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> wrote:
> From: Eric Lemmon <wb6...@verizon.net>
> Subject: [Repeater-Builder] Reed vs. Reedless Boards in Mitrek Radios
> To: Repeater-Builder@yahoogroups.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
