  We use Floquet formalism to study fluctuations in periodically modulated
continuous quantum thermal machines. We present a generic theory for such
machines, followed by specific examples of sinusoidal, optimal, and circular
modulations respectively. The thermodynamic uncertainty relations (TUR) hold
for all modulations considered. Interestingly, in the case of sinusoidal
modulation, the TUR ratio assumes a minimum at the heat engine to refrigerator
transition point, while the Chopped Random Basis (CRAB) optimization protocol
allows us to keep the ratio small for a wide range of modulation frequencies.
Furthermore, our numerical analysis suggests that TUR can show signatures of
heat engine to refrigerator transition, for more generic modulation schemes. We
also study bounds in fluctuations in the efficiencies of such machines; our
results indicate that fluctuations in efficiencies are bounded from above for a
refrigerator, and from below for an engine. Overall, this study emphasizes the
crucial role played by different modulation schemes in designing practical
quantum thermal machines.
