ADRF6801 datasheet(17/36 Pages) AD

17 / 36 page

ADRF6801

Rev. 0 | Page 17 of 36

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 37.

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 Bit DB18 of Register 4). When

using an external resistor, the value of ICP,BASE can be varied

according to

[]

250

217

−

⎥⎦

⎤

⎢⎣

⎡

BASE

SET

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 the

DB11 and DB10 bits 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-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 the DB17 bit 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 ADRF6801 also provides a MUXOUT pin that can be

programmed to output a selection of several internal signals. The

default mode provides a lock-detect output that allows users to

verify when the PLL has locked to the target frequency. In addition,

several other internal signals can be routed to the MUXOUT pin as

described in Figure 37.

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