ATR2731 Datasheet, PDF (9/26 Pages) ATMEL Corporation – DAB One-chip Front End

6. PLL Part
ATR2731
The purpose of the PLL part is to perform a phase lock of the voltage-controlled RF oscillator to
an on-chip crystal reference oscillator. This is achieved by means of a special
phase-noise-shaping technique based on the fractional-N principle. It concentrates the phase
detector's phase-noise contribution to the spectrum of the controlled VCO at frequency positions
where it does not impair the quality of the received DAB signal. A special property of the trans-
mission technique used in DAB is that the phase-noise-weighting function measuring the
influence of the LO's phase noise on the phase information of the coded signal in a DAB receiver
has zeros; that is, if phase noise is concentrated in the position of such zeros as discrete lines,
the DAB signal is not impaired as long as these lines do not exceed a set limit. For DAB mode I,
this phase-noise- weighting function is shown in Figure 6-1.
Figure 6-1. Phase-noise-weighting Function
1,80
1,60
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0,00
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 ₁₀₀₀₀
df/Hz
It is important to realize that this function shows zeros in all distances from the center line which
are multiples of the carrier spacing. The technique of concentrating the phase noise in the posi-
tions of such zeros is patent protected.
6.1 Reference Oscillator
An on-chip crystal oscillator generates the reference signal which is fed to the reference divider.
As already described in “IF Gain-controlled Amplifier/Mixer Combination” on page 8, the LO sig-
nal for the mixer in the IF section is derived. By applying a crystal to the pins OSCI and OSCO
(Figure 13-2 on page 21), this oscillator generates a highly stable reference signal. If an external
reference signal is available, the oscillator can be used as an input buffer. In such an application
(Figure 13-3 on page 22), the reference signal has to be applied to the pin OSCI and the pin
OSCO must be left open.
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4904A–DAB–03/06