LMX2541 Datasheet, PDF (11/64 Pages) National Semiconductor (TI) – Ultra-Low Noise PLLatinum Frequency Synthesizer with Integrated VCO

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LMX2541SQ2060E, LMX2541SQ2380E
LMX2541SQ2690E, LMX2541SQ3030E
LMX2541SQ3320E, LMX2541SQ3740E
SNOSB31I – JULY 2009 – REVISED FEBRUARY 2013
Typical Performance Characteristics (Not Guaranteed)
LMX2541SQ3740E Raw Phase Noise Measurement
-80
-85
-90
-95
-100
-105
-110
-115
-120
1.E+02
1/f Noise
1.E+03
Measured Noise
1.E+04
Noise Floor
1.E+05
1.E+06
1.E+07
OFFSET (Hz)
The above plot demonstrates the PLL phase noise of the LMX2541SQ3700E operating at 3700 MHz output frequency, phase detector
frequency of 100 MHz, and charge pump gain of 32X. The loop bandwidth was made as wide as possible to fully expose the PLL phase
noise and reference source was a 100 MHz Wenzel crystal. This measurement was done in integer mode. To better understand the impact of
using fractional mode, consult the applications section.
The measured noise is the sum of the PLL 1/f noise and noise floor. At offsets below 1 kHz, the PLL 1/f noise dominates and changes at a
rate of 10 dB/decade. The noise at 1 kHz is dominated by this 1/f noise and has a value of -103 dBc/Hz. In the 100 - 200 kHz offset range,
the noise is -113.7 dBc/Hz and is dominated by the PLL noise floor. It can be shown that if the effects of the loop filter peaking and the 1/f
noise are subtracted away from this measurement, it would be about 0.6 dB better.
If the phase detector frequency is changed with the VCO frequency held constant, the PLL noise floor will change, but the 1/f noise will
remain the same. If the VCO frequency is changed, both the 1/f noise and PLL noise floor change at a rate of 20 dB/decade.
Figure 2.
LMX2541SQ2690 System Phase Noise
-70
V_I = 4.7 mRad
-80
(F_PD = 50 MHz)
-90
-100
-110
V_I = 4.2 mRad
(OSCin Scaled to Freq.)
-120
-130
-140
V_I = 4.4 mRad
(F_PD = 100 MHz)
-150
-160
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
OFFSET (Hz)
For this plot, a third order modulator with dithering disabled was used with a fractional denominator of 500000. The charge pump gain was
32X and the loop filter components were C1 = 2.2 nF, C2 = 22 nF, R2 = 470 Ω. The internal loop filter components were C3_LF = 20 pF,
C4_LF = 100 pF, R3_LF = 1 kΩ, R4_LF = 200 Ω. The VCO frequency is 2720.1 MHz. The OSCin signal was a 500 MHz differential LVPECL
output of the LMK04033.
Figure 3.
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E