0:1 SO that accurate level stability 2Hz. (CCITT, 1985.) To achieve this the carrier generation master of the carrier is not required. oscillators receive regular maintenance adjustment using a refer- ence frequency comparison pilot (see Section 20.6.2.2) or in later developed systems are phased locked to the frequency comparison 20.4.1.2 Carrier and signal suppression pilot. The recommended stability of the carrier frequencies are given in This is also known as carrier 'leak' and signal 'leak'. Perfect balance Table 20.2. With this carrier accuracy the end to end difference in of modulator referred to above is not possible in practice. The best virtual carriers will exceed 3Hz for 1% of the time and 4Hz for 0.1% balance that can be achieved in a manufacturing environment is in of the time over a hypothetical international link of 2500km. the order of 30dB. There will therefore be some unwanted products at the output of the modulator that will have to be removed by filtering before presentation to the combined path. Filter Fa (see Figure 20.1) Table 20.2 Recommended stability of carrier frequencies requirements are shown in Table 20.1. The requirement is higher for the signal leak and the upper sideband as these products will give rise to crosstalk. Carrier Stability Channel carriers 1 1 x 10 6 Carriers Group and supergroup 1 1 x 10-7 Hypergroup mastergroup and * 5 x 10 8 20.5.1 Carrier frequency accuracy supermastergroup associated with 12MHz systems For some services, particularly data circuits, it is desirable that the 'virtual' carrier difference over a network is maintained to within 20/8 Frequency division multiplexing 20.5.2 Carrier purity Table 20.3 Some common reference pilots This is as follows: Pilot Frequency 1. Harmonics less than -20dBmO. Group reference pilot 84.080kHz 2. Purity against other carriers and side products related to 4kHz less than -80dBmO. Supergroup reference pilot 411.920kHz Hypergroup or mastergroup reference 1552kHz Spurious sidebands are produced by the carrier impurities during 