User's Manual Part 1

AT7400 Owner Manual Rev. 2.0 – October 2006-10-23 Section 3 - 10/39
3.2.2. 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 signal generation out from the local oscillator, 4
VCO’s (voltage controlled oscillator) were implemented with a shift frequency range of 125MHz each. Just one
VCO works at the time to avoid interferences. On this arrangement, a 102dBc@20Hz phase noise is achieved.
The synthesizer on the feedback loop select the desired frequency inside the choose VCO band.
The VCO’s signal outputs are isolated among them via a sum and inverted circuits. The reference frequency
generated by a DDS circuitry complete the Up-conversion frequency process. The output local oscillator signal
power is +5dBm.
3.2.3. PCB CIM3443 – Local Oscillator – Block Diagram
3.3. PCB CIM3442 – UHF Mixer
The mixing operation translates the IF modulated signal frequency up to the assigned UHF broadcasting channel,
the operation utilizes the complex approach considering the orthogonal pair of signals (I-, I+) and (Q-,Q+) allowing
rejection of one side band, and the oscillator itself. The rejection is around 40dB, facilitating the post filtering
steps.
Right after the mixing operation, the now RF signal is 10dB broadband (VHF and UHF) amplified under a typical
2dB roll-off characteristic. The RF output pass through a 25dB dynamic range variable attenuator. The control of
this attenuator is related with the DC level generated by the ALC (Automatic Level Control). In case of absence of
a IF signal or valid IF signal (locked via PLL) of any of the 4 possible IF signals, the attenuator assumes it higher
level of attenuation, and shutting down the RF chain of amplification.