What is PL?

PL (or Private Line) is a signaling used in analogue (FM) two-way radios. Radios which use PL will superimpose a unique, continuous tone on the transmitted signal. Since all analogue two-way radios only use the audio-frequency range of 300 to 3000Hz to pass audio; and this tone is somewhere between 67 and 254Hz, it will not be heard by the radio users.

In order for radios to use PL and communicate with each other, they all need to be configured to encode and decode the same unique PL tone (often called a PL code). If the transmitted PL tone does not match that which is configured into the receiving radio, it will not un-mute (i.e. the transmitted audio will not make it through to the receiving radio's speaker).

PL permits a frequency to be reused on neighboring radio systems.

PL permits a frequency to be reused on neighboring radio systems. Users on these neighboring systems will be able to communicate within the system but will not hear any communications from the neighboring system(s). PL will not prevent the two systems from interfering with each other so therefore should not be used if there is a significant amount of overlap between the two systems.

Outside of Motorola, PL is known as Quiet Tone; Channel Guard and others. The correct technical name is CTCSS (or Continuous Tone Coded Squelch System). Nearly all analogue two-way radios are able to support it. For example, in Europe, there are only 16 frequencies (channels) allocated to licence free PMR446 use - two-way radios which operate in this band, are however able to support over a hundred channels, because each frequency is subdivided into further channels by means of PL tones.

The frequencies used for PL are defined by EIA RS-220 and are so chosen to avoid any falsing caused by harmonic distortion or due to AC mains. For example, a distorted 60Hz sine wave might produce a harmonic at 120Hz and 180Hz (2nd and 3rd harmonic). In the USA, an earth loop (mains noise getting into an audio line) might cause it to be transmitted over the air.
So, for example, PL code 7A has a freuqncy of 192,8Hz - the harmonics of which do not match any of the other PL tones and vice versa.

PL tones can also be used to divide the use of a radio channel into time slots in such a way that each user group gets a fair chance to communicate on a shared channel for a defined amount of time. This typically occurs when used with a repeater system. Such repeaters are also known as Community Repeaters and employ an external controller to manage the repeater operation. This controller is often referred to as the Tone Panel.

The tone panel carries out the following functions:

Prevents one user group from hogging the channel: by limiting the amount of time the group can talk and by preventing the group from using the channel again immediately after the call has timed out.
Decoding and then re.encoding the PL tone - this is an important function as simple passing the PL tone through will often result in interruptions due to phase error/distortion.
Managing additional features like telephone interconnect (a.k.a phone patch) and system control (adding and deleting groups).

Systems which use PL will often also use PL Reverse Burst. A radio which has this configured will continue transmitting for at few additional milliseconds but with the PL tone phase-shifted by 120°. This phase shift causes all the receiving radios to mute thereby preventing the squelch tail (or squelch crash) from being heard.
The Zetron Model 735 operates as a Community repeater but also has functions like phone patch and ANI calling.

Below is a list of  standard PL tones. It is also possible to configure custom (non-standard) tones in Motorola two-way radios. PL code 1Z should not be used in almost all countries as its a 2nd harmonic of the 50Hz mains supply frequency.

PL Code PL Tone (Hz)
XZ 67.0
WZ 69.3
XA 71.9
WA 74.4
XB 77.0
WB 79.7
YZ 82.5
YA 85.4
YB 88.5
ZZ 91.5
ZA 94.8
ZB 97.4
1Z 100.0
1A 103.5
1B 107.2
2Z 110.9
2A 114.8
2B 118.8
3Z 123.0
3A 127.3
3B 131.8
4Z 136.5
4A 141.3
4B 146.2
5Z 151.4
5A 156.7
5B 162.2
6Z 167.9
6A 173.8
6B 179.9
7Z 186.2
7A 192.8
8Z 206.5
9Z 229.1
0Z 254.1
M1 203.5
M2 210.7
M3 218.1
M4 225.7
M5 233.6
M6 241.8
M7 250.3

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