WIDE-BAND PHASE-LOCKED LOOP SYNTHESIZER USING LINEAR FREQUENCY VARIATION DIRECT DIGITAL SYNTHESIZER AS REFERENCE OSCILLATOR

A wideband phase-locked loop (PLL) synthesizer is proposed in which a linear frequency variation direct digital synthesizer (LFDDS) is used as a reference oscillator and a frequency variation band can be widened up to the full tunable range of voltage controlled oscillator (VCO) by frequency hopping within the lock-in range of PLL. A theoretical study is made and experimental results ale explained. At first, assuming continuous frequency variation of LFDDS, transient behavior of frequency difference between two input signals of phase comparator of PLL is determined. Next, the condition for frequency variation speed of LFDDS is derived so that phase locking can be obtained using the maximum value of this frequency difference. Then the quantized frequency variation of LFDDS is assumed, transient behavior of the frequency difference is determined, and its difference from the continuous variation is clarified. A 6-GHz band PLL synthesizer is fabricated, and the theory's validity is confirmed through experiments. Using LFDDS, a frequency setting time for a PLL synthesizer increases less than 10 percent for five times wider operation bandwidth. Performing experiments in 6-GHz band, we obtained an operation bandwidth of 23 percent and so frequency hopping can be performed. Moreover, a frequency setting time of 50 mu s and spurious level of -55 dBc were obtained.