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AUDIO BOARD 800–194.
The 600 ohm balanced audio input to the transmitter is connected to the inputs of unity gain differential amplifier
IC–1A. Potentiometer R9 is provided for adjusting the amplifier to produce the maximum common mode rejection
(CMR). The output of the differential amplifier is directly coupled to the pre–emphasis amplifier. The negative
feedback circuit of this amplifier has two capacitors selected by switches S1 and S2 which provides options of 0, 25,
50, or 75 microsecond pre–emphasis. Following the pre–emphasis circuit are two stages of active Butterworth low–
pass filtering. The signal is then routed to the MODULATOR audio input and to metering amplifiers IC–3A and
IC–3B. The subcarrier input is routed to the MODULATOR subcarrier input.
MODULATOR 800–195.
The MODULATOR consists of voltage controlled crystal oscillator Q2, buffer amplifier Q3, subcarrier modulator
D2, precision voltage regulator IC–1 and oven heater control Q1. Precise frequency control is obtained by thermally
coupling components Y–1, Q1, Q2, and IC–1 to temperature regulated frequency control module 800–160–1.
Coarse and fine frequency adjustment is provided by L1 and L2 while the main channel modulation is provided by a
voltage variable capacitance diode in the frequency control module. Bias for this diode is obtained through voltage
divider resistors R4, R5, and R7. Diode D1 temperature compensates the bias supply. Components L3, C13 and D2
permit phase modulation of the subcarrier information upon the RF output of transistor Q3.
MULTIPLIER BOARD 800–163.
The modulator RF output is frequency–multiplied in the multiplier board to obtain the various operating frequencies
listed on the STL–10A specification sheet. The power level at the input to the multiplier is approximately 5 mw.
The output level is 0.7 to 1.2 watts.
RF POWER AMPLIFIER 800–374B AND 800–373B.
The RF power amplifier consists of two boards; the PA Interface board (800–374B) and the Two–Stage RF Power
Amplifier board (800–373B). Up to 1 watt RF power from the Multiplier board (800–163) at a frequency ranging
from 445 to 480 MHz is fed into the 10/15C PA Interface board (800–374B). The purpose of this board is to double
the incoming frequency, amplify the doubled frequency and filter out all other frequencies. This board also provides
a means for controlling the RF power level via the front panel POWER ADJUST POT. An automatic level control
(ALC) circuit stabilizes and maintains an accurate output power level. A 50 mW (max), 890–960 MHz signal from
this board is fed into the two stage RF Power Amplifier board. The RF goes through two stages of RF amplification.
The first stage (U2) is a 1 watt (max) pre–driver. It has an input and output transformer (T1 and T2) for achieving
optimum 50 ohm matching between the stages. The output of T2 is fed into the final PA module (U3) for an output
of 15 watts max. The signal is then low–passed filtered through FL1 and then fed through a directional coupler for
monitoring forward and reflected power. This board also provides regulated B+ for powering the PA and the rest of
the chassis. Finally, there also exist circuitry for regulating the fan and monitoring PA current.
DUAL LINK STEREO STL.
Two STL–10A Links can share the same 500 KHz, 300 KHz, or 200 KHz channel for transmission of left and right
stereo components, as shown in the STEREO STL SYSTEM BLOCK DIAGRAM, Drawing No. 702–058. A system
operating in this manner uses no more spectrum (and possibly less) than "COMPOSITE" systems, while providing
much lower channel cross–talk (72 dB). Since the two links are independent, if one link fails, continued operation in
mono service is assured. Phase shift between channels can be less than 1 degree. The two carriers are offset above
and below the channel center frequency as shown in the following FCC EMISSION PROFILE Drawings. Offsets are
125 KHz for 500 KHz channels, 75 KHz for 300 KHz channels, and 50 KHz for 200 KHz channels. Using a Marti
HRC–10 Transmitter Combiner having 40 dB isolation, all sidebands and IM products are well within the limits of
Subpart E of FCC Rules Part 74.535. Optional transmitter combiners are available which provide 70 dB or 100 dB
of isolation. Receiver IF selectivity required in this system is 200 KHz for 500 KHz channels, 85 KHz for 300 KHz
channels and 75 KHz for 200 KHz channels.
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