STW81100 Datasheet, PDF (13/23 Pages) STMicroelectronics – MULTI-BAND RF FREQUENCY SYNTHESIZER WITH INTEGRATED VCOS

STW81100
7.8 Voltage Controlled Oscillators
7.8.1 VCO Selection
Within STW81100 two low-noise VCOs are integrated to cover a wide band from 3280MHz to 4400MHz
(direct output), from 1640MHz to 2200MHz (selecting divider by 2) and from 820MHz to 1100MHz (select-
ing divider by 4).
VCO A frequency range 3280MHz-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 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 Comparison Frequency.
The maximum allowed f_comp to perform the calibration process is 1 MHz. Using an higher f_comp the follow-
ing procedure should be adopted:
1. Calibrate the VCO at the desired frequency with an f_comp less than 1 MHz
2. Set the A, B and R dividers ratio for the desired f_comp
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