I recently acquired a Kenwood HM-250 Audio Distortion Analyzer, and I have been experimenting with various CTCSS tone encoders to find which produce the purest tones. Since I am putting together a 6m repeater using Mitrek radios, I wanted to compare the older HLN4020B reed board to the newer HLN4181A reedless board. What an eye-opener!
My gut feeling was that the reed board would produce a purer tone than the digital reedless board, since the reeds are essentially tuning forks. That turned out to be a false assumption. With two known-good tone boards hooked up on the bench, the 4020B reed board consistently produced a 127.3 Hz tone with distortion ranging from 0.75% to 1.52%, while the 4181A reedless board produced the same tone with only 0.43% distortion. I adjusted the output level pot (R23) on the 4181A board to match the output level of the 4020B board. I tested the 4020B board with six 127.3 Hz reeds. Another interesting fact emerged from my experiment: Although the PL tone reeds can be plugged into their sockets in either of two positions, I found that there was definitely a difference in the amount of distortion produced. The differences ranged from 0.1% up to 0.6%- not much, but surprising, since the reeds are supposedly symmetrical. I got similar results with KLN6209A, KLN6210A, and TLN6824A reeds. For comparison, I measured the distortion at 127.3 Hz from several pieces of test equipment, with the following results: HP 204B Audio Generator: 0.24% Motorola R2600D Service Monitor: 0.26% Wavetek 188 Audio Generator: 0.19% CSI TE-64D Tone Generator: 0.76% My next step is to evaluate the purity of the CTCSS tones after passing through an RF link. Some radios- cheap ones especially- use rather coarse tone synthesis techniques to generate PL tones, and the resulting tones are prone to falsing and talk-off problems. Stay tuned... 73, Eric Lemmon WB6FLY
