STW81100

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7.8 Voltage Controlled Oscillators

7.8.1 VCO Selection

Within STW81100 two low-noise VCOs are integrated to cover a wide band from 3300MHz to 4400MHz

(direct output), from 1650MHz to 2200MHz (selecting divider by 2) and from 825MHz to 1100MHz (select-

ing divider by 4).

VCO A frequency range 3300MHz-3900MHz

VCO B frequency range 3800MHz-4400MHz

7.8.2 VCO Frequency Calibration

Both VCOs can operate on 32 frequency ranges that are selected by adding or subtracting capacitors to

the resonator. These frequency ranges are intended to cover the wide band of operation and compensate

for process variation on the VCO center frequency.

An automatic selection of the range is performed when the bit SERCAL rises from “0” to “1”. The charge

pump is inhibited and the pins ICP & VCTRL are at VDD/2 volts.

Then the ranges are tested to select the one which with this VCO input voltage is the nearest to the desired

output frequency (Fout = N*Fref/R). When this selection is achieved the signal ENDCALB (which means

End of Calibration) falls to “0”, then the charge pump is enabled again and SERCAL should be reset to “0”

before the next channel step.

The reference clock signal at the REF_IN input terminal must be running before starting the calibration.

The PLL has just to perform fine adjustment around VDD/2 on the loop filter to reach Fout, which enables

a fast settle.

Figure 16. VCO Sub-Bands Frequency Characteristics

The SERCAL bit should be set to “1” at each division ratio change. It should be noted that in order to reset

the autocalibrator State Machine after a power-up, and anyway before the first calibration, the INITCAL bit

should be set to “1” and back to “0” (this operation is automatically performed by the Power On Reset cir-

cuitry). The calibration takes approximately 7 periods of the PFD Frequency.

procedure should be adopted:

1.

2.