SI4770-77-A20 Datasheet, PDF (25/54 Pages) Silicon Laboratories – HIGH-PERFORMANCE CONSUMER ELECTRONICS

Si4770/77-A20
4.2. Clocking
The Si4770/77-A20 generates all internal clocking from
an external crystal using an on-chip oscillator or an
external programmable reference clock. The reference
clock of Si4770/77-A20 is a sinusoidal or rectangular
clock provided by an external source on pin RCLK. The
supported crystal and external clock source frequencies
are selected frequencies in the 36–38 MHz range.
The power up command enables the selection of an
external crystal or reference clock. The reference clock
and/or crystal accuracy should be ±100 ppm. In a multi-
receiver system, a single crystal can be shared between
all Si4770/77-A20 receivers. The Si4770/77-A20 family
features programmable loading capacitors for the on-
chip crystal oscillator, eliminating external loading
capacitors.
4.3. Tuning
The Si4770/77-A20 includes a complete on-chip PLL-
VCO frequency synthesizer to generate the quadrature
LO input to the image-reject AM and FM mixers. The
Si4770/77-A20 employs a single-conversion mix (down
conversion) to a fixed low IF center frequency. An
innovative high-performance image reject mixer
architecture allows for IF center frequencies below
300 kHz, thereby eliminating ceramic filters required in
10.7 MHz IF tuner architectures. The tune command
automatically programs the LO frequency to the center
of the desired channel plus (minus) the output center IF
frequency when using a high-side (low-side) mix. The
Si4770/77-A20 supports 50, 100, or 200 kHz channel
spacing for FM, 9 or 10 kHz for AM.
CLK
Si477x
Audio Receiver 1
CLK
Si477x
Audio Receiver 2
Figure 8. Xtal Share between Two Tuners
Rev. 0.9
25