November 16, 2006
6881094C31-E
3-14
Theory of Operation:
Transceiver Board
700–800 MHz:
Three VCOs generate the first LO and transmit-injection signals.
NOTE:
Refer to
Table 8-1, “List of Transceiver Schematics and Board Overlays,” on page 8-1
for a
listing of FGU-related schematics that will aid in the following discussion.
3.1.4.1 Reference Oscillator Y200
The radio's frequency stability and accuracy derive from the Voltage-Controlled Temperature-
Compensated Crystal Oscillator (VCTCXO), Y200. This 16.8 MHz oscillator is controlled by the
voltage from the WARP pin of the FracN (fractional-N frequency synthesizer) IC, U202, that can be
programmed through a serial peripheral interface (SPI). The oscillator output at pin 3 is coupled
through capacitor C234 to the FracN synthesizer reference oscillator input and through C236 (C237
for VHF and UHF Range 2) to the non-invertive input of the op-amp, U201.
Op-amp U201 buffers the 16.8 MHz output to the VOCON board. Components L205 and C214
(C213, L205, and C214 for VHF) form a low-pass filter to reduce harmonics of the 16.8 MHz.
The Digital-to-Analog Converter (DAC) IC, U203, and Switched Capacitors Filter (SCF) IC, FL200,
form the interface between radio's DSP and the analog modulation input of the FracN IC.
3.1.4.2 Fractional-N Frequency Synthesizer (FracN) IC U202
The FracN IC, U202, is a mixed-mode, Motorola-proprietary, CMOS, fractional-N frequency
synthesizer with built-in dual-port modulation. The XTS 5000 radio uses a low-voltage version of the
device, sometimes called LVFracN, for compatibility with the 3 V logic used throughout the radio.
The FracN IC incorporates frequency division and comparison circuitry to keep the VCO signals
stable. The FracN IC is controlled by the MCU through a serial bus. All of the synthesizer circuitry is
enclosed in rigid metal cans on the transceiver board to reduce interference effects.
Separate power supply inputs are used for the various functional blocks on the IC. Inductors L203
and L204 provide isolation between supply pins 20 (AVDD) and 36 (DVDD) connected to Vdd3. Host
control is through a three-wire, smart SPI interface (pins 7, 8, and 9) with a bi-directional data pin.
FracN functions include frequency synthesis, reference clock generation, modulation control, voltage
multiplication and filtering, and auxiliary logic outputs.
3.1.4.2.1 Synthesizer
Frequency synthesis functions include a dual-modulus prescaler, a phase detector, a programmable
loop divider and its control logic, a charge pump, and a lock detector output. Fractional-N synthesizer
IC principles of operation are covered in detail in the manufacturers' literature. No similar discussion
will be attempted here.
3.1.4.2.2 Clocks
U202, pin 23 (XTAL1), is the 16.8 MHz reference oscillator input from the VCTCXO (Y200).
3.1.4.2.3 Modulation
To support many voice, data, and signaling protocols, XTS 5000 radios must modulate the
transmitter carrier frequency over a wide audio frequency range, from less than 10 Hz up to more
than 6 kHz. The FracN supports audio frequencies down to zero Hz by using dual-port modulation.
The audio signal at pin 10 (MODIN) is internally divided into high- and low-frequency components,
which modify both the synthesizer dividers and the external VCOs through signal MODOUT (pin 41).
The IC is adjusted to achieve flat modulation frequency response during transmitter modulation
balance calibration using a built-in modulation attenuator.
Summary of Contents for ASTRO XTS-5000
Page 7: ...vi Table of Contents November 16 2006 6881094C31 E Notes ...
Page 11: ...x List of Figures November 16 2006 6881094C31 E Notes ...
Page 17: ...November 16 2006 6881094C31 E xvi CommercialWarranty Notes ...
Page 31: ...November 16 2006 6881094C31 E xxx Portable Radio Model Numbering System Notes ...
Page 83: ...November 16 2006 6881094C31 E 3 44 Theory of Operation Encryption Module ...
Page 135: ...November 16 2006 6881094C31 E 5 44 Troubleshooting Charts Secure Hardware Failure Notes ...
Page 175: ...November 16 2006 6881094C31 E 7 32 Troubleshooting Tables List of Board and IC Signals Notes ...
Page 367: ...November 16 2006 6881094C31 E Glossary 10 Notes ...
Page 373: ...Index 6 November 16 2006 6881094C31 E Notes ...