Technical Manual

767-870 MHz Exciter Module
Appendix A - 700/800 MHz Circuit Descriptions
A-3
ATLAS 4500 Multimode Station Technical Manual July 2021
Synthesizer IC28 and associated components provide a reference of 345.6 MHz for
IC31 DDS IC. This 345.6 MHz signal is frequency locked to the 10 MHz main
reference clock. The 345.6 MHz clock is divided by 8 within IC31 and outputs a
43.2 MHz clock that drives IC32 FPGA.
Synthesizer IC24 and associated components provide a reference of 120.45 MHz
for the error loop second mixer LO amp IC21. This 120.45 MHz signal is frequency
locked to the 10 MHz main reference clock.
Synthesizer IC22 and associated components provide a reference of 40.32 MHz for
the MCLK line to the A/D IC 31. This 40.32 MHz signal is frequency locked to the
43.2 MHz FPGA ADLMX line.
A.1.2.2 TX VCO Board
The plug-on VCO board consists of a 2-band dielectric resonator oscillator with
common base oscillator for low phase noise. This is contained in a shielded
compartment in the case. The VCO provides frequencies over 879.64 – 894.64
MHz and 969.64 – 989.64 MHz ranges.
The main RF VCO OUT on SKJ-6 is first buffered by a very high isolation circuit
consisting of the MMIC on the VCO, then on main board, a 3 dB pad and an
amplifier IC17. It is then up-converted at mixer X8 and used to generate the main
transmit RF amplifier signal source.
Each band is switched under control of the Controller. 2 MMIC buffers feed the
synthesizer and main outputs. The power supply to the VCO consists of an 8-V
regulator and active filter for maximum noise rejection located on the exciter PCB.
The 8-V supply consists of IC11, TR6 and associated components.
The PLL control line varies VCO frequency (SKT – 1).
A.1.2.3 Reference Clocks
A stable 10 MHz reference is generated by X1—a temperature controlled crystal
oscillator, and is fed to the synthesizer IC6 through select switch IC7. A detector
circuit senses the presence of an external 10 MHz reference > ~ 0 dBm (0.6 Vpp)
and informs the Controller through EXT REF DET CN3 – 10. IC7 switches
between this internal oscillator and an external 10 MHz reference source fed into
CN4 under control of the Controller through EXT OSC SW CN3-13.