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9129AS–DAB–04/08
ATR2732N3
3.9 Fast Fractional PLL
The frequency of the VHF VCO is locked to a reference frequency by an on-chip fractional-N
PLL circuit which guarantees superior phase-noise performance. The reference frequencies for
the two PLL blocks are generated by an on-chip oscillator.
The VCOs are fully integrated, which simplifies the design of the device and reduces the bill of
materials of the application.
The LO signal for the first L-band mixer is derived from a PLL-controlled on-chip VCO. The
down-converting to an IF frequency of 38.912 MHz for VHF or converted L-band signal is done
by an additional on-chip VCO using an internal fractional-N PLL.
Due to the digital tuning option of the reference frequency, the ATR2732N3 is able to support the
single reference clock design if the baseband can support such a feature (as the ATR2740
does).
3.10 Reference Oscillator
An on-chip crystal oscillator generates the reference signal which is fed to the reference divider.
By applying a crystal to the pins XTALA and XTALB, this oscillator generates a highly stable ref-
erence signal.
Furthermore, the frequency of this reference oscillator can be digitally tuned via the SPI bus bits
XOTi (i = 11, ..., 0) with a 12-bit step size.
3.11 Reference Divider
Starting from a minimum value, the scaling factor of the 6-bit reference divider is arbitrarily
programmable by means of the SPI bus bits Ri (i = 5, ..., 0).
The output of this first programmable divider typically provides a 2.048 MHz reference frequency
for the L-band PLL.
A second programmable divider (dividing by 8 to 128) then outputs 64 kHz, which is a useful ref-
erence frequency for the VHF PLL.
Together with the fractional-N PLL, a step size of 16 kHz for the frequency setting of the VHF LO
is ensured.
3.12 Main Divider
The main divider consists of a fully programmable 13-bit divider which defines a division ratio N.
The applied division ratio is either N or N + 1, as specified by a special control unit. On average,
the scaling factors SF = N + k / 4 can be selected where k = 0, 1, 2, or 3.
3.13 Phase Comparator and Charge Pump
The tri-state phase detectors cause the charge pumps to source or sink currents at the output
pins PFDOUTV (for VHF) and PFDOUTL (for L-band) depending on the phase relation of its
input signals, which are provided by the reference and the main dividers, respectively.
