LMX2541 Datasheet, PDF (2/64 Pages) National Semiconductor (TI) – Ultra-Low Noise PLLatinum Frequency Synthesizer with Integrated VCO

LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
SNOSB31I – JULY 2009 – REVISED FEBRUARY 2013
System Diagram
www.ti.com
Dirty Input
Clock
PLL
Clean
Reference
Clock
PLL
Ultra-Clean
Local Oscillator
LMK04000
(Clock Jitter Cleaner)
LMX2541
(RF Synthesizer)
LMX2541 Frequency Coverage
VCO
_DIV
1
2
3
4
5
6
7
8
…
63
2060E
Start
Stop
1990.0 2240.0
995.0 1120.0
663.3 746.7
497.5 560.0
398.0 448.0
331.7 373.3
284.3 320.0
248.8 280.0
…
…
31.6
35.6
2380E
Start
Stop
2200.0 2530.0
1100.0 1265.0
733.3 843.3
550.0 632.5
440.0 506.0
366.7 421.7
314.3 361.4
275.0 316.3
…
…
34.9
40.2
2690E
Start
Stop
2490.0 2865.0
1245.0 1432.5
830.0 955.0
622.5 716.3
498.0 573.0
415.0 477.5
355.7 409.3
311.3 358.1
…
…
39.5
45.5
3030E
Start
Stop
2810.0 3230.0
1405.0 1615.0
936.7 1076.7
702.5 807.5
562.0 646.0
468.3 538.3
401.4 461.4
351.3 403.8
…
…
44.6
51.3
3320E
Start
Stop
3130.0 3600.0
1565.0 1800.0
1043.3 1200.0
782.5 900.0
626.0 720.0
521.7 600.0
447.1 514.3
391.3 450.0
…
…
49.7
57.1
3740E
Start
Stop
3480.0 4000.0
1740.0 2000.0
1160.0 1333.3
870.0 1000.0
696.0 800.0
580.0 666.7
497.1 571.4
435.0 500.0
…
…
55.2
63.5
All devices have continuous frequency coverage below a divide value of 8 (7 for most devices) down to their
minimum frequency achievable with divide by 63. The numbers in bold show the upper end of this minimum
continuous frequency range. For instance, the LMX2541SQ3740E option offers continuous frequency coverage
from 55.2 MHz to 571.4 MHz and LMX2541SQ2060E offers continuous frequency coverage from 31.6 MHz to
280 MHz. If using the part in External VCO mode, all parts have roughly the same performance and any option
will do.
Determining the Best Frequency Option of the LMX2541 to Use
When there are multiple devices that can satisfy the frequency requirement, performance characteristics can
sometimes be used to make a decision. Consider the following example of where an output frequency of 1200 to
1250 MHz is desired with a channel spacing of 100 kHz. From the frequency table, the LMX2541SQ2380E could
be used with a divide value of 2, or the LMX2541SQ3740E option could be used with a divide value of 3. This
raises the question: Which one has better performance? The following table is helpful in comparing the
performance.
Performance
Characteristic
Fractional Spurs
and
Fraction Noise
What Makes it
Better
Why
Larger Value
of VCO_DIV
Fractional spurs at the VCO are independent of VCO frequency, but when the VCO frequency is
divided down by a factor of VCO_DIV, the fractional spurs improve by a factor of 20·log(VCO_DIV).
Also, the fractional channel spacing can be made wider at the VCO, which makes the fractional spurs
farther from the carrier.
The fractional noise of the modulator is divided down in a similar way as fractional spurs. In
applications where this is dominant, this larger division can have an impact. Consult the applications
section for more information on the fractional phase noise.
2
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E