User's Manual

AT7120 Owner Manual Rev. 1.1 – March 2009 28/76
The already modulated IF carrier I and Q feed the 2 inputs of the up-converter – Module 4492. The
output signal of this module is a FCC/6MHz/UHF channel (14-69). Besides this main function, it also
controls the potential overdrive into the final 60W amplifier.
The up-conversion circuit consists of an orthogonal mixer fed by the IF carriers, I and Q respectively
with a fixed center frequency and 6 MHz bandwidth. The mixer converts the IF signal using the local
oscillator CW (Carrier Wave) signal. This mixing results in two RF lateral bands. The superior lateral
band is not desirable and is attenuated by 35dB.
The resultant RF signal is filtered and amplified. The serial data stream that program the PLL circuit is
transferred via MICROWIRE interface, composed by 3 high speed transfer lines of 20MHz. The
microcontrollers inside the up-converter perform the following functions: {needs clarification}
PCB CIM3443 – Local Oscillator
The local oscillator is designed based on a PLL circuit. This oscillator is able to synthesize frequencies
within the band from 450MHz up to 900MHz. To guarantee a high quality local oscillator signal, 4
VCO’s (voltage controlled oscillator) are implemented with a shift frequency range of 125MHz each.
Just one VCO works at the time to avoid interference. This approach achieves a 102dBc@20Hz phase
noise level. The synthesizer on the feedback loop selects the desired frequency inside the chosen VCO
band.
The VCO’s signal outputs are isolated via sum and inverted circuits. The reference frequency generated
by a DDS circuitry completes the up-conversion frequency process. The output local oscillator signal
power is +5dBm.
PCB CIM3443 – Local Oscillator – block diagram
Fig.4.3: CIM3443 local oscillator – block diagram