Datasheet

LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
SNOSB31I –JULY 2009–REVISED FEBRUARY 2013 www.ti.com
Action
Using
Increase
Noise/Spur Type
Increase Larger
Modulator
Dithering Equivalent
Order
Fractions
WORSE
Phase Noise (But only for larger fractions or WORSE WORSE
more dithering)
Primary Fractional Spur BETTER NO IMPACT NO IMPACT
WORSE
Sub-Fractional Spurs (Creates more sub-fractional BETTER BETTER
spurs)
Modulator Order
In general, the fractional mode of the PLL enables the use of a higher phase detector frequency relative to the
channel spacing, which enables the in-band noise of the PLL to be lower. The choice of modulator order to be
used in fractional mode is based on how much higher f
PD
can be made relative to the channel spacing and the
acceptable spur levels. The LMX2541 has a programmable modulator order which allows the user to make a
trade-off between PLL noise and primary and sub-fractional spur performance. The following table provides some
general guidelines for choosing modulator order: Note that the spurs due to crosstalk will not be impacted by
modulator order.
ORDER Guidelines for use
• Use if f
PD
can be made very high without using a fractional N value.
Integer Mode
• Use if it is not desired to make f
PD
higher using a fractional N value. This could be the
case if the loop bandwidth is very narrow and smaller loop filter capacitors are desired.
• Use 1st order if f
PD
can be increased by at least a factor of four over the integer case
and fractional spur frequencies and levels are acceptable.
1st Order Modulator
• If the channel spacing is 5 MHz or greater, the 1st order modulator may provide better
spur performance than integer mode.
• If the spurs of the 1st order modulator are unacceptable, use a higher order modulator. If
the spurious components are due to crosstalk they will not be improved by increasing
modulator order. In this case , use the lowest order modulator that gives acceptable
2nd Order Modulator
performance.
3rd Order Modulator
• Use if the spurs of the 1st order modulator are unacceptable.
4th Order Modulator
• In general, use the lowest order modulator unless a higher order modulator yields an
improvement in primary fractional spurs. If the spurious components are due to crosstalk,
they will not be improved by increasing the modulator order.
Programmable Output Power with On/Off
The power level of the RFout pin is programmable, including on/off controls. The RFoutEN pin and RFOUT word
can be used to turn the RFout pin on and off while still keeping the VCO running and in lock. In addition to on/off
states, the power level can also be programmed in various steps using the VCOGAIN, DIVGAIN, and OUTTERM
programming words. There are tables in the Typical Performance Characteristics (Not Guaranteed) section that
discuss the impact of these words on the output power. In addition to impacting the output power, these words
also impact the current consumption of the device. This data was obtained as an average over all frequencies. In
general, it is desirable to find the combination of programming words that gives the lowest current consumption
for a given output power level. All numbers reported are relative to the case of VCOGAIN = OUTTERM = 12.
According to this data, using a VCOGAIN or OUTTERM value of 12 or greater yields only a small increase in
output power, but a large increase in current consumption.
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E