[Lowfer] BPSK31

[Stewart Nelson]

Hi Ed and all,

Some possible ways to generate PSK31 at LF with little hardware:

1. Use the serial port at a relatively high baud rate to control the timing.
   For example, set it to 9600 bps, 8 data bits, 1 stop bit, no parity.
   To send a "1" bit, send 31 characters of 0xFF. To send a "0" bit, send
   31 characters of 0x00.  On every fourth bit, send it only 30 times
   instead of 31.  This gives an average bit time of 32.03125 ms, within
   0.1% of the nominal value.  The start and stop bits, which you cannot
   control, will cause glitches of about 100 us every millisecond.  Use a
   simple RC filter (time constant ~0.5 ms) between the Tx data line and
   the gate that controls the carrier phase.

2. Use the sound card for timing.  For example, generate a .wav file with
   32 ms of a 2 kHz tone for a "1", and 32 ms of silence for a "0".  A
   rectifier and trivial filter on the sound card output will provide the
   needed phase control signal.

3. An exciter like TAG, but do the audio phase shift in software, using
   the stereo outputs of the sound card.

4. As above, but use a 4052 mux as an image reject mixer.  Feed the two
   address lines with quadrature carrier signals.  These can be generated
   by a dual flip-flop clocked at four times the desired frequency.  Now,
   if the soundcard channels are I and Q, the four analog inputs should
   be +I, +Q, -I, -Q.  You can use op amps for the phase splitters, or
   use both muxes in the 4052 package, with a transformer coupled output.

5. Generate the PSK audio at a high frequency, e.g. 18 kHz, at a 48 kHz
   sample rate.  Combine with carrier in a double-balanced mixer.  If
   you are trying to Tx on 185 kHz, your LO would be at 167 kHz, so the
   image is at 149 kHz.  This is far enough away from the desired output,
   that one high-Q tuned circuit should be an adequate filter.  The
   second filter is your antenna :)

Hope one of these works for you.  If you have a choice, options 3-5 can
also generate Jason, DFCW, and other not-yet-invented signal formats.
The first two are limited to BPSK modes.

73,

Stewart KK7KA

----- Original Message ----- 
From: "John Andrews" <w1tag@charter.net>
To: <lowfer@mailman.qth.net>
Sent: Saturday, May 31, 2003 8:17 PM
Subject: Re: [Lowfer] BPSK31


> Ed,
> 
> > Question:  How is TAG generating the BPSK31 signal?
> 
> It's actually a phasing SSB exciter. The carrier is generated by a DDS chip,
> then buffered and fed to two SBL-3 mixers. The output of my sound card is
> routed through a 90 degree audio phase shift network to the mixers, and the
> combined mixer output is amplified and sent to the PA. It's a pretty broad
> signal, since the PA runs in the usual lowfer switching mode. But for this
> application, there's nobody to bother.
> 
> John Andrews