he atomic clock PRC might have some input ports on it to connect other timing equipment, and the GNSS receiver might have an input to allow it to be guided by an atomic clock (see the follow- ing section "10-MHz Frequency"). The GNSS receiver always has an antenna port, which is commonly a subminiature version A (SMA), or a bayonet Neill-Concelman (BNC) connector. It can also be a threaded Neill-Concelman (TNC) connector, which is a threaded form of the BNC. This antenna port commonly supplies a DC voltage up the antenna cable to a pre-amplifier in the base of the antenna (to boost the signal over the length of the antenna cable). Both the TNC/BNC input/output and SMA input connectors on the frequency source are normally female. A PRC/PRS, although it may not be phase aligned with UTC, may also include the capability to output a 1PPS phase signal. Some devices can recover frequency from this single pulse, or perhaps a network that requires phase synchronization but does not need it aligned with UTC can use it. The ITU-T has specified performance characteristics for the PRC in G.811 and has recently added the ePRC performance level to that in G.811.1. For more details on these recommendations, see Chapter 8. Now that the timing distribution network has a source of frequency, how is it used? 10-MHz Frequency The frequency reference signal that is output from a PRC/PRS is normally a 1-, 2-, 5-, or 10-MHz sine wave, with 10 MHz being the most common. However, 2 MHz is also a popular choice, espe- cially outside North America. Typically, these devices may offer optional modules or product vari- ants that support other types of signal outputs, such as E1/T1. These reference signals not only are used to frequency synchronize (syntonize) networking and telecommunications equipment but are also applied to other applications. As illustrated in Figure 3- 17, any device whose measurement accuracy depends on using an oscillator, or counting over time, will normally make provision for a 10-MHz reference signal as input (photos