Controller Section; Analog Mode Of Operation - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

2-12
The voltage-controlled oscillator (VCO) module employs a Colpitts configuration with two bipolar
stages in a common-base, common-collector configuration. The LC tank circuit's capacitive portion
consists of a varactor diode, coupling capacitor, and a laser-trimmed capacitor for frequency
adjustment. The inductive portion consists of microstrip transmission line resonators for TX VCO and
coaxial resonators for RX VCO. Tuning is performed by the module manufacturer and is not field
adjustable. The varactor changes the oscillator frequency when the DC voltage of the steering line
changes. The output of the common base is coupled to the second transistor for impedance
buffering, and its output is coupled to respective TX/RX buffer amplifiers.
In TX mode, the transmitter VCO output is coupled to a three-stage buffer before being injected into
the power amplifier. In RX mode, the receiver VCO output is buffered and amplified with a two-stage
transistor/microwave monolithic IC (MMIC) circuit. The output of the first-stage transistor is split into
two paths. One path feeds back to the synthesizer prescaler; the other path is injected into the
second-stage MMIC. The output of the MMIC provides the proper signal level for the LO port of the
RX front-end mixer.
The superfilter supplies the voltage to the first two stages of the TX buffer and to the first-stage
transistor of the RX buffer/amplifier. The voltage for the third stage of the TX buffer is supplied by a
keyed 9.1 V source to conserve current drain while the radio is receiving. The second-stage MMIC of
the RX buffer/amplifier is supplied by a 9.3 V regulator.
2.7

Controller Section

This section provides an explanation of radio operating modes and an overview of the controller
section components and circuits.
2.7.1

Analog Mode of Operation

When the radio is receiving, the signal comes from the antenna/antenna-switch to the front-end
receiver. The signal is then filtered, amplified, and mixed with the first local-oscillator signal
generated by the voltage-controlled oscillator (VCO). The resulting intermediate frequency (IF)
signal is fed to the IF circuitry, where it is again filtered and amplified. This amplified signal is passed
to the digital back-end IC, where it is mixed with the second local oscillator to create the second IF at
2.25 MHz. The analog IF is processed by an analog-to-digital (A/D) converter inside the digital back-
end IC where it is converted to a digital bit stream and divided down to a baseband signal, producing
digital samples. These samples are converted to TTL logic signals and sent to the DSP. The DSP
digitally filters and discriminates the signal, decodes the information in the signal, and identifies the
appropriate destination for it. For a voice signal, the DSP will route the digital voice data to the coder/
decoder (CODEC) for conversion to an analog signal. The CODEC will then present the signal to the
audio power amplifier, which drives the speaker. For signalling information, the DSP will decode the
message and pass it to the microcomputer.
When the radio is transmitting, microphone audio is passed to an adjustable gain circuit, then to the
CODEC where the signal is digitized. The CODEC passes digital data to the DSP where pre-
emphasis and low-pass (splatter) filtering are done. The DSP sends this signal to the modulation
digital-to-analog (D/A) converter where it is reconverted into an analog signal and scaled for
application to the voltage-controlled oscillator as a modulation signal. Transmitted signalling
information is accepted by the DSP from the microcomputer, coded appropriately, and passed to the
modulation D/A converter, which handles it the same as a voice signal. Modulation information is
passed to the synthesizer along the modulation line. A modulated carrier is provided to the power
amplifier (PA), which transmits the signal under dynamic power control.
May 25, 2005
Product Overview: Controller Section
6881096C74-B