Direct synthesis transmitter
View Patent ↗A direct synthesis transmitter. A very efficient and highly linear direct synthesis transmitter is provided that can be used to directly synthesize and transmit any type of modulated signal. The direct synthesis transmitter includes a phase-locked loop (PLL) with controls phase modulation plus an accompanying variable gain amplifier for amplitude modulation. Also included is a system to align the phase and amplitude modulation signals. A transmitter constructed in accordance with the present invention is very efficient and suitable for use in portable radio equipment.
1. Apparatus for direct synthesis of a modulation waveform, the apparatus comprising:
a fractional-N phase-locked loop circuit that includes a counter controlled by a ΔΣ modulator, wherein a sequence output produced by the ΔΣ modulator alternates between integer values to resolve a fractional N value;
a two-port voltage-controlled oscillator, the first port receives a control voltage for the phase-locked loop and the second port receives a direct modulation signal;
a second input to the fractional-N phase-locked loop that operates to remove the modulation applied at the voltage-controlled oscillator;
a variable gain amplifier that receives the sequence output of the phase-locked loop and an amplitude-control signal, wherein an envelope characteristic of the phase-modulated signal produced by the phase-locked loop is modulated;
a phase data input circuit that receives the phase modulation data, differentiates the data, and routes the differentiated data to the second input of the fractional-N phase-locked loop as well as the second port of the voltage-controlled oscillator; and
an amplitude data input circuit that receives amplitude data and produces a signal that is coupled to a variable gain amplifier, wherein the phase data and the amplitude data experience substantially the same signal delay.