Doug, Since the resistor array in the TRN4224A serves only to "program" the digital synthesizer in the encoder/decoder chip, changing resistors should not make any difference in the tone purity. However, I once built up an array for testing using discrete resistors, and- if I can find it- I'll run a purity test on it. Mike Morris has a very informative article about the HLN4181A board here: <www.repeater-builder.com/mitrek/mitrek-hln4181-info.html>
73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of rb_n3...@tds.net Sent: Monday, December 29, 2008 9:28 PM To: Repeater-Builder@yahoogroups.com Cc: Eric Lemmon Subject: Re: [Repeater-Builder] Reed vs. Reedless Boards in Mitrek Radios With the reedless board chip being nothing more than a simple resister network, I wonder what your purity results might be if you built-up a 127.3 network using 1, 5,and 10% 1/8w resistors for comparison as well. -- Doug N3DAB/WPRX486/WPJL709 ---- Eric Lemmon <wb6...@verizon.net <mailto:wb6fly%40verizon.net> > wrote: ============= 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! At the outset, 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
