DIY FM Bandstop filter - dips into HF also?

wowologist

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Jul 21, 2013
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Any filter guru's about?

I used ELSIE to make an FM bandstop filter and then monte carloe'd it out and it looked great...then after making it andfm bandstop sweep filter 2.jpgfm bandstop sweep filter 2.jpg swept it, its great at the FM bandstop portion of the spectrum but it also keeps coming up with sharp dips into the HF band...so basically making it useless to me. I tore it down, re- did it and ended up with an identical sweep. I then made another with the almost identical results. The markers 2 & 3 are the ones I'm concerned about. (see attached sweep screenshot). Any comments appreciated!
 

MDScanFan

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Jan 23, 2010
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What filter topology are you using? And what kind of response are you expecting to see from hf through 120 MHz.

From this plot, and your statement of it working well at FM rejection, are you trying to reject frequencies just below the FM BCB or within it?
 

Ubbe

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Replace the filter with an adapter and sweep again. It must be the cables you are using, or the measuring method, that are causing the dip in the HF band.

No way that a FM band stop filter can have anything to do to HF frequencies. The coils in the filter are like a short circuit at those frequencies at an almost 0 ohm impedance straight thru the filter.

How does the design look like? You have made a filter that attenuates at 55Mhz, then it it only attenuates 30dB at 60MHz then it attenuates again at 68Mhz and 70Mhz. It is for the russian broadcast FM band?

Can you try this calculator? RF Tools | LC Filters Design Tool
Choose Buterworth type as that are not so picky about exact component values and use series first and 5th order should be enough for FM band.

/Ubbe
 
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