A Phasing Type SSB Transmitter!
I have been bitten by the bug, struck by lightning, entered into an age of enlightenment or just maybe gone stark raving mad! But, I am taking on the building of a phasing type SSB transmitter.
There is a long history of phasing transmitters both from a homebrew and also commercial ventures. One homebrew transmitter in particular goes back to 1956 with a project from Tony Vitale, W2EWL who in the true ham spirit took a war surplus ARC-5 Command Transmitter and made it into a Phasing SSB Transmitter. This was amazing!
Commercial ventures such as the Central Electronics 10A or the hallicrafters HT-37 (and HT-44) all used the phasing method. Another homebrew type rig was featured in the GE Ham News known as the SSB Jr.
Below is a typical 1950's style Phasing Transmitter with the 90 Degree Audio Phase Shift Network typically being deisgnated as the 2Q4.
The theory of phasing generation of SSB signals involves mathematical relationships that take advantage of phase differences in signals to cancel the carrier, diminish one sideband and to embellish the other. This is acheived through the use of a phase shift network which has a daunting task to do the following:
Easy to say; but harder to do!
There are many ways of generating a Phasing type SSB signal with the more simple forms (read less elegant) producing perhaps only at best (on a clear day with all the planets aligned) about 40 dB of opposite sideband suppression.
This notably was in the form of a commercial audio phase shift network (PSN) manufactured by B&W and known as the Model 350 2Q4. James Millen also produced such a network; but it seemed to be less popular perhaps because the 2Q4 was packaged like an 8 Pin tube and simply plugged into a standard octal socket.
The 2Q4 Network consisted of 4 resistors and 4 capacitors. The values were close tolerance with standard values for the caps but "oddball" (not common) values of resistance. The Model 350 was used in W2EWL's rig as well as the SSB Jr.
Moving up the food chain we have the Polyphase Network that uses 4 inputs and 4 outputs at 0, 90, 180, and 270 Degrees. These Polyphase networks are a complex array of resistors and capacitors; but do provide a far greater opposite sideband supression and are better suited to receiving as well as transmitting with better opposite sideband suppression. Such networks can be purchased for a modest sum from QRP Labs. That said you need more sophisticated front end and back end electronics to accommodate the four phased inputs/outputs.
Another method is the use of op-amps to do the phase shifting and Rick Campbells (KK7B) R2 and T2 boards are a good example of this approach.
Really high up on the food chain is the use of the Teensy 3.5 (or later) microcontroller (ala ZL2CTM) or the Raspberry Pi3 or ASUS Tinkerboard such as I have done with my RADIG board.
Since I have two of the high end radios (Teensy 3.5 and the RADIG with either a RPi3 or the Tinkerboard), why not a hardware only transceiever (actually transreceiver) as I intend to have a separate transmitter and receiver linked with a common Arduino / Si5351 for the LO. But first the transmitter part.