Datasheet

LMX2541SQ2060E, LMX2541SQ2380E

LMX2541SQ2690E, LMX2541SQ3030E

LMX2541SQ3320E, LMX2541SQ3740E

www.ti.com

SNOSB31I –JULY 2009–REVISED FEBRUARY 2013

FDM - Extended Fractional Denominator Mode Enable

Enabling this bit allows the fractional numerator and denominator to be expanded from 10 bits to 22 bits. In 10-bit

mode, only the first 10 bits of the fractional numerator and denominator are considered. When using FSK mode,

this bit has to be disabled.

FDM Fractional Mode

0 10-bit

1 (Default) 22-bit

ORDER[2:0] -- Delta Sigma Modulator Order

This word determines the order of the delta sigma modulator in the PLL. In general, higher order fractional

modulators tend to reduce the primary fractional spurs that occur at increments of the channel spacing, but can

also create spurs that are at a fraction of the channel spacing. The optimal choice of modulator order is very

application specific, however, a third order modulator is a good starting point. The first order modulator has no

analog compensation or dithering

Delta Sigma

ORDER Mode Comments

Modulator

0 Disabled Integer Allows larger N Counter

1 First Order This has no analog compensation or dithering

2 Second Order

Fractional

3 Third Order Traditional Delta Sigma Operation

4 Fourth Order

5-7 Illegal States n/a n/a

DITH[1:0] -- Dithering

Dithering randomizes the delta sigma modulator output. This reduces sub-fractional spurs at the expense of

adding phase noise. In general, it is recommended to keep the dithering strength at None or Weak for most

applications. Dithering should never be used when the device is used in integer mode or a first order modulator.

When using dithering with the other delta sigma modulator orders, it is beneficial to disable it in the case where

the fractional numerator is zero, since it can actually create sub-fractional spurs.

DITH Dithering Strength

0 Weak

1 Medium

2 Strong

3 Disabled

CPT - Charge Pump TRI-STATE

When this bit is enabled, the charge pump is at TRI-STATE. The TRI-STATE mode could be useful for open loop

modulation applications or as diagnostic tool for measuring the VCO noise, but is generally not used.

CPT Charge Pump

0 Normal Operation

1 TRI-STATE

Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E

LMX2541SQ3320E LMX2541SQ3740E