A block upconverter (BUC) is used in the transmission (uplink) of satellite signals.
The 10 MHz reference frequency is usually sent on the same feedline as the main carrier.
BUCs are generally used in conjunction with low-noise block converters (LNB).
The block upconverter is a block shaped device assembled with the LNB in association with an OMT, orthogonal mode transducer to the feed-horn that faces the reflector parabolic dish.
This is opposed to other types of frequency upconverter which may be rack mounted indoors or not co-located with the dish.