ATLAS 4500 Multimode Station Technical Manual
A-7
135-175 MHz Receiver Module
Appendix A - VHF Circuit Descriptions
A.2.2 Schematic Description
A.2.2.1 Frequency Synthesizer
RF is output from the VCO board on SKU2-1 is applied to the fractional-N
synthesiser IC6 main divider input. This signal is compared with the reference
oscillator frequency and the correction voltage from the synthesiser’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. 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 so as to provide
a wide tuning range voltage and good phase noise to frequency control varicap
diodes located on the VCO board. Due to the wide VCO range used different PLL
charge pump values are set across the tuning range by the Controller.
A second RF output from the VCO on SKT2-6 is mixed with the incoming RF and
generates an IF output of 55 MHz at CN2. This VCO signal is first buffered by a
very high isolation circuit consisting of a MMIC on the VCO and a 1 dB pad and a
MMIC 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 4 band high Q inductor oscillators with a
common base oscillator for low phase noise. This is contained in a shielded
compartment in the case. The VCO provides frequencies for high side injection in 2
bands for the 55 MHz IF. The bands are 190 – 210 MHz and 210 – 230 MHz.
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 the
55 MHz IF signal.
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 the VCO frequency (SKT – 1).
