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Here's a circuit that allows one to drive a Class D push-pull amplifier with the output from a linear transmitter or transverter. It should be noted that this is only usable for frequency modulated modes such as RTTY, Pactor, Amtor, MFSK, Olivia, Domino, Throb, WSJT, WSPR etc. It can not be used with amplitude modulated signals such as PSK, BPSK, QPSK, WOLF etc.
The circuit is a frequency doubler that provides the required Class D drive at two times the final output frequency. For example, supply a 505 kHz carrier at the input and get a 1010 kHz carrier at the output. The modulation (or frequency shift) is also doubled ... which is exactly what is required. The input to the Class D push-pull amplifier utilizes a flip flop (divide by 2) that provides the two phases to drive the push pull arrangement. This divide by two restores the proper frequency shift.
X2LinearClassDinterface2.pdf
The frequency doubler is a full wave rectifier that uses a bifilar wound transformer and two diodes. The five-element, low-pass filter that follows provides a clean sinewave to the 74HCT4046 that squares up the X2 frequency signal. A clean sinewave helps ensure an accurate 50% duty cycle signal at X2 frequency.
The input to the multiplier should be in the +7 to +20 dBm range. If the drive level is in excess of +20 dBm consider adding an attenuator between the output of the transceiver / transverter and the input to the doubler.
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