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 in a PLL circuit, the apparatus comprising:
a voltage controlled oscillator (VCO) that produces a VCO output, said VCO comprising a first VCO port that receives a control voltage and a second VCO port that receives a modulation signal;
a phase detector having a phase output that is coupled to the VCO;
an N-counter coupled to receive the VCO output and produce a counter output that is input to the phase detector; and
a modulation circuit coupled to modulate the N-counter to produce fractional values of N;
wherein said PLL is configured so that the modulation signal is applied to the PLL through the second VCO port and not through a feedback loop of the PLL.
2. The apparatus of claim 1 , wherein the modulator circuit operates to remove modulation applied by the modulation signal at the second VCO port.
3. The apparatus of claim 1 , further comprising a phase data input circuit that receives phase modulation data and outputs differentiated data that is coupled to the second port of the voltage-controlled oscillator.
4. The apparatus of claim 1 , further comprising a variable gain amplifier (VGA) that receives the VCO output and an amplitude-control signal to produce a VGA output having a selected envelope characteristic.
5. The apparatus of claim 4 , further comprising an alignment circuit to time align the amplitude control signal with the modulation signal.
6. The apparatus of claim 5 , wherein the alignment circuit is configured to provide the amplitude-control signal through a signal path substantially approximating the time delay of the modulation signal path.
7. The apparatus of claim 1 , wherein said modulation signal is a direct modulation signal.
8. A method for direct synthesis of a modulation waveform in a PLL circuit, the method comprising:
modulating a reference signal with a phase-locked loop circuit that comprises a two-port voltage-controlled oscillator (VCO) and a fractional-N counter;
adjusting the fractional-N counter to obtain fractional values of N;
applying a frequency control signal to a first port of the two-port VCO; and
applying a modulation signal to a second port of the two-port VCO;
wherein said modulation signal is applied to the PLL through the second VCO port and not through a feedback loop of the PLL.
9. The method of claim 8 , further comprising removal of substantially all components of the modulation signal applied at the second port of the two-port VCO by a modulation circuit coupled to the fractional N-counter.
10. The method of claim 8 , wherein said modulation signal comprises a signal provided by an output of a phase data input circuit, said phase data input circuit receiving phase modulation data and providing as said output differentiated data.
11. The method of claim 8 , further comprising providing an amplitude-control signal to a variable gain amplifier (VGA) to provide a selected output amplitude envelope.
12. The method of claim 11 , wherein said amplitude-control signal is time aligned with said modulation signal.
13. A direct synthesis transmitter comprising:
a PLL circuit including:
a voltage controlled oscillator (VCO) that produces a VCO output, said VCO having a first input port configured to receive a control signal, and a second input port configured to receive a modulation signal;
a phase detector having a phase output coupled to the VCO; and
an N-counter coupled to receive the VCO output and produce a counter output that is input to the phase detector;
a modulation circuit coupled to modulate the N-counter to produce fractional values of N;
an amplitude control circuit configured to produce an amplitude control signal associated with said modulation signal; and
a variable gain amplifier (VGA) output circuit disposed to receive the VCO output and the amplitude control signal and provide an output signal having a selected output envelope;
wherein said PLL is configured so that the modulation signal is applied to the PLL through the second VCO input port and not through a feedback loop of the PLL.
14. The transmitter of claim 13 , wherein the modulator circuit operates to remove modulation applied by the modulation signal at the second input port of the VCO.
15. The transmitter of claim 14 , further comprising a phase data input circuit that receives phase modulation data and outputs differentiated data that is coupled to the second input port of the VCO.
16. The transmitter of claim 15 , further comprising an alignment circuit, said alignment circuit configured to time align the amplitude-control signal to substantially approximate the time delay of the modulation signal.
17. The transmitter of claim 15 , further comprising a power amplifier (PA) coupled to the output of the VGA to provide a modulated transmit signal having a selected power level.