Playing with Phasing - part 1

      Every so often it seems that some folks are lured into the less-populated territory of SSB reception and transmission using the phasing method.  Maybe it's the promise of simpler (?) circuitry, eliminating the need for crystal/mechanical filters, or perhaps as a gateway drug into SDR-land.   Rick Campbell's (KK7B) articles describing a single-signal direct conversion receiver (QST - Jan 1993), followed by a phasing transmitter (QST - April 1993) are seminal works that detail the implementation of the phasing method with modern (for the 1990's, at least) circuitry.  These papers also discuss design tradeoffs in terms that are easily understood.  Quite a few years ago, I actually built a R2/T2 transceiver, shamelessly copying those designs and adding a Si5351 quadrature LO along the lines of that described by ZL2CTM in his blog.  It worked OK, but the opamp-based, all-pass phase shifters required hand matching of the RC components for decent performance.  It looked like this:

 

 

So I decided to use the same designs, but replace the analog phase-shifters with digital equivalents based on the Teensy 4.0 and SGTL5000 audio shield.

     The Teensy 4.0 and SGTL5000 form a compact, but very capable (IMO) audio processing unit.  The "new" (actually built a year ago) rig's concept is illustrated below:

 

The front end and LO borrow from existing designs.  The quadrature LO was implemented using a Si5351 and ZL2CTM's code (lifted from his blog).  I added a polled switch to manually reset the Si5351 PLL because I found that it would randomly lose the desired 90 degree phase offset while tuning over a large frequency  range.   This avoids resetting the PLL for every frequency change and the attendant "pops" that accompany such a reset.  Now, a PLL reset is only needed when the inter divisor changes or when the Si5351 decides to throw one of its random snizz-fits.  An ugly hack, I'll admit, but it works. 

The audio processing code was also "adapted" from ZL2CTM's work. I added post-detection audio filtering, switch select-able between typical SSB and CW bandwidths.  On the hardware side, I used a FST3235 mux as a DPDT RF switch between the LO and the  mixers.  The "other" pole is used to route the LO to a companion TX when desired.

The basic RX doesn't include any in-board RF filtering - bandpass or lowpass filters are added extenally as needed.  I did include a Noisy Cricket audio output amp (Sparkfun) for comfortable low-Z headphone or speaker use and manual volume control.  

 

 

Opposite sideband suppression is acceptable to my tastes and at least as good as the older, analog version of the RX.  (The slight background noise in the video is the clothes dryer running in the background!)