User's Manual

AT7120 Owner Manual Rev. 1.1 – March 2009 29/76
PCB CIM3442 – UHF Mixer
The mixing operation translates the IF modulated signal frequency up to the assigned UHF broadcasting
channel, the operation utilizes 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 RF broadband signal is (VHF and UHF) amplified 10dB 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 an IF signal, a valid IF signal (locked via PLL) or any of the 4
possible IF signals; the attenuator assumes its highest level of attenuation, and shuts down the RF
amplification chain.
PCB CIM3442 – UHF Mixer – block diagram
Fig.4.4: CIM3442 UHF mixer – block diagram
PCB CIM3442 – UHF Mixer – adjustment procedures:
The mixer-ALC has two types of adjustments. The first one is a DC level that polarizes each branch of
the input of the complex mixer. Each one of the DC levels must be adjusted to assure maximum
rejection of the undesired vestigial side band, in conjunction with the LO rejection as well. The level