled together?

No representation of the Q for the inductors?

Odd.

Suggest you open a book on filter design as a starting point. The two tuned circuits are going to affect the performance of each other.
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Jerry Twomey
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rfcooltools.com
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Re: LC filter
Reply #3 - Jun 20th, 2011, 2:58pm
 
frank_23,

your filter looks like a coupled tank bandpass.  These type of filters are very sensitive to component placement ,accuracy of the coupling elements of which non zero impedance in the ground is a coupling element.  If your filter is wider than expected you may want to reduce the coupling factor some how because to much coupling will push the tanks frequency apart.  
First convince your self that each resonant circuit is resonant at the desired frequency
Next I suggest making your coupling capacitors incrementally smaller or even remove them altogether and see if the ground plane is the cause of the problem.

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