AX5243 datasheet(29/65 Pages) ONSEMI

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Circuit Description

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AX5243

It is also possible to use a fully external VCO by setting bits VCO2INT=0 and VCOSEL=1

in the

PLLVCODIV register. A differential input at a frequency of double the desired RF

frequency must be input at device pins L1 and L2. The control voltage for the VCO can

be output at device pin FILT when using external filter mode. The voltage range of this

output pin is 0 – 1.8 V.

This mode of operation is recommended for special applications where the phase noise

requirements are not met when using the fully internal VCO or the internal VCO with

external inductor.

VCO Auto-Ranging

The AX5243 has an integrated auto-ranging function, which allows to set the correct

VCO range for specific frequency generation subsystem settings automatically. Typically

it has to be executed after power-up. The function is initiated by setting the RNG_START

bit in the

PLLRANGINGA or PLLRANGINGB register. The bit is readable and a 0 indicates

the end of the ranging process. Setting RNG_START in the

PLLRANGINGA register

ranges the frequency in

FREQA, while setting RNG_START in the PLLRANGINGB register

ranges the frequency in

FREQB. The RNGERR bit indicates the correct execution of the

auto-ranging.

VCO auto-ranging works with the fully integrated VCO and with the internal VCO with

external inductor.

Loop Filter and Charge Pump

The AX5243 internal loop filter configuration together with the charge pump current

sets the synthesizer loop band width. The internal loop-filter has three configurations

that can be programmed via the register bits FLT[1:0] in registers

PLLLOOP or

PLLLOOPBOOST the charge pump current can be programmed using register bits

PLLCPI[7:0] in registers

PLLCPI or PLLCPIBOOST. Synthesizer bandwidths are typically 50

- 500 kHz depending on the

PLLLOOP or PLLLOOPBOOST settings, for details see the

section 4.3: AC Characteristics.

The AX5243 can be setup in such a way that when the synthesizer is started, the

settings in the registers

PLLLOOPBOOST and PLLCPIBOOST are applied first for a

programmable duration before reverting to the settings in

PLLLOOP and PLLCPI. This

feature enables automated fastest start-up.

Setting bits FLT[1:0]=00 bypasses the internal loop filter and the VCO control voltage is

output to an external loop filter at pin FILT. This mode of operation is recommended for

achieving lower bandwidths than with the internal loop filter and for usage with a fully

external VCO.

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