CTCSS CODEC schematic|
Page last updated: 01/07/2009 |
CTCSS encoder / decoder
Continuous tone coded squelch system CTCSS, also known as private line PL at Motorola, tone lock at Pye, and sub audible tone is a system used to reduce the problems associated with co channel interference. Instead of the squelch simply opening when the received signal level exceeds the squelch threshold, the squelch threshold must be exceeded and a pre determined tone must also be present on the received signal.
There are 47 commonly used tone frequencies as shown in the table below:
| 67.0Hz | 77.0Hz | 88.5Hz | 100.0Hz | 114.8Hz | 131.8Hz | 151.4Hz | 167.9Hz | 186.2Hz | 199.5Hz | 218.1Hz | 241.8Hz |
| 69.3Hz | 79.7Hz | 91.5Hz | 103.5Hz | 118.8Hz | 136.5Hz | 156.7Hz | 173.8Hz | 189.9Hz | 203.5Hz | 225.7Hz | 250.3Hz |
| 71.9Hz | 82.5Hz | 94.8Hz | 107.2Hz | 123.0Hz | 141.3Hz | 159.8Hz | 179.9Hz | 192.8Hz | 206.5Hz | 229.1Hz | 254.1Hz |
| 74.4Hz | 85.4Hz | 97.4Hz | 110.9Hz | 127.3Hz | 146.2Hz | 162.2Hz | 183.5Hz | 196.6Hz | 210.7Hz | 233.6Hz | --- |
The CTCSS encoder decoder
And so onto the CTCSS codec hardware, there are numerous dedicated CTCSS codec IC's available on the market today. Manufacturers such as CML Microcircuits, and Maxim/Dallas Semiconductors to mention but two. However for a simple single frequency CTCSS encoder decoder ideally suited to implementation into a repeater and made from conventional readily available components its difficult to better the design by Eric van de Weyer VK2KUR, for the original documentation I found on the internet goto VK2KUR's CTCSS encoder however the schematic redrawn is included below: