LMX2581_16 Datasheet, PDF (47/56 Pages) Texas Instruments – LMX2581 Wideband Frequency Synthesizer with Integrated VCO

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9.3 Do's and Don'ts
LMX2581
SNAS601G – AUGUST 2012 – REVISED SEPTEMBER 2014
CATEGORY
DO
DON'T
WHY
Output Pull-Up
Components
Place pull-up components close
to RFoutA and RFoutB
Go through a Via before getting
to the pull-up component.
The output impedance is determined by this
component and if it is far away, there will be loss in
output power.
•
Fractional Spurs ^•
•
•
Take advantage of TI tools
that can simulate these.
Read the section on spurs to
better understand them.
•
Use a systematic process to
optimize them
Assume that raising the
phase detector frequency
always improves the integer
boundary spur.
Fractional spurs can have more than one
Assume that changing the ^{mechanism, especially the integer boundary spur.}
loop bandwidth will always
impact integer boundary
spurs.
Dithering
• Understand the trade-offs
and when it is appropriate to
use.
Use on simple fractions .
• Combine with larger
equivalent fractions.
Dithering is very effective in eliminating some
spurs, but useless for eliminating others. Dithering
adds PLL phase noise, so it should be only used
for appropriate situations.
VbiasCOMP
Put as much capacitance as
and VbiasVCO possible, up to 32 µF
• Use less than 10 µF of
capacitance
This capacitance impacts the VCO phase noise.
• Ignore capacitor de-rating
factors.
10 Power Supply Recommendations
10.1 Supply Recommendations
Low noise regulators are generally recommended for the supply pins. It is OK to have one regulator supply the
part, although it is best to put individual bypassing as shown in the Layout Guidelines for the best spur
performance. The most noise sensitive components are the pull-up components for the output buffers since
supply noise here will directly go to the output. For purposes of bypassing, below is how the current consumption
is approximately distributed through each pin. For this table, default mode is with internal VCO mode with one
output buffer powered up with OUTx_PWR=15. External VCO mode assumes the VCO divider and output buffers
are off.
PIN NUMBER
Pin 6
Pin 10
Pin 16
Pin 17
Pin 28
Pin 32
n/a
TOTAL
Table 16. Current Consumption by Pin
PIN NAME
VccCP
VccPLL
VccBUF
VccVCO
VccDIG
VccFRAC
Output Pull-Up
Component
DEFAULT MODE
12
28
23
83
10
<<1
22
178
CONDITION
DEFAULT MODE
with VCO DIVIDER
ENABLED
12
28
43
83
10
<<1
22
198
EXTERNAL VCO MODE
with OUTPUT BUFFER
DISABLED
12
48
1
14
10
<<1
0
85
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