Page 432 - Satellite Communications, Fourth Edition

412 Chapter Thirteen

For space stations transmitting in the frequency range 3400 to
7750 MHz, the limits are specified in terms of power flux density for
any 4- kHz bandwidth. Denoting the angle of arrival as degrees,
measured above the horizontal plane as shown in Fig. 13.7c, these
limits are
2
■ 152 dB(W/m ) in any 4-kHz band for 0° 5°
2
■ 152 0.5 ( 5) dB(W/m ) in any 4-kHz band for 5° 25°
■ 142 dB(W/m ) in any 4-kHz band for 25° 90°
2
Because the specification is in terms of power or flux density in any
4-kHz band, not the total power or the total flux density, a carrier may
be within the limits when heavily frequency-modulated, but the same
carrier with light frequency modulation may exceed the limits. An
energy-dispersal waveform is a low-frequency modulating wave which
is inserted below the lowest baseband frequency for the purpose of dis-
persing the spectral energy when the current value of the modulating
index is low. In the INTELSAT system for FDM carriers, a symmetri-
cal triangular wave is used, a different fundamental frequency for this
triangular wave in the range 20 to 150 Hz being assigned to each FDM
carrier. The rms level of the baseband is monitored, and the amplitude
of the dispersal waveform is automatically adjusted to keep the over-
all frequency deviation within defined limits. At the receive end, the dis-
persal waveform is removed from the demodulated signal by lowpass
filtering.
With television signals the situation is more complicated. The dis-
persal waveform, usually a sawtooth waveform, must be synchronized
with the field frequency of the video signal to prevent video interfer-
ence, so for the 525/60 standard, a 30-Hz wave is used, and for the
625/50 standard, a 25-Hz wave is used. If the TV signal occupies the
full bandwidth of the transponder, known as full-transponder television,
the dispersal level is kept constant at a peak-to-peak deviation of 1 MHz
irrespective of the video level. In what is termed half-transponder tel-
evision, where the TV carrier occupies only one-half of the available
transponder bandwidth, the dispersal deviation is maintained at 1-MHz
peak to peak when video modulation is present and is automatically
increased to 2 MHz when video modulation is absent. At the receiver, video
clamping is the most commonly used method of removing the dispersal
waveform.
Energy dispersal is effective in reducing all modes of interference but par-
ticularly that occurring between earth and terrestrial stations (A mode)
2
and between space and terrestrial stations (C ). It is also effective in reduc-
1
ing intermodulation noise.