[Lowfer] Phasing exciters

[Ed Phillips]

Stewart Nelson wrote:
> 
> 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

	Very interesting thoughts.  How does an ignoramous do "the phase
shifting in software"?  The rest is straightforward enough.

Ed