Low noise generator for frequency swept signals
A signal generation system suitable for use in a radar system comprises a local oscillator (LO) and an intermediate frequency (IF) oscillator, wherein the IF oscillator is a Direct Digital Synthesizer (DDS), and the LO is a free running oscillator not itself locked to another oscillator but which acts as a clock reference for the DDS and is the highest frequency oscillator in the system. The LO may also act as a reference for a receive chain digitizer. The invention exploits phase noise advantages of a free running oscillator at some distance from the carrier whilst maintaining coherency with other system components. The system typically finds application in FMCW radars.
1. A signal generation system for a high resolution coherent radar comprising at least a local oscillator (LO) and an Intermediate Frequency (IF) signal generator utilising a direct digital synthesiser (DDS), wherein the local oscillator is a free running oscillator not itself controlled by a further reference oscillator, is the highest frequency oscillator in the system, and which is adapted to be a reference source for a clock signal for the IF signal generator, and further wherein the local oscillator is operative in the microwave or millimeter wave regions.
2. A signal generation system as claimed in claim 1 , wherein the local oscillator is a free running dielectric resonant oscillator.
3. A signal generation system as claimed in claim 1 , wherein the radar is a frequency modulation continuous wave radar system.
4. A signal generation system as claimed in claim 1 , wherein frequency down-conversion means is located in a receive chain of the radar and is arranged to reduce the frequency of a received signal using mixing means, the mixing means having an input derived from the local oscillator.
5. A signal generation system as claimed in claim 1 , wherein a receive chain of the radar incorporates a digitisation system having a clock derived from an output of the local oscillator.
6. A signal generation system as claimed in claim 1 , wherein a frequency mixer in a transmit path is adapted to receive the output from the local oscillator and the output from the IF signal generator to produce a mixer output signal.
7. A signal generation system as claimed in claim 6 , wherein the system forms part of a radar, and further wherein the mixer output produces a signal to be transmitted by the radar.
8. A signal generation system as claimed in claim 6 , wherein the system forms part of a radar, and further wherein a frequency multiplier is connected to the mixer, with an output of the frequency multiplier producing a signal to be transmitted by the radar.
9. A signal generation system as claimed in claim 6 , wherein the mixer is a quadrature mixer, with inputs from the local oscillator and the IF signal generator, the IF signal generator being adapted to have an in-phase (I) and a quadrature (Q) output.
10. A signal generation system as claimed in claim 9 , wherein the IF signal generator is adapted to generate an output signal starting at a frequency F s , that sweeps linearly to zero Hz, and then sweeps linearly to a stop frequency F p , wherein at the point of reaching zero Hz the DDS is adapted to swap the phase of the I channel with that of the Q channel.
11. A radar system incorporating a signal generation system as claimed in claim 1 .
12. A system as claimed in claim 1 , wherein the DDS is adapted to produce a signal comprising a frequency sweep.
13. A method of processing signals within a radar system comprising the steps of:
i. arranging a free running oscillator (FRO) to provide a transmit local oscillator signal, the FRO being operative in the microwave or millimeter wave regions;
ii. deriving a reference clock for an IF oscillator from an output of the FRO, wherein the IF oscillator is a direct digital synthesiser (DDS);
iii. producing a transmit signal by mixing a signal derived from the output of the FRO with a signal derived from the output of the DDS.
14. A method as claimed in claim 13 , wherein the method includes the additional step of incorporating an analogue to digital converter (ADC) in a receive chain of the radar, the ADC having a clock reference derived from an output of the FRO.
15. A signal generation system comprising at least a local oscillator (LO) and an Intermediate Frequency (IF) signal generator utilising a direct digital synthesiser (DDS), wherein the local oscillator is a free running oscillator not itself controlled by a further reference oscillator, is the highest frequency oscillator in the system, and which is adapted to be a reference source for a clock signal for the IF signal generator, and further wherein a quadrature frequency mixer in a transmit path is adapted to receive the output from the local oscillator and in-phase (I) and quadrature (Q) outputs from the IF signal generator to produce a mixer output signal, and further wherein the IF signal generator is adapted to generate an output signal starting at a frequency that sweeps linearly to zero, and then sweeps linearly to a stop frequency, wherein at the point of reaching zero the DDS is adapted to swap the phase of the I channel with that of the Q channel.