LMX2485E EVAL National Semiconductor, LMX2485E EVAL Datasheet - Page 4

• Manufacturer Part Number: LMX2485E EVAL
• Description: BOARD EVALUATION LMX2485E
• Manufacturer: National Semiconductor
• Series: PLLatinum™r

Specifications of LMX2485E EVAL

• Main Purpose: Timing, Frequency Synthesizer
• Utilized Ic / Part: LMX2485E
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -

L M X 2 4 8 5 E
E V A L U A T I O N
B O A R D
O P E R A T I N G
I N S T R U C T I O N S
Evaluation Hints
It is strongly recommended that the user reviews sections 1.9 FRACTIONAL SPUR AND PHASE NOISE
CONTROLS and 2.1.2 RF_N[10:0] – RF N Counter Value in the LMX2485/LMX2485E Datasheet. These
sections will specify minimum divide ratio limits for the LMX2485E.
This will determine how high the
comparison frequency can be. Also, it points out that lower order modulators have better performance. It
seems that the best performance at these low frequencies is with the dithering disabled. Also, it seems that
there is improved and consistent performance with lower fractional denominators at these low frequencies.
Page 11 shows Impact of Large Fractional Denominator on fractional spurs.
It is recommended that the minimum slew rate specification of 100 V/µs be maintained. If the power level is 1
dBm or higher, the slew rate specification will be met at 50 MHz. Slew rates less than 100 V/µs could have an
negative effect on the performance of the LMX2485E.
nd
The default state for the LMX2485E Evaluation board in these instructions are 2
delta sigma modulator with
dithering disabled and charge pump current set to 8X (0.76 mA). Also RF_P is set to 1 for a Prescaler value of
8 to achieve low divide ratios while not violating minimum divide ratio specifications.
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