LMX2541SQ2060E_NOPB Datasheet, PDF (3/64 Pages) Texas Instruments – LMX2541 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
Performance
Characteristic
VCO Phase
Noise
What Makes it
Better
Why
Operating in
the lower
frequency
range of the
VCO
At the lower end of the tuning range, the VCO phase noise is less because the tuning gain is less. This
provides better phase noise, even accounting for the difference in frequency.
Considering the fractional spurs and phase noise, the channel spacing at the 2380E VCO would be 200 kHz.
When this is divided by two, the offset of these spurs does not change and the spurs at the VCO output would be
reduced by a factor of 20·log(2) = 6 dB. The channel spacing at the 3740E VCO would be 300 kHz and these
spurs would be reduced by a factor of 20·log(3) = 9.5 dB. So the spurs of the 3740E option would probably be
better by virtue of the fact that they are farther from the carrier and easier to filter and also that they are divided
down more by the VCO divider. The fractional phase noise would also be (9.5 - 6) = 3.5 dB better by the same
reasoning.
Now consider the VCO phase noise. For the 3740E option, 1200 - 1250 MHz corresponds to a VCO frequency of
3600 - 3750 MHz, which is closer to the lower end of the tuning range for this device. For the 2380E option, this
would correspond to 2400 - 2500 MHz, which is closer to the higher end of the tuning range. To verify this, take
the phase noise numbers from the electrical specifications, extrapolate them to the actual frequencies, and then
subtract a factor of 20·log(VCO_DIV). For the 2380E option, this works out to -116 dBc/Hz at 1200 MHz and -
115.4 dBc/Hz at 1250 MHz. For the 3740E option, this works out to -117.4 dBc/Hz at 1200 MHz and -116.9
dBc/Hz at 1250 MHz.
Functional Block Diagram
VregVCO
VrefVCO
VCO
VREG
ExtVCOin
2-63
Divider
RFout
VREG
RFout
RFoutEN
MUX
OSCin
OSCin*
N Divider
4/5
Prescaler
Modulus
Control Comp
2X
R
Divider
MUX
Vtune
Charge
Pump
CPout
I
Fast
Lock
FLout
Ftest/LD
FRAC
VREG
VregFRAC
Serial Interface
Control
DATA
CLK
LE
CE
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Product Folder Links: LMX2541SQ2060E LMX2541SQ2380E LMX2541SQ2690E LMX2541SQ3030E
LMX2541SQ3320E LMX2541SQ3740E