Phase-Locked Loop 137
6.2 60-GHz PLL Design
The propos ed new inductive tuning in Chapter 4 is utilized for VCO design
in the 60-GHz PLL. To achieve a wide FTR with maintained phase noise
performance, the VCO prototype presented in Section 4.4.3 with symmetric
inductive tuning is selected.
Various frequency dividers have been explored for diﬀerent applications.
Generally, frequency dividers can be classiﬁed into digital and analog classes.
The digital class of dividers c an be s ub-divided into current-mode logic (CML)
dividers  and dynamic divider ; and the analog class of divider s can be
sub-divided into Miller dividers  and injection-locked frequency dividers
(ILFD) [183, 184, 185]. Dynamic dividers are ea sy to design that operate at
low frequency with wide range and variable division-ratio. CML and Miller di-
viders can operate at higher operating frequency with wide locking range, but
usually suﬀer from high power consumption. Compared with Miller frequency
dividers and CML dividers, ILFD can achieve hig he r operating frequency with
lower power consumption. Its major limitation, however, is the limited locking
In the de sign of the divider chain in 60-GHz PLL, accor ding to diﬀerent
operating frequencies, the choice o f the divider structure in each dividing stage
will be diﬀerent. The ﬁrst stage divider opera tes at the highest fre quency in the
divider chain, and thus the ILFD is usually employed. The last stage divider
operates at the lowest frequency in the chain, and thus a dynamic divider is
usually employed due to the simple str uc ture and va riable division-ra tio. The
remaining sta ges can be designed with CML and Miller dividers for their wide
locking range. The major challenge is to design the ﬁrst stag e divider with a
wide locking range to cover the wide freque nc y range at 60 GHz. To increase
the locking r ange of the ILFD, varactors are usually deployed to tune the self-
oscillation frequency of ILFD. However, a large varactor will severely degrade
the phase noise of ILFD and PLL . Capacitor banks can b e used to realize
multi-band operation . Howe ver, as frequency scales up to 60-GHz, the
large parasitic capacitance from the capacitor bank becomes too large and the
quality factor of capac itor decrea ses signiﬁcantly [145, 186].
As introduced in Chapter 4, inductive tuning has recently become a
promising substitute to replace the capacitive tuning, and is used in 60-GHz
VCO design to realize a wide FTR. Besides a wide FTR, inductive tuning can
also pr ovide the beneﬁt of isolated DC noise from the tuning element. The
same mechanism could be used in high-frequency divider design to realize a
large locking range. In this section, an inductor-loaded transformer is intro-
duced into the conventional ILFD structure to improve its locking range by
creating multiple frequency bands.
The locking range (LR) for an injectio n locke d oscillator is given by