A Technique for Reference Spur Eradication in Fractional-N Frequency Synthesis

Abstract

This thesis details a technique of eradicating reference spurs (in theory). It involves a Fixed Width Variable Amplitude Charge Pump (FWVACP) which is inherently insensitive to current mismatch. Charge pump current pulses are modified such that they have a fixed width and occur on the falling edge of the reference signal. Phase error information is embedded in the amplitude of the current pulses. The periodic nature of these pulses creates nulls at integer multiples of the reference frequency. At the VCO output, this translates into nulls at offsets equal to integer multiples of the reference from the carrier effectively eradicating the reference spurs. The spectrum of the current pulses has a sync null whose position depends on their duty cycle. This means that in addition to getting rid of the reference spurs, any spur at an offset greater than the reference frequency can be targeted by choosing the appropriate duty cycle. Charge pump noise contribution can be reduced by using narrow current pulses which limit its on-time when the loop is locked.The FWVACP and a standard charge pump were fabricated using IBM 0.13 micron CMOS technology. The prototype occupies less than 1mm 2 . The measured reference spur with the FWVACP is more than 10 dB lower than with the standard CP.

Optimal period design of fixed position variable amplitude loop current in FN-PLL for low spurs and fast locking

“…However, these techniques undergo complicated structure and have limited ability to eradicate spurs. R. Katumba indicates the variations of loop current's position and width are the main reason of generating spurs, and proposed a fixed width variable amplitude charge pump (FWVA-CP) so that loop current has fixed position and variable amplitude [8]. Unfortunately, fractional spurs couldn't be reduced.…”

Current pulse shaping technique for low spur fractional‐N phase‐locked loop

“…As pulse position modulation (PPM) and pulse width modulation (PWM) exist in the above studies, spurs could not be eradicated [4]. Katumba [5] proposed a fixed width variable amplitude CP (FWVA-CP) so that current pulse has fixed width, fixed position and variable amplitude, thus both the PPM and the PWM are removed. However, only the reference spurs are eradicated.…”

“…And, fractional spurs at the other fractional frequencies of the proposed FN-PLL is much less than that with the traditional FN-PLL and the FWVA-CP based FN-PLL. FWVA-CP based FN-PLL [5] 35 40 45 50 frequency, MHz…”