Injection Spurs due to Reference Frequency Retiming by a Channel Dependent Clock at the ADPLL RF Output and its Mitigation

This paper describes the mechanism of Retimed Reference Frequency Clock (CKR)-induced spurs at the ADPLL radio frequency (RF) output. These spurs appear not only at the average CKR or reference frequency (FREF) and its harmonics, called the reference spurs, but also at the inter-modulation locations related to the amount of inherent jitter in the retimed clock, termed as side spurs. The amplitude of these side spurs, which appear centered around the FREF frequency and its harmonics can be greatly reduced by decreasing the amount of inherited jitter in the generated CKR clock. This paper explains the relationship between the various retiming choices and the location of these side-spurs. Further, specific to a GSM transmitter realized in 90 nm CMOS, a performance analysis of using the channel dependent Variable Clock Frequency (CKV) for retiming without any frequency division is presented.