MICOR PL Filter Modification
For flatter frequency response
Article by Kevin K. Custer
Text and Engineering additions by Jeff DePolo WN3A
Images and testing by Tom Gunderson W9SRV

Someone posted to the Repeater-Builder email list that they were experiencing a peak in the repeated audio response in the low end of the audio spectrum on a MICOR repeater.

Jeff DePolo replied with the following:

You're probably seeing the peak in the PL filter's response.  If you remove the PL filter and config the Audio & Squelch board for carrier squelch operation, you'll probably see relatively flat response up to about 2.5 kHz where the "splatter filter" starts to take over.

                                                                      --- Jeff WN3A

I then replied with a bit more information:

Jeff is right on the nose here....
I have swept the MICOR and MOTRAC PL filters and there is a defined peak around 400 Hz.  It is likely caused by the self resonance of the 6 Henry chokes and associated support components which create the tuned network used in the filter.  This can be tamed down (filter made to be flatter in response) to some degree by placing a 220K resistor across the choke(s).

See Here:

Kevin Custer

Tom Gunderson read these replies and since he had a MICOR repeater (real station) on the bench he was working on, decided to do a little testing.  Here is Tom's reply:

Thank you for this little nugget! I suspected the PL filter as well....

Threw those resistors on tonight, and while the difference in response measured is not that great, the difference to the ear is huge!  This really cleaned up the audio on my box on the bench. Measuring a pure sine wave will probably not show the real effects the ringing has on speech and other complex waveforms, likely validating my observation that the speech is significantly cleaned up.

Thanks again for the great experience shared with the group!


Tom has documented his research to illistrate the differences to the response with and without the 220K shunt resistors.   Please refer to the images below:


Credits and Acknowledgements:MICOR is a trademark of Motorola Inc.
Original text by Kevin K. Custer W3KKC
Photos of the MICOR PL Deck provided by Tom Gunderson W9SRV
Images of the response sweeps provided by Tom Gunderson W9SRV
Edited text, artistic layout, and the hand-coded HTML is © Copyright November 24, 2009 and date of last update by Kevin Custer W3KKC.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors.
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