ADRF6655ACPZ-R7 datasheet(22/44 Pages) AD

22 / 44 page

ADRF6655

Rev. 0 | Page 22 of 44

ADRF6655

+5V

EXTERNAL VCO

VTUNE LINE

CHARGE PUMP

LOOP FILTER

EXTERNAL

REFERENCE

RSET

VCC1

DECL1

GND

RSET

REFIN

GND

USING AN EXTERNAL VCO

The ADRF6655 has the necessary provisions for interfacing an

external VCO. A high performance discrete VCO may be desirable

in applications that call for the very best phase noise performance.

The basic circuit connections for interfacing an external VCO

are included in Figure 65. It is important to select a VCO with a

frequency tuning voltage range that covers the available charge

pump output compliance range of 1 V to 2.8 V. The external VCO

waveform needs to pass through the on-chip divide-by-2/divide-

by-3 programmable dividers before reaching the mixer. As a result,

the VCO center frequency should be selected to be roughly 2×

or 3× the desired LO signal frequency. The available output power

for the selected VCO should be greater than −10 dBm to ensure

adequate signal levels into the mixer core. The charge pump loop

filter components should be designed to provide adequate phase

margin for the given KVCO tuning sensitivity of the selected VCO.

It is important to properly configure the digital registers for

external VCO operation. When using an external VCO, the

internal VCO should be disabled using DB17 in Register 6.

Other register programmable LDOs, including the VCO LDO

(DB18 in Register 6), should be enabled. For more information

on programming the ADRF6655, see the ADRF6655 Control

Software section.

Figure 65. External VCO Connections

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