Service manual

STP 11-25R13-SM-TG
U - 3
(5) Weatherproof housing for portable equipment.
(6) Ruggedized packaging for vehicular mobile equipment.
c. Baseband Amplifier. The baseband amplifier is wideband (10 MHz or so) for best linearity and
incorporates liberal feedback for stability. A 13 dB preemphasis is used which together with the receiver
de-emphasis improves the overall signal to noise performance. Provision is also made for combining one
or more audio voltage-controlled oscillator (VCO). The gain of the video amplifier is matched to the
modulation sensitivity of the oscillator so that 8 MHz P-P deviation is produced on the RF carrier for a 1V
P-P input video signal program subcarrier modulators with the video before frequency modulating the.
The low frequency response of the amplifier extends below 1 Hz as required for tilt performance.
d. Voltage-Controlled Oscillator (VCO). The transistorized VCO typically runs at 2 GHz where
transistors are available with enough power capacity to produce an adequate transmitter output level. As
new transistors are developed, the trend will result in higher output power and greater power conversion
efficiency (lower DC input power). A number of solid-state devices are available that operate directly at
the output carrier frequency.
e. Power Amplifier. The power amplifier consists of one or more broadband stages to provide a
gain of 25 to 30 dB at a power output of 8 to 10 watts at 2 GHz falling to 1 watt or less at 13 GHz. As with
other transmitter components, the mark of quality is linearity, wide bandwidth (10% or more of the
operating frequency) and long-term stability. The amplifier should operate on any assigned frequency
channel in the band without retuning.
f. Frequency Multipliers.
(1) Present generation solid-state equipment at 7 and 13 GHz use frequency multipliers and the
degree of multiplication used depends upon the output operating frequency. This is typically x3 or x4 for
7 GHz and x6 for 13 GHz. These multipliers consist of varactors, impedance matching circuits, cavities,
and filters to efficiently operate over a wide band of frequencies with minimum spurious output signals.
No tuning is required to cover up to 10 assigned frequency channels in one of the three operating bands.
Typical power output from the varactor multiplier is 1 to 2 watts at 7 GHz and 0.8 watts at 13 GHz.
(2) At the present time, operating frequencies of solid-state components are increasing so that the
need for frequency multipliers will not exist for any frequency band in the near future, as they are not
presently required at 2 GHz.
g. Common Approaches for AFC.
(1) One is a crystal-controlled source that feeds one side of a mixer. The other side of the mixer is
fed with a sample of the transmitter frequency. The mixer output is an IF signal that is amplified and
demodulated by a temperature-compensated discriminator. The DC output from the discriminator is the
carrier frequency error voltage that, when applied through a DC amplifier to the VCO, keeps it on its
assigned frequency.