ADRF6655ACPZ-R7 datasheet(29/44 Pages) AD

29 / 44 page

ADRF6655

Rev. 0 | Page 29 of 44

REFERENCE PATH CONTROL

With R4[2:0] set to 100, the on-chip charge pump, PFD, and

reference path control register is programmed as shown in

Figure 74.

The charge pump current is controlled by the base charge

pump current (ICP, BASE) and the value of the charge pump

current multiplier (ICP, MULT).

The base charge pump current can be set using an internal or

external resistor (according to DB18 of Register 4). When using an

external resistor, the value of ICP, BASE can be varied according to

[]

250

217

−

⎥

⎦

⎤

⎢

⎣

⎡

BASE

RSET

(3)

When using the internal resistor, the base charge pump current

is 250 μA. The actual charge pump current can be programmed

to be a multiple (1, 2, 3, or 4) of the charge pump base current.

The multiplying value (ICP, MULT) is equal to 1 plus the value of

Bit DB11 and Bit DB10 in Register 4.

The PFD phase offset multiplier (θPFD,OFS), which is set by Bit DB16

to Bit DB12 of Register 4, causes the PLL to lock with a nominally

fixed phase offset between the PFD reference signal and the

divided-down VCO signal. This phase offset is used to linearize

the PFD-to-CP transfer function and can improve fractional

spurs. The magnitude of the phase offset is determined by

MULT

OFS

PFD

[deg]

ΔΦ

Finally, the phase offset can be either positive or negative

depending on the value of DB17 in Register 4.

The reference frequency applied to the PFD can be manipulated

using the internal reference path source. The external reference

frequency applied can be internally scaled in frequency by 2×, 1×,

0.5×, or 0.25×. This allows a broader range of reference frequency

selections while keeping the reference frequency applied to the

PFD within an acceptable range.

The ADRF6655 also provides a MUXOUT pin that can be

programmed to output a selection of several internal signals.

The default mode is to provide a lock-detect output to allow the

user to verify when the PLL has locked to the target frequency.

In addition, several other internal signals may be passed to the

MUXOUT pin, as described in Figure 74.

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