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akrcal
Joined: 14 Aug 2006
Posts: 25
Helped: 2
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 13 May 2008 7:04 Band PAss filter at 70MHz |
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I am designing band pass filter with
centre frequency : 70 MHz
pass band : 62-68 MHz
Rejection : 20dB(at)58 MHz, 20dB(at)72MHz
I have used air core inductor and silver mica capacitors. I am taking fifth order for designing it. When I am simulating it with ADS I am getting good results but when I am fabricating and testing with VNA then I am getting pass band more than what I have achieved in simulation. What could be the reason and how to make precise and accurate band pass filter at VHF using lumped element? Is there any good reference material for that?
Regards,
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tyassin
Joined: 22 Jul 2004
Posts: 243
Helped: 30
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| 13 May 2008 7:18 Re: Band PAss filter at 70MHz |
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Hi
Maybee you are already doing it, but one thing you can/must do, is to use models for your inductors and capacitors. They can sometimes be found from vendors.
Regards
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FvM
Joined: 22 Jan 2008
Posts: 2633
Helped: 431
Location: Bochum, Germany
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| 13 May 2008 8:09 Re: Band PAss filter at 70MHz |
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| I think, the capacitors could be regarded ideal except for series inductance (that may be modelled with systematic value), but the inductor limited Q due to series resistance (DC and frequency dependant skin effect losses) reduces the filter Q and increases bandwith. A Q value at the filter frequency from the datasheet (could be converted into a series resistance) is usually sufficient for a realistic filter calculation. Additionally an inductor parallel capacitance could be calculated from self resonance datasheet values and used in the simulation.
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akrcal
Joined: 14 Aug 2006
Posts: 25
Helped: 2
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| 13 May 2008 8:42 Re: Band PAss filter at 70MHz |
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| But I have used air core inductor which I have made myself by coil winding so how to obtain Q of that? And other problem is that How can I tune the filter to improve return loss? How will I know which Inductor to squeeze or expand to improve the respone. Any help?
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VSWR
Joined: 07 Feb 2002
Posts: 639
Helped: 53
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| 13 May 2008 22:50 Re: Band PAss filter at 70MHz |
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Have you taken into account parasitics of your layout? There are stray capacitance from the component connection pads to ground. And there is a stray inductance as well of component interconnections and wires, about 1 nH/mm.
Can you post a schematic diagram and a layout drawing of your design?
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biff44
Joined: 24 Dec 2004
Posts: 1295
Helped: 169
Location: New England, USA
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| 14 May 2008 14:02 Re: Band PAss filter at 70MHz |
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Whatsa "silver mica" capacitor? Something my grandpa used to listen to the ole fireside theatre?
Get with the program and use some chip ceramic, or if high power some porcelain caps.
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