  We have carried out full imaging simulation studies to explore the impact of
frequency standards in millimeter and sub-millimeter Very Long Baseline
Interferometry (VLBI), focusing on the coherence time and sensitivity. In
particular, we compare the performance of the H-maser, traditionally used in
VLBI, to that of ultra-stable cryocooled sapphire oscillators over a range of
observing frequencies, weather conditions and analysis strategies. Our
simulations show that at the highest frequencies, the losses induced by H-maser
instabilities are comparable to those from high quality tropospheric
conditions. We find significant benefits in replacing H-masers with cryocooled
sapphire oscillator based frequency references in VLBI observations at
frequencies above 175 GHz in sites which have the best weather conditions; at
350 GHz we estimate a 20-40% increase in sensitivity, over that obtained when
the sites have H-masers, for coherence losses of 20-10%, respectively. Maximum
benefits are to be expected by using colocated Water Vapour Radiometers for
atmospheric correction. In this case, we estimate a 60-120% increase in
sensitivity over the H-maser at 350 GHz.
