Technical Manual

A-2 ATLAS 4500 Multimode Station Technical Manual July 2016
767-870 MHz Exciter Module Appendix A - 700/800 MHz Circuit Descriptions
A.1.1 General Description
The Exciter module generates the low level RF transmitter signal that is amplified
to nominal output power level by the Power Amplifier. The exciter module consists
of two circuit boards. A Voltage Controlled Oscillator (VCO) board and an RF
board with associated frequency synthesizers, a modulation controller, filtering
circuits and internal power supply circuits. Also included is circuitry for an error
loop system to improve transmitter linearity.
Output frequency programming is achieved by using serial data connections to an
external controller.
If an external frequency reference is desired, the exciter can be fed with an external
10 MHz signal. Otherwise it has an internal 10 MHz reference oscillator.
A.1.2 Schematic Description
A.1.2.1 Frequency Synthesizers
There are 4 synthesizers in the MDM Exciter: The main synthesizer, and 3
synthesizers used in the error loop section.
The Synthesizer O/P from the VCO board on SKU-1 is split to the dual fractional-N
synthesizer IC6 main divider input and the error loop first mixer LO amp IC16.
This signal is compared with the 10 MHz reference oscillator frequency and the
correction voltage from the synthesizer’s charge pump output is filtered then
amplified by the non-inverting low noise op amp IC10. This correction voltage is
fed back to the VCO to maintain loop lock as well as being fed to the Controller
through TX VCO VOLTS CN3-14. A lock detect signal from IC6 is also fed to the
Controller through TX LD CN3-16. The op amp IC10 uses a 25-V power supply
generated by IC4 to provide a wide tuning range voltage to frequency control
varicap diodes located on the VCO board. The VCOs power is supplied by a
dynamic filter TR6 and 8 V regulator IC11.
A serial flash IC47 contains synthesizer band, charge pump settings and
configuration data.
This is read by the Controller through the I2C bus to set the synthesizer.
Frequency programming data for the exciter is sent to Synthesizer IC6 from the
Controller through serial data (CN3-18) under the control of the Clock (CN3-15)
and Strobe (CN3-17) lines.