Technical Manual

792-825 MHz Receiver Module
Appendix A - 700/800 MHz Circuit Descriptions
A-7
ATLAS 4500 Multimode Station Technical Manual July 2021
The receiver module consists of two circuit boards. A Voltage Controlled
Oscillator (VCO) board and an RX board with associated frequency synthesizers,
down converter, TCXO, filtering circuits and internal power supply circuits.
Receiver input frequency programming is achieved by using serial data connections
to an external controller. A serial flash IC contains synthesizer band, charge pump
settings and configuration data read by the Controller.
A.2.2 Schematic Description
A.2.2.1 Frequency Synthesizer
RF is output from the VCO board on SKU2-1. It gets applied to the fractional-N
synthesizer IC6 main divider input. This signal is compared with the reference
oscillator frequency and the correction voltage from the synthesizer’s charge pump
output, which 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. The
VCO control voltage is also buffered and sent to the Controller through on RX
VCO VOLTS (SK1-14). The op amp uses a 25-V power supply generated by IC4
to provide a wide tuning range voltage and good phase noise-to-frequency control
varicap diodes located on the VCO board.
A second RF output from the VCO on SKT2-6 is mixed with the incoming RF and
generates an IF output of 90 MHz at CN2. This VCO signal is first buffered by a
very high isolation circuit consisting of a MMIC on the VCO and a 1dB pad and an
amplifier (IC17) at 22 dBm.
Frequency programming data for the exciter is sent to IC6 from the Controller
through a serial data (SK1-18), Clock (SK1-15), Strobe (SK1-17). A lock detect
signal from IC6 is also fed to the Controller through RX LD (SK1-16).
A.2.2.2 RX VCO Board
The plug on VCO board consists of 2 band dielectric resonator oscillators with a
common base oscillator for low phase noise. This is contained in a shielded
compartment in the case. The VCO provides frequencies in 2 bands for 90 MHz
high side injection. The bands are 911.64 – 924.64 MHz and 924.64 – 944.64
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 the main board, a 3 dB pad and an
amplifier IC17. It feeds LO to mixer X8 used to down convert the RX signal to a 90
MHz IF signal.