Direct QAM modulator

A direct QAM modulator for modulating baseband data, the direct QAM modulator comprising:

• a microwave oscillator having an output;
• a control signal source of pre-distorted and pre-filtered control signals corresponding to baseband data;
• a divider for dividing the output of the microwave oscillator into in phase and quadrature components;
• a first variable attenuator comprising a first coupler having a first input port, first and second reflection ports and a first output port, the first and second reflection ports each terminating in a forward biased PIN diode, the forward biased PIN diodes having resistance controlled by the control signals, and first means connected to the first output port for compensating for phase variations induced by the first coupler to produce amplitude modulated in phase signals;
• a second variable attenuator comprising a second coupler having a second input port, third and fourth reflection ports and a second output port, the third and fourth reflection ports each terminating in a forward biased PIN diode, the forward biased PIN diodes having resistance controlled by the control signals, and second means connected to the second output port for compensating for phase variations induced by the second coupler to produce amplitude modulated quadrature signals;
• a first single pole double throw switch for selecting a first phase of the in phase output from the divider when baseband data is positive and a second phase of the in phase output from the divider when baseband data is negative, the first single pole double throw switch being connected to provide the first and second phases to the first input port of the first variable attenuator;
• a second single pole double throw switch for selecting a first phase of the quadrature output from the divider when baseband data is positive and a second phase of the quadrature output from the divider when the baseband data is negative, the second single pole double throw switch being connected to provide the first and second phases of the quadrature component to the second input port of the second variable attenuator;
• the control signal source comprising a pre-distortion and prefiltering circuit connected to the PIN diodes for compensating for non-linearity of the PIN diodes;
• and
• a combiner for combining the amplitude modulated in phase and quadrature signals.