Multiloop Synthesizer With Optimal Spurious Performance

This invention relates generally to phase locked loops, and more specifically to phase locked loops with optimal spurious performance.

The tuning bandwidth of a conventional single loop analog synthesizer often has a narrow tuning bandwidth (below 20% bandwidth), due to limitations in the tuning range of the varactor diode. More particularly applicable to low band and mid band ranges, if a high resonator Q is desired for good sideband noise performance, then a large and bulky resonating coil is usually required. The typical techniques used to diminish these problems involves the use of the divide down synthesis method or the mix down synthesis method.

Use of the divide down synthesis method generally reduces the size of the coil when a higher frequency voltage controlled oscillator (VCO) is used, but fails to provide an adequate tuning range due to the limits of the divider ratio. The conventional mix down method eliminates the problems with coil size and limits in tuning range, but adds the additional problem of inherent in-band spurious response signals due to Able-Baker mixing products. The Able-Baker spurious response signals cannot be filtered out due to their proximity to the desired signal over a wide frequency band. Therefore, a need for a mix down synthesizer method that reduces the in-band spurious response signal problems is required.

Accordingly, it is an object of the present invention to provide an improved mixed down synthesizer scheme having a first phase locked loop and a second phase locked loop providing a first frequency and a second frequency respectively. The first phase locked loop has at least a first programmably tunable filter and a mixer. The second phase locked loop has an output received by the mixer in the first phase locked loop. A controlling means controls the first and second frequencies and tunes the first programmably tunable filter.

The first and the second phase locked loops are programmed to maintain a narrow frequency ratio range between the first and second frequencies in order to maintain a minimum frequency offset of the spurious products from the desired frequency.

FIG. 1 is a schematic diagram of a dual phase locked loop in accordance with the present invention.

Referring to FIG. 1, there is shown a mixed down synthesizer scheme 10 in accordance with the present invention. As illustrated, the dual looped mixed down synthesizer scheme 10 has a single frequency source, preferably a reference oscillator 16 that provides a reference signal to a first loop 12 and a second loop 14, although a separate frequency source may optionally be provided for each loop. The first loop 12 and the second loop 14 provide a first frequency and a second frequency respectively. The first loop 12 has a conventional phase detector 22 that receives the reference signal and a first prescaled signal from a prescaler (programmable divider) 32, and provides a phase detected signal. The phase detected signal is applied as an input into a loop filter 24. The loop filter 24 provides a filtered voltage signal and is applied as a controlling voltage input to a voltage controlled oscillator 26 for developing an output useful as an injection signal input F.sub.x to a mixer 28. After the injection signal from the VCO 26 is mixed in the mixer 28, the output of the mixer 28, a desired signal F.sub.d, is applied to a first programmable tunable filter, preferably a varactor tuned filter 30 within the first loop 12. The output of the varactor tuned filter 30 is applied to the prescaler 32, preferably a dual modulus prescaler. Note that the desired signal F.sub.d from the mixer 28 can be split to provide the desired signal F.sub.d to the filter 42 in the output stage of the mixed down synthesizer scheme 10.

The mixer 28 also receives an injection signal F.sub.r from a VCO 34 in the second loop 14. The mixer 28 mixes the injection signals F.sub.x and F.sub.r from the VCOs 28 and 34 to provide the desired signal F.sub.d to varactor tuned filter 30 and, optionally, to another varactor tuned filter 42 in the output stage of the mixed down synthesizer scheme.

The second loop 14 has a phase detector 38 that receives the reference signal from the reference oscillator 16 and a prescaled signal from a prescaler 40 and provides a phase detected signal. The phase detected signal is applied as an input to a loop filter 36. The loop filter 36 provides a filtered voltage signal and is applied as a controlling voltage input to a voltage controlled oscillator 34. The output or the injection signal from the VCO 34 is then applied to the prescaler 40, preferably another dual modulus prescaler which provides a second prescaled signal. The second prescaled signal is then fed back to the phase detector 38. The output or injection signal from the VCO 34 is also applied to the reference injection port of the mixer 28. The second loop 14 acts as a programmable frequency reference source for the mixer 28. The output F.sub.d from the mixer 28 after mixing a signal injection signal F.sub.r from (the VCO 34 in) the second loop 14 with the injection signal F.sub.x from (the VCO 26 in) the first loop 12 provides a desired signal. The output of the mixer 28, the desired signal, is applied to the programmable tunable filter 42 in the output stage of the mixed down synthesizer scheme 10. The output stage of the mixed down synthesizer 10 preferably has at least one buffer 44 for buffering the output signal from the varactor tuned filter 42.

Varactor tuned filters 30 and 42 are both programmably tuned by a processor 18, preferably a MC68HC11 microcontroller manufactured by Motorola. Processor 18 also can control the divisors of the prescalers 32 and 40. The filters 30 and 42 are controlled by the processor 18 via a digital-to-analog convertor 20. The filters 30 and 42 are programmably tuned so that an almost constant frequency ratio between the first frequency and second frequency is maintained in order to maintain a minimum frequency offset from mixed-in spurious product or the spurious frequencies. The constant frequency ratio is dependent on the operating frequency and the desired spurious performance. Typically, there are numerous ratio ranges providing good spurious performance, but the range between 0.83 and 0.9 is preferred. By keeping the spurious frequencies at a minimum offset frequency from the desired signal's frequency, a constant bandwidth filter can be designed to easily filter out Able-Baker type and other in-band spurious responses. This provides an improved in-band spurious response over a wide range of frequencies.

In an alternative embodiment (not shown), all programmably tunable filters (42 and 30) can be located in the output stage, preferably receiving the desired signal from the mixer 28. A low pass filter (not shown) can replace the filter 30 in loop 12. When a low pass filter is used, a smaller bandwidth is obtained. Again, the filters (42 and 30) can be controlled by a microprocessor 18 via a D-to-A convertor. The outputs from the filters (42 and 30) are preferably buffered.