PL / CG / CTCSS Filter Information Index

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Information Index
The links below will allow you to build several working subaudible tone filters.
Information compiled by Mike Morris WA6ILQ
Maintained by Robert Meister WA1MIK

No warranty as to the quality or usefulness of the filter circuits below is expressed by Repeater Builder.

Note: All audio filters have a slope to their response curve. There are no 90 degree corners in the response curve. As a result, if you have, for example, a 24dB per octave high pass filter starting at 300Hz, the received audio will be down by 24db at 150Hz (1 octave), 48dB at 75Hz (2 octaves), but only 12dB at 225Hz (half an octave). As a result, you will hear more of the higher frequency PL tones in the audio that passes through the high-pass filter.

From the owner of the Scom company:

There was a discussion on Repeater-Builder a while ago about the advantages and disadvantages of various commercial CTCSS highpass filters. S-COM's analog guru, Virgil, WØINK, did a careful analysis of five common highpass filter designs: Communications Specialists TS-32, Communications Specialists TS-64, the stock Motorola MICOR PL filter, the Motorola MSR2000 PL filter, and the GE CG filter. The results are certainly interesting. His paper can be found at http://www.scomcontrollers.com/downloads/ctcsssreject&hpfilters.pdf.

A repeat audio board designed for the GE Master Exec II from Micro Computer Concepts donated by Ron Wright N9EE. This image is oriented horizontally for viewing, not printing. Same image as above but oriented vertically for printing, not viewing.

A High Pass PL Tone Filter By Bob Dengler NO6B

A High Pass PL Tone Filter By Kevin Custer W3KKC

The PL filter from a Motorola Motrac receiver audio-squech board By Kevin Custer W3KKC. We use it in MICORs too.

GE's MASTR Exec II tone board from LBI30854 which replaced LBI4143. This board has the tone encoder, tone decoder and tone filter. Look for the section on the "Tone Reject Filter".

MASTR Pro Aux Receiver chassis This link goes to LBI4172 which is for a rack-mount power supply chassis for the MASTR Pro series of receivers. One option was a line amplifier that came in tone and non-tone versions. Look for the section that describes "Line Amplifier A503/A504". A503 does not have the filter, A504 has it. Just print those two schematic pages and compare them.

MASTR Pro base station / repeater power supply chassis This is LBI4323, and contains the line amplifier information for the 4EA24A12 and A13 chassis: the A12 version is used in carrier squelch stations (non-tone), and the A13 is for stations with Channel Guard tone decoders, filters and line amplifiers. Simply compare the two schematics.

Another one of GE's tone filter boards Another look at how GE did it. This fits onto the MASTR II audio board; see LBI30705.

This article, from WØINK, evaluates and compares various commercial CTCSS filters. A local copy can be retrieved here as a 280 kB PDF file.

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Page created 14-Sep-2004 by Kevin K Custer W3KKC