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SLWS245B – MAY 2014 – REVISED FEBRUARY 2017
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In Fractional mode, the accuracy of the final frequency is set by 25-bit resolution. The RF stepsize is f
PFD
/2
25
which is less than 1 Hz for f
PFD
up to 33 MHz. The appropriate fractional control bits in the serial register must be
programmed. Optimal performance may require tuning the MOD_ORD, ISOURCE_SINK, and ISOURCE_TRIM
values according to the chosen frequency band.
8.4.4 Selecting the VCO and VCO Frequency Control
To achieve a broad frequency tuning range, the TRF3722 integrates multiple VCOs. Each VCO tank uses a bank
of coarse tuning capacitor to bring VCO frequency within a few MHz of the desired value. For a given LO
frequency an appropriate VCO and capacitor array must be selected. The device integrates logic that
automatically selects an appropriate VCO and capacitor array, such that in closed loop V
(TUNE)
is approximately
equal to the open loop calibration reference voltage set by VCO_CAL_REF. An on-chip temperature sensor
automatically adjusts this reference voltage so that proper lock can be maintained over the temperature range.
The calibration logic is driven by a CAL_CLK signal which is scaled version of the reference frequency according
to CAL_CLK_SEL. For optimum accuracy It is recommended to limit the CAL_CLK frequency to 600 kHz.
When VCO_SEL_MODE is '0', the device automatically selects the VCO and the capacitor array. When
VCO_SEL_MODE is '1', the VCO selected by VCO_SEL is used and the logic automatically selects the capacitor
array. The VCO and capacitor array settings resulting from the calibration can be read from Register 0 - read
back register.
Automatic calibration can be disabled by setting CAL_BYPASS to '1'. In this manual calibration mode, the VCO is
selected through register bits VCO_SEL, while the capacitor array is selected through register bits VCO_TRIM.
Calibration modes are summarized in
Table 3. VCO Calibration Modes
CAL_BYPASS
VCO_SEL_MODE
MAX CYCLES
CAL_CLK
VCO
CAPACITOR
ARRAY
0
0
46
Automatic
0
1
34
VCO_SEL
Automatic
1
don't care
N/A
VCO_SEL
VCO_TRIM