LMX2541SQE2060E/NOPB National Semiconductor, LMX2541SQE2060E/NOPB Datasheet - Page 54

• Manufacturer Part Number: LMX2541SQE2060E/NOPB
• Description: IC PLL FREQ SYNTH W/VCO 36LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Clock/Frequency Synthesizer (RF)r

Specifications of LMX2541SQE2060E/NOPB

• Pll: Yes
• Input: Clock
• Output: Clock
• Number Of Circuits: 1
• Ratio - Input:output: 2:2
• Differential - Input:output: No/No
• Frequency - Max: 2.24GHz
• Divider/multiplier: Yes/No
• Voltage - Supply: 3.15 V ~ 3.45 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-LLP
• Frequency-max: 2.24GHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LMX2541SQE2060ETR

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3.5 IMPACT OF MODULATOR ORDER, DITHERING, AND
LARGER EQUIVALENT FRACTIONS ON SPURS AND
PHASE NOISE
To achieve a fractional N value, an integer N divider is mod-
ulated between different values. This gives rise to three main
degrees of freedom with the LMX2541 delta sigma engine:
the modulator order, dithering, and the way that the fractional
portion is expressed. The first degree of freedom, the modu-
lator order, can be selected as zero (integer mode), one, two,
three, or four. One simple technique to better understand the
impact of the delta sigma fractional engine on noise and spurs
is to tune the VCO to an integer channel and observe the im-
pact of changing the modulator order from integer mode to a
higher order. A higher fractional modulator order in theory
yields lower primary fractional spurs. However, this can also
give rise to sub-fractional spurs in some applications. The
second degree of freedom is dithering. Dithering seeks to im-
prove the sub-fractional spurs by randomizing the sequence
of N divider values. In theory, a perfectly randomized se-
quence would eliminate all sub-fractional spurs, but add
phase noise by spreading the energy that would otherwise be
contained in the spurs. The third degree of freedom is the way
that the fraction is expressed. For example, 1/10 can be ex-
pressed as a larger equivalent fraction of 100000/1000000.
Using larger equivalent fractions tends to increase random-
ization similar to dithering. In general, the very low phase
noise of the LMX2541 exposes the modulator noise when
dithering and large fractions are used, so use these with cau-
tion. The avid reader is highly encouraged to read application
note 1879 for more details on fractional spurs. The following
table summarizes the relationships between spur types,
phase noise, modulator order, dithering and fractional ex-
pression.
3.6 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
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 pro-
Phase Noise
Noise/Spur
Fractional
Fractional
Primary
Spurs
Type
Spur
Sub-
(But only for
fractions or
Modulator
more sub-
dithering)
Increase
fractional
BETTER
WORSE
WORSE
(Creates
Order
spurs)
larger
more
NO IMPACT
Dithering
Increase
BETTER
WORSE
Action
PD
can be made relative
NO IMPACT
Equivalent
Fractions
BETTER
WORSE
Larger
Using
54
vides some general guidelines for choosing modulator order:
Note that the spurs due to crosstalk will not be impacted by
modulator order.
3.7 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 pow-
er level can also be programmed in various steps using the
VCOGAIN, DIVGAIN, and OUTTERM programming words.
There are tables in the typical performance characteristics
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.
ORDER
— Use if f
— Use if it is not desired to make f
— Use 1st order if f
— If the channel spacing is 5 MHz or greater, the
— If the spurs of the 1st order modulator are
— Use if the spurs of the 1st order modulator are
— In general, use the lowest order modulator
a fractional N value.
a fractional N value. This could be the case if
the loop bandwidth is very narrow and smaller
loop filter capacitors are desired.
a factor of four over the integer case and
fractional spur frequencies and levels are
acceptable.
1st order modulator may provide better spur
performance than integer mode.
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
performance.
unacceptable.
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.
PD
can be made very high without using
Guidelines for use
PD
can be increased by at least
PD
higher using