Service manual

STP 11-25R13-SM-TG
U - 4
(2) A more state-of-the-art approach uses a phase-locked oscillator as the microwave source.
Here a microwave IC divider samples the oscillator output directly at 2 GHz. The output of the divider is
at a frequency where standard transistor-transistor logic (TTL or T
2L
) ICs and techniques can be used to
lock the oscillator to a crystal reference. The phaselock approach involves less hardware and is thus less
expensive.
h. Off-air Monitor. The off-air monitor is essentially a microwave receiver that demodulates a
sample of the transmitter output signal for monitoring. To be of maximum value as a transmitter test tool,
the monitor signal should be of high quality even though this approach is slightly more expensive than
slope detectors and other lower cost devices that are sometimes used.
i. Frequency Switcher. The frequency switcher shifts the operating bias on the oscillator to
change its frequency and, when AFC is used, switches crystals in the reference oscillator. Since all
circuits are wideband, no tuning is necessary when frequency is changed.
j. Transmitter Output Filter. The transmitter output filter is used when the transmitter is diplexed
with another transmitter or duplexed with a receiver, or to reduce spurious levels. The filter may be
broadband covering 5 percent or more of the spectrum, or narrow band covering a single assigned
channel.
k. Portable Applications.
(1) For portable applications, a weatherproof housing is required which is easily fitted, with an
antenna, to a tripod. If remote control is used, special wiring and a connector are installed for mating with
the remote control cable.
(2) Equipment for mobile applications must be packaged ruggedly to withstand the high degree of
shock and vibration experienced in most vehicles. Also size and weight become more important so
special precautions must be taken to keep the high-density packaged components from overheating.
l. Preventing Interference. To prevent interference to or from broadcast microwave equipment,
special precautions must be taken for radio-frequency interference (RFI) filtering and shielding. The total
unit is usually enclosed in an RFI-resistant case with critical circuitry completely shielded. Additionally, all
input and output electrical connections must be RFI filtered. Without these precautions, considerable
difficulty can be expected, since broadcast microwave equipment often operates in close proximity to
high-powered transmitters.
m. Receivers.
(1) All TV microwave receivers are typically super-heterodyned with a 70 MHz IF amplifier as
shown in Figure U-2. For broadcast work, the local oscillator and mixer should be broadband to eliminate
the need for tuning when switching to a variety of different frequencies.