U2731B Datasheet, PDF (8/25 Pages) ATMEL Corporation – DAB ONE- CHIP FRONT END

PLL Part
Reference Oscillator
Reference Divider
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 spe-
cial phase-noise-shaping technique based on the fractional-N principle which is already
used in Atmel's U2733B frequency synthesizer series. It concentrates the phase detec-
tor'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 prop-
erty of the transmission technique which is used in DAB is that the phase-noise-
weighting function which measures the influence of the LO's phase noise to the phase
information of the coded signal in a DAB receiver has zeros, i.e., if phase noise is con-
centrated 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 3.
Figure 3. 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 positions of such zeros is patent protected.
An on-chip crystal oscillator generates the reference signal which is fed to the reference
divider. As already described in the section “IF Gain-controlled Amplifier/Mixer Combi-
nation” on page 7, the LO signal for the mixer in the IF section is derived. By applying a
crystal to the pins OSCI, OSCO, see Figure 8 on page 20, 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, see Figure 9 on page 21, the
reference signal has to be applied to the pin OSCI and the pin OSCO must be left open.
Starting from a minimum value, the scaling factor SFref of the 9-bit reference divider is
freely programmable by means of the two-wire bus bits ri (i = 0, ..., 8) according to
∑ ^SFref ^{= r}i ^{× 2i}
8 U2731B
4671C–DAB–06/04