We present a low phase noise 4.5-to-6.5 GHz injection-locked oscillator-based frequency tripler (ILT) from an ultra-low jitter 1.5-to-2.16 GHz clock source. Class-C biasing is employed in the digitally controlled LC oscillator (LC-DCO) and the injection circuit to simultaneously achieve low phase noise in the LC-DCO and improve the third harmonic injection strength. Using BJT in the injection transistors and DCO greatly improves the low-frequency phase noise performance of the ILT. Fabricated in a 0.13 mu mathrm{m} BiCMOS process, the ILT has a measured tuning range of 4.5-to-6.5 GHz, with a jitter tracking bandwidth of 25 MHz for sub-harmonic injection. The ILT demonstrates good sub-harmonic rejection ratios SHRR1 and SHRR2 as 48 dB and 58 dB, respectively. The ILT adds 15 fs additional rms jitter to an input clock source with rms jitter of 68 fs. © 2022 IEEE.