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[aprssig] APRS tone encode question

scott at opentrac.org scott at opentrac.org
Mon Aug 23 20:58:20 UTC 2004


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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