[aprssig] APRS tone encode question

[scott at opentrac.org]

I do all of my schematics and board layouts in Eagle PCB, by Cadsoft.  The
library system is awful, but I've gotten used to it, and it's a good program
overall.  Hard to beat what it does for the price.

I find it's often quicker to sketch out a schematic, generate a board
layout, and etch and drill the board than it would be to build the same
circuit on a protoboard.  I just did one today that worked right on the
first try, despite me reversing the top and bottom layers.  It's a small
board (2.5" by 3") that serves as a carrier for a surface mount OpenTracker,
and provides APP power connectors (in and out), a power MOSFET for radio
power control, radio connector, serial connector, and RJ11 connector for a
1-wire weather station.  The firmware needs some debugging, but it makes for
a very compact, low-power (8 ma @ 12 volts) weather station.  (Power
consumption is down to about 4 ma for some test boards and firmware, but it
still needs work.)

Scott
N1VG
  -----Original Message-----
  From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org]On Behalf Of Marsh Jas III
  Sent: Monday, August 23, 2004 8:38 PM
  To: TAPR APRS Mailing List
  Subject: RE: [aprssig] APRS tone encode question


  Hi Scott:

  What program program did you  use to make your schematic? It looks neat.

  Tnx.

  Jim.  w9nje

  scott at opentrac.org wrote:
    The schematic is here: http://n1vg.net/opentracker/ot1_schematic.png.
The
    PWM waveform comes from pin 13 (timer channel 0) of the MCU. The output
is
    a 57.6 KHz square wave with the duty cycle determined by the sine wave
    lookup table. The PWM frequency and its harmonics are far enough out of
the
    audio passband to not be a problem.

    Scott
    N1VG

    -----Original Message-----
    From: Andrew Rich [mailto:vk4tec at tech-software.net]
    Sent: Monday, August 23, 2004 1:46 PM
    To: scott at opentrac.org; 'TAPR APRS Mailing List'; 'TAPR PIC Development
    Special Interest Group'
    Subject: RE: [aprssig] APRS tone encode question


    Scott how do you get this waveform ? RC ?


    -----Original Message-----
    From: Scott Miller [mailto:scott at 3xf.com]On Behalf Of scott at opentrac.org
    Sent: Tuesday, 24 August 2004 4:17 AM
    To: vk4tec at tech-software.net; 'TAPR APRS Mailing List'; 'TAPR PIC
    Development Special Interest Group'
    Subject: RE: [aprssig] APRS tone encode question


    Yeah, I'd expect 16 levels. Haven't had a chance to look at my TT3 to
see
    what it's actually doing.

    Here's a snapshot of an OpenTracker's output. Steps aren't really
visible
    on this 'scope...

    http://n1vg.net/opentracker/images/waveform.jpg

    The OpenTracker uses PWM for D/A. In AFSK mode it uses 4 bits
resolution,
    and in BPSK mode it's 8. Works pretty well. I'll have to do a comparison
    of the output waveforms of the TT3, OpenTracker, and MX614 sometime. For
    audio stuff, Yoshimasa Electronic's RAL package is great. It's got a
nice
    oscilloscope and FFT analyzer display, plus a signal generator and
assorted
    other features.

    Scott
    N1VG

    -----Original Message-----
    From: aprssig-bounces at lists.tapr.org
    [mailto:aprssig-bounces at lists.tapr.org]On Behalf Of Andrew Rich
    Sent: Monday, August 23, 2004 2:40 AM
    To: TAPR APRS Special Interest Group; TAPR PIC Development Special
    Interest Group
    Subject: [aprssig] APRS tone encode question


    I threw the tinytrak audio onto a CRO and compared it to a sine wave
from a
    sig gen.

    How come a DAC ladder network of 4 resistors only produces 8 steps ?

    Should it not end up being 16 steps ?

    0000
    0001
    0010
    0011
    0100
    0101
    0110
    0111
    1000
    1001
    1010
    1011
    1100
    1101
    1110
    1111

    image

    http://www.tech-software.net/139_3935.JPG


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