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LeakSt20 25th July 2005 11:18 PM

LC filter combination for 19mhz signal

Am trying to build a stereo decoder for my Leak Troughline II tuner, using the following circuit diagram at:

My problem is there is no L C combination shown on the diagram. I have worked out that if I use a

1mh inductor I can use a 17.5nf cap:

If L, is 1milliHenry, then

C = 1 / ( L* ((Fo * 2 * pi )^2))
= 1 / (0.001 *( (19000 * 2 * 3.14 )^2))
= 17.5 nanoFarads

but am not sure, my maths insn't great. Also could I use a 20nf cap with 20% tolerance to get the 17.8 - 22.2 nf range I need with the 1mh inductor?

Has anyone out there created this circuit? If so what combo did you use, if you used one.

I have also heard that I could use an RC filter instead of an LC filter, and this would be easier. Any help here would be appreciated.



LeakSt20 26th July 2005 08:00 AM

Sorry I meant to say 19Khz not 19mhz.

Frank Berry 26th July 2005 08:22 AM

1 mH and 70 nanoFarads will resonate the circuit at 19 kHz.

Frank Berry 26th July 2005 11:06 AM

2 Attachment(s)
Below is a plot of the filter.

LeakSt20 26th July 2005 01:29 PM

Thanks very much Frank

LeakSt20 8th August 2005 02:58 PM

The 1mh 70nf LC filter assumes less then 1% tolerance in order to resonate at 19Khz.

The capcitor I have has is 68nf with a tolerance of 5% (anywhere from 64nf to 71nf ) and so does the 1mh choke.

Is there any way I can gurantee 19khz, perhaps using a smaller capacitor value, trimmer etc.

I appreciate any advice you can give.


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