You're partially correct. It certainly is true to two different PL tones each deviating at 300Hz are going to generate a combined deviation of about 700-750 hz (because there will be some third order/mixing entered into it also).
In my situation, I used 2 Comm-Spec TS-32 boards. One board was generating 100 Hz and the other board was generating 179.9 Hz. I took the output of each board and ran them thru an R/C network which should have rolled off anything above about 220 Hz. I experimented with varying the level of signal generation of my service monitor to see how weak a signal I could make most receivers reliably decode PL tones (using a variety of receivers for testing. I found that, in general terms, a weak signal (like .25 uV) had trouble decoding much below 300 Hz deviation. So I set my deviation for 350 Hz on each encoder (being sure to kill power to the other encoder as disconnecting the encoder output would change the impedence of the circuit). That gave me a combined deviation of about 750-800 Hz when BOTH PL tones were active. I never had any receiver PL decoders that had a problem with this setup and nearly all the radios used were amateur radio grade radios (Yae-Com-Wood) from the 1990's. The transmitter was a GE Mastr Pro. That same repeater is still operating today but I am no longer using the second PL tone feature. (probably one of the few Mastr Pro repeaters still operating today!) In my opinion, 750-800 Hz deviation for a PL tone is the high end of acceptable. Motorola used to recommend 750-900 Hz deviation for PL tones. My other repeater transmitters usually run about 500 Hz deviation for PL. ------ Original Message ------ Received: Fri, 06 Mar 2009 09:09:11 PM PST From: [email protected] To: [email protected] Subject: Re: [Repeater-Builder] PL vs. DPL > At 3/6/2009 12:40, you wrote: > >If you are careful about which tones you use and careful about level setting, > >you won't need any extra bandwidth. > > You'll always need more bandwidth than what's needed for a single tone. If > you can turn down the deviation of each tone to, say 300 Hz for a total > deviation of 600 Hz, then one tone by itself will work @ 300 Hz > deviation. However, decoding under weak signal conditions will be poor. > > Bob NO6B > >
