User Manual

SECTION 1: THEORY OF OPERATION
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This section displays the DC power supplies, frequency reference, and RF output circuitry. Regulator
VR1 provides 9 volts DC for VCO module VCO1, and RF amplifier (U7). Regulator (VR2) provides a low
noise 5-volt DC output for inverter (U3), synthesizer (U2), and reference (Y1).
Reference module (Y1) provides a high-stability 10 MHz reference frequency. Y1 is a voltage controlled,
temperature controlled crystal oscillator (VCTCXO). This device also has a VC input which accepts a
control voltage from pot R23. The pot permits a slight shift in the reference frequency which enables the
three (3) receivers to be tuned precisely to the assigned receive frequency. A diode (CR2) provides
additional voltage regulation, improving the frequency stability of reference Y1.
The RF output circuitry consists of RF amplifier (U7), and power splitters (U5 and U6). U7 increases the
signal level to correct for losses in the splitters. One output drives splitter U5, which provides local
oscillator injection for Receivers 2 and 3. The other output drives splitter (U6), which drives receiver 1
and the PLL_FEEDBACK input on chip U2.
Exciter Board
This section displays the input/output interface, transmitter keying, and power supply circuitry. The
input/output interface is built around terminal block (TB1) and Schmidt Trigger inverters (U7). Incoming
clock, serial data, and chip select signals on block TB1 are squared up by U7. Then they are sent to the
appropriate inputs on the transmitter synthesizer (U2). The EXCDATA source comes from the receive
synthesizer on the Injection Synthesizer Board. A Schmidt Trigger chip is used here because of a cable
ran to the System Controller Board. The synthesizer returns a lock detect output to the Injection
Synthesizer Board via U6 and EXCLD.
A regulator (VR2) powers the T/R switch circuitry. When the System Controller Board makes TXKEY*
low, inverter U6D goes high, turning on transistor Q2 and FET Q1. This applies 5-volt power to the
TXENABLE output, turning on the T/R switch on the Power Amplifier Board. At the same time, transistor
Q3 conducts, grounding the KEY* input of the Power Amplifier Board. Finally, inverter U6C goes high
and turns on RF switch U1, connecting the VCO output to the Power Amplifier Board for transmission.
The power supply consists of two (2) voltage regulators. A regulator (VR1) provides 5-volt power for the
VCO.
Analog Modulation
This section displays the analog modulation circuitry. Incoming modem audio from the System Controller
Board appears at TXMOD, and is buffered by op amp U5C. If an external modulation source (modem or
amplified microphone) is connected to the base station’s DB25 connector, audio appears at EXTMOD.
From there the audio passes through low pass filters U10, U4DCBA, and on to the input of op amp U5B.
The audio is inverted and amplified by an op amp (U5B). It then passes on to the VCO module via
VCOMOD.
The 10 MHz reference is also modulated in order to counteract the corrective effects of the synthesizer
loop circuitry. For example, if only the VCO were modulated, the synthesizer would try to compensate for
the frequency “error,” caused by the modulation. This effectively reduces the amount of modulation
available. Modulating the reference and the VCO simultaneously effectively cancels this effect when the
reference frequency goes high, the VCO frequency goes high, and vice-versa.
An op amp (U9A) amplifies the AUDIO output from another op amp (U5D) and applies it to jumper block
JMP1. Pot R30 adjusts the gain of U9A. Op amp (U9B) inverts the phase of the audio and applies it to
the other side of jumper block JMP1. The purpose of the jumper block is to select the proper phase of the
audio. If the wrong phase is used, on modulation peaks the reference will swing in the same direction as