10.7 A 6.75-to-8.25GHz 2.25mW 190fs<inf>rms</inf> integrated-jitter PVT-insensitive injection-locked clock multiplier using all-digital continuous frequency-tracking loop in 65nm CMOS (2015)

Sub-harmonically injection locked oscillators provide a simple means for generating very-low-noise high-frequency clocks in a power, and area efficient manner [1-5]. Ideally, a free-running oscillator can be locked to the N <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> harmonic of a reference clock simply by injecting narrow pulses at reference frequency (F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">REF</inf> ) into the oscillator, such that F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OUT</inf> =NF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">REF</inf> . In the locked state, the oscillator tracks the reference clock and its close-in phase noise is greatly suppressed. As such, the phase noise of an injection-locked clock multiplier (ILCM) is limited only by the noise of reference clock. However, in practice, there are several design challenges that limit usage of ILCMs. First, lock-in range (ΔF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> ) of the injection-locked oscillator is limited. Therefore, separate frequency tuning, typically performed using a phase-locked loop (PLL) is needed to bring the oscillator free-running frequency (F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FR</inf> ) to be within the lock-in range, i.e., F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ERR</inf> = F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FR</inf> -NF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ref</inf> <ΔF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> [5]. If F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ERR</inf> ≠0 (but <ΔF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> ), injection ensures phase locking but causes a reference spur whose magnitude is proportional to F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ERR</inf> [1]. The second major challenge is the voltage and temperature (VT) sensitivity of ILCM. F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ERR</inf> increases as F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FR</inf> drifts due to VT variations, which degrades phase noise and spurious performance and may even lead to loss of lock once F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ERR</inf> exceeds ΔF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> [4]. This is especially problematic in the case of high-Q LC oscillators because of their relatively small ΔF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> . Techniques to extend ΔF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> by reducing N or lowering Q are undesirable as smaller N mandates higher F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">REF</inf> and lower Q incurs a large power penalty.