Service manual

STP 11-25R13-SM-TG
U - 9
b. In all uses, the deviation is generally 20 dB below the peak video deviation of 4 MHz.
c. Program Audio Subcarriers.
(1) The program audio subcarriers are generally modulated 75 kHz peak deviation at a 1 kHz test
tone. Maximum audio bandwidth is almost universally 15 kHz. To allow adequate filtering of one
subcarrier from the next, subcarriers are generally spaced at least 600 kHz apart (though occasionally a
400-kHz spacing is used). Moreover, successive subcarriers are spaced at successively larger distances
apart. This avoids a multiple difference beat of 600 kHz in the video baseband.
(2) Up to four audio subcarriers are commonly used. But the more subcarriers used, the greater
receiver bandwidth required, with its tradeoffs. When 25 MHz wide channels (7 and 13 GHz bands), the
four subcarriers are often located at 6.2, 6.8, 7.5 and 8.3 MHz. If 17 MHz wide channels are used, (2
GHz band) the four subcarriers might be located. at 5.8, 6.2, 6.8 and 7.5 MHz. However, such a four-
subcarrier system in a 17 MHz bandwidth will show compromises in audio and video quality.
Occasionally a subcarrier at 4.83 MHz will be used. However, this generally requires more severe video
filtering, and the resultant degradation of video performance. The 4.83 MHz subcarrier is generally used
in ENG systems where a narrow IF filter is desirable due to transmitter power limitations and band
congestion.
(3) Often a single-sideband (SSB) suppressed carrier supervisory and control system is added to
the baseband. If less than four subcarriers are used, this SSB modulation system might be located at
one of the above mentioned subcarrier frequencies. In four subcarrier systems, the SSB system might be
located at 8.59 or 9.023 MHz.
d. Order Wire and Supervisor-Y Signals. The supervisory and control system can include both an
engineer's order wire and remote repeater alarm. The order wire allows two-way communication among
the terminals and the repeaters. The alarm allows automated fault reporting from these unattended
repeater locations. The requirements for "party-line" transmission from multiple sites forces the use of a
suppressed carrier, avoiding carrier beats which would otherwise occur.
e. Continuity Pilot. In addition, a continuity pilot may be included in the baseband spectrum.
Generally the pilot is the highest frequency in the baseband; 8.59 and 9.023 MHz are commonly used.
f. Modulators and Demodulators. Depending on manufacturers and models, the program audio
modulator and demodulator might be packaged separately or be integral to the microwave transmitter or
receiver.
(1) The subcarrier modulator is actually a FM transmitter operating in the range of 4.83 to 8.3 MHz.
The subcarrier oscillator is generally frequency stabilized. Previous techniques had included AFC for the
oscillator using a built-in FM discriminator as the frequency reference. Current practice is to phase lock
the oscillator to a crystal reference using IC phase-lock components. Usually 75-usec preemphasis is
employed in the audio circuit.
(2) The subcarrier demodulator is similarly a FM receiver operating in the range of 4.83 to 8.3 MHz.
These demodulators are available fixed tuned to one subcarrier frequency (analogous to a TRF receiver)
or frequency agile, able to be tuned to several subcarrier frequencies (the super-heterodyne approach).
In either case, the RF or IF filters must be sharp enough to reject adjacent subcarriers as well as the 20-
dB stronger video signal.