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Old 11th December 2005, 08:32 PM   #1
mcs is offline mcs  Denmark
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Default FM MPX filter

I'm working on an FM tuner/MPX decoder design, and I'm trying to figure out how to design the audio output filters.

I found a schematic in the LA3410 MPX chip datasheet, but when I simulated it in PSpice, I found out it wasn't that good... Here's a frequency plot:

http://stiftsbogtrykkeriet.dk/~mcs/MPXFilter.gif

Not a lot of attenuation at 19kHz and -3dB somewhere around 13kHz.

The next thing I tried was "expanding" that filter with some extra sections. That produced this result:

http://stiftsbogtrykkeriet.dk/~mcs/F4V10.gif

Good attenuation of 19kHz and above junk, but some ripple in the audio band. Here's a close-up of the 1-50kHz range:

http://stiftsbogtrykkeriet.dk/~mcs/Not_nice.gif

To get a nicer looking curve I tried calculating a standard low-pass filter (6th order Linkwitz-Riley at 25kHz in this case), and then I added some caps accross the coils to create notches. This is the result I got from that:

http://stiftsbogtrykkeriet.dk/~mcs/Nice.gif

Looks very nice in the audio band, but the -3dB point has moved lower, and the 19kHz and above attenuation isn't as good.

So which filter would actually work best and sound best?

I have looked at some Toko datasheets for MPX filters also. They have about 25-30dB attenuation at 19kHz, and some of them higher at 38kHz. But I have no idea what circuit they use. Would they be better, or would a DIY filter with good quality parts (rather than ceramic chip caps) be a better idea?

If anybody has suggestions or experience in this field I would like to hear about it

Best regards,

Mikkel C. Simonsen
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Old 11th December 2005, 09:04 PM   #2
moamps is offline moamps  Croatia
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Hi,
here are filters I use on the tx side. The passive sounds better, IMO.
For more info check the old Revox and Studer tuners schematics.

http://www.prodigy-pro.com/forum/vie...=8107&start=15

Regards,
Milan
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Old 12th December 2005, 01:25 AM   #3
mcs is offline mcs  Denmark
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Your schematic is exactly the same as mine

The values are very different though. How did you choose those exact values?

Best regards,

Mikkel C. Simonsen
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