I'm in the process of designing a WWVB Frequency Receiver and Clock that can receive either the amplitude modulated time signals or the phase modulated time signals. What I needed to go ahead with this project is a modulator that can make the phase and amplitude modulation. I racked my brain for days until I came up with a schematic of this simple concept. It is a combined phase and amplitude modulator that uses a quadrature signal generator. It sounds complicated. It is really quire simple. Have a look at the schematic. The schematic shows just the bare basics. There is no amplification or filtering or control processor. Those will be added later as I develop the design. But I just had to share my idea because it is so cute. Note that for phase reversal modulation both the I and Q channels must be switched simultaneously. If you only switch one you get a 90 degree shift. I may use that to see how the receiver behaves, but the receiver is not designed for quadrature phase modulation. Just Binary Phase Shift Keying (BPSK).
As I further develop the concept I will post more details here. Eventually I will be designing and building a board. The control microprocessor will be run off the master clock on the board so switching will be roughly coherent. With some tuned circuits to eliminate harmonics and such the exact phasing (such as WWVB does) is probably not strictly necessary. At least for testing purposes.