Service manual

STP 11-25R13-SM-TG
U - 1
APPENDIX U – PRINCIPLES OF MICROWAVE
U-1. Introduction.
Microwave transmitters play an important part of Armed Forces Radio and Television Service
(AFRTS) transmission capabilities. The studio-to-transmitter link (STL) has long been and will continue to
be the backbone of the commercial broadcast industry. Equipment design today allows for essentially
transparent, full color video transmission, as well as subcarriers capable of full frequency modulation
(FM) audio bandwidth and multiple voice and remote control functions.
a. FM of the microwave carriers has almost exclusively carried out transmission of video and
audio signals over the past several years by an analog signal (the exact waveform to be transmitted
rather than a sampled and coded version of that waveform).
b. Since the broadcast television transmitter uses the analog signal it is probable that analog video
transmission will predominate for at least another decade. Also, audio information is usually frequency
modulated onto a subcarrier, though digital modulation in the sync period or on a subcarrier is also used.
c. The microwave transmitter terminal is comprised of a baseband processor and frequency
modulator connected directly to the microwave or via an intermediate frequency (IF). The receiver
terminal comprises a down converter, frequency demodulation (invariably at IF) and baseband
processing.
U-2. Repeaters.
a. Multihop System.
(1) In a multihop system, two approaches may be taken at the intermediate repeater sites:
(2) The received signal may be demodulated to baseband using a conventional terminal receiver
and then this baseband applied to a conventional terminal transmitter thus regenerating the microwave
signal at the desired output level. This is the remodulation approach.
(3) The second approach is to take the 70 MHz IF off the receiver and apply this to a transmitter,
where it is mixed with a local oscillator 70 MHz away from the desired radio frequency (RF) output. The
correct side band is extracted by filtering, amplified at microwave and forms the desired repeater output
signal. This is the heterodyne approach.
b. Advantages and Disadvantages of the Two Systems.
(1) The basic advantage to a remodulating system is cost. The heterodyne approach involves both
microwave power amplifiers and tight RF (microwave) filters to reject the up-conversion LO and unwanted
sideband. Both these items are costly.
(2) Heterodyne systems have a significant advantage in preserving the purity of the video signal.
Because the video is not demodulated and remodulated, extra low-frequency corners are not added to
the video path. This improves the tilt and bounce performance, particularly over a large number of hops.
Another feature is video gain stability. Since the signal is never demodulated and remodulated, the 4
MHz peak deviation is invariant over any number of hops. This directly translates to video gain stability.
Only the modems at the terminal transmitter and receiver affect the video gain.
c. Choosing a Repeater System.
(1) In long-haul microwave systems, heterodyne repeaters are a necessity to keep end-to-end
video degradation down to reasonable levels.