[Lowfer] Phasing exciters

[Stewart Nelson]

Hi Ed, John, and all,

By using both output channels of the sound card, and
doing the audio phase shift in software, a phasing
network is not needed.  I believe that the exciter
should only require two chips, a dual flip-flop and a
dual 4-input analog mux, e.g. 4052. (That does not
include the LO and output amp.)

Unfortunately, I have not yet built/tested this
design, but it is pretty straightforward, and I
wouldn't expect any serious problems.  I'm traveling
and don't have a decent drawing program; here is
a textual description:

Start with a source at 4 times the desired LO freq.
Feed it into the clock inputs of the flip-flops.
Connect the flip-flops as a two-bit Gray counter.
Drive the multiplexer address inputs from this
counter.

Let's call the soundcard outputs I and Q.  Wire the
multiplexer inputs to cycle through them as below:

(TIME)  ADR   IN 1Y   IN 2Y
------  ---   -----   -----
  0      0      I      GND
  1      1      Q      GND
  2      3     GND      I
  3      2     GND      Q

The outputs 1Z and 2Z, taken differentially, form the
SSB signal.  You just need a transformer or an op-amp
to convert to single ended.  You also need some low pass
filtering, because there will be strong spurious signals
near odd harmonics of the LO.

There are several ways one could use such a design.
For the 160-190 kHz band, you could have a fixed LO
near 175 kHz, and (using audio frequencies up to 15 kHz)
tune the entire band in software.  A possible problem
with higher audio frequencies is that sound card time
base stability becomes more important.

Alternatively, you can use an audio frequency of a
few hundred hertz, and vary the LO for coarse tuning.
A synthesizer with, say, 100 Hz steps should be ok,
with fine tuning done by varying the audio.

73,

Stewart KK7KA
 
> 
> I've never reduced it to electronic form. Will have to do that. The catch is
> that my audio phasing network is quite elaborate. That's only because it's a
> small board from a product we used to make at work. Always being willing to
> take the low road, I used that rather than rolling my own. Somewhere here,
> though, I've got a sketch of a simpler network tailored for the 800 Hz
> stuff. Will do some research!