Bob, I am very pleased to see that I am not the only one studying this issue! I got started on this track by evaluating the waveform purity of backup power systems, AKA "mini-UPS", that sum several frequencies to approximate a 60 Hz sine wave. Such devices produce very crude approximations of a sine wave, and some equipment doesn't work too well when fed that power.
Depending upon the technology used, and the intelligence used in the design, a PL tone generator can range from a square-wave oscillator to a nearly-pure sine wave oscillator. As Kevin has pointed out numerous times on this list, a PL tone that is audible to a repeater user may be the result of distortion, as well as over-deviation. I always set PL deviation to the 400-500 Hz range, but still hear a buzz when some radios transmit. As expected, some inexpensive radios from Asia seemed to be far more prone to "PL tone buzz" than commercial radios. Based upon my limited experimentation to date, I have found that the really cheap handheld radios use a CTCSS generation technique that results in greater distortion than the technique used in high-tier commercial radios. I'm not sure how beneficial to the repeater community this knowledge will be, but I personally feel that minimizing distortion of audio signals within a repeater system is a good thing. I am still stunned that a reed tone board has more than twice the distortion of a digital tone board. Conventional wisdom would predict that a tuning fork design would produce a pure sine wave. As Bob suggests, the distortion in the reed board may be caused by excessive feedback, and that's a topic for further study. Any ideas why the reed has more distortion in one orientation than in the other? 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Bob M. Sent: Tuesday, December 30, 2008 5:12 AM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Reed vs. Reedless Boards in Mitrek Radios You may find that after the PL signal goes through the modulator, the distortion is even lower. It also probably doesn't matter too much; 1% is still 40dB down. You probably don't know (yet) what component is producing that little bit of signal either; it could be some high-frequency noise coming out of the circuit itself, rather than actual distortion of the sine wave or an actual harmonic of the PL tone. If the reed is being overdriven in an attempt to make sure it oscillates, it could be bumping into something inside and causing distortion that way. Don't they use wide-band noise to get the reed going in the first place? Some synthesized radios (i.e. MaxTracs) use two output lines from the microprocessor that get summed in a two-resistor D/A converter. This feeds a low-pass filter before joining the modulation path. The filtering has to allow for all PL tones up to about 250 Hz, but it doesn't have to be flat; radio alignment can make up for some of that. Yet the PL tone is fairly pure. MSF5000s use a four-bit D/A converter for their PL encoder, so I would expect it to be even more pure than a MaxTrac. Spectras and GTXs do everything inside an IC, so only "The Minds of Motorola" truly know what goes on in them. Every time I try to measure the distortion of a MaxTrac's transmit audio, I find it to be in the 3% range, even with low levels of modulation, at 400 and 1000 Hz. Since you're in California, I'm sure your readings will be lower! Sounds like a bit more research and investigation is in order. Could be a nice topic for a Repeater-Builder article. Bob M. ====== --- On Mon, 12/29/08, Eric Lemmon <wb6...@verizon.net <mailto:wb6fly%40verizon.net> > wrote: > From: Eric Lemmon <wb6...@verizon.net <mailto:wb6fly%40verizon.net> > > Subject: [Repeater-Builder] Reed vs. Reedless Boards in Mitrek Radios > To: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > Date: Monday, December 29, 2008, 8:26 PM > 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
