A simple technique for analog tuning of frequency synthesizers

An analog implementation of the fractional N-phase-locked-loop variable-frequency synthesis technique is presented. In addition to its simplicity, this implementation allows tuning over broad frequency ranges. The synthesizer was developed in response to a need for a compact, low-power, local oscillator for a swept heterodyne, low-frequency, battery-operated, portable spectrum analyzer. The resulting prototype synthesizer was constructed on a 4-in×4-in circuit board using standard CMOS integrated circuits. The total power requirements were +7 V at 8 mA and -7 V at 1 mA when the synthesizer is operated in the 380-580-kHz frequency range. Further reductions in size may be expected from the use of surface mount devices. Spectral data are presented for the prototype circuit serving as the local oscillator in a prototype swept-frequency spectrum analyzer. That is, instead of a spectral analysis of a fixed synthesizer frequency, the synthesizer was swept through a range of frequencies about a stable reference applied to the input of the prototype analyzer. Thus the results are conservative , since they include the effects of noise coupled to the sweep voltage from other circuitry within the prototype analyzer. This method of evaluation demonstrates one of the distinct advantages of this circuit