LMX2430EVAL National Semiconductor, LMX2430EVAL Datasheet - Page 5

• Manufacturer Part Number: LMX2430EVAL
• Description: EVALUATION BOARD FOR LMX2430
• Manufacturer: National Semiconductor
• Series: PLLatinum™r

Specifications of LMX2430EVAL

• Main Purpose: Timing, Frequency Synthesizer
• Utilized Ic / Part: LMX2430
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Lead Free Status / Rohs Status: Not Compliant
• Other names: *LMX2430EVAL

2.0 Measurement Considerations
2.1
The phase noise characteristics of the PLL can be measured on a spectrum analyzer or a phase
noise test set. The spectrum analyzer test technique is described here. Phase noise is measured
in units of dBc/Hz. In this evaluation, the phase noise is measured at 1 kHz offset from the output
signal. For accurate close-in phase noise measurements, the offset selected should be on the flat
portion of the curve “inside the loop”. For integrated phase noise measurements a phase noise
analyzer is recommended.
The CodeLoader software is used to set the desired frequency and to program the LMX2430SLE
device. Refer to Appendix E for more details. The spectrum analyzer is then tuned to the desired
center frequency and the span is adjusted so that the appropriate offset frequency can be viewed.
For the spectrum analyzer used here, the difference between the carrier and the noise level
minus 10 * log
bandwidth is used for the RF and IF phase noise evaluation. The measurements do not take into
account any errors from the spectrum analyzer. The phase noise result is a negative number.
Since phase noise is measured in dBc/Hz, the measurement is always normalized to a 1 Hz
bandwidth. Modern spectrum analyzers have a feature that automatically normalizes the phase
noise measurements to a 1 Hz bandwidth. The video averaging feature of the spectrum analyzer
is used to better determine the noise level.
2.2
The loop bandwidth is the bandwidth of the closed loop PLL system. It is, by definition, the
frequency that makes the forward loop gain equal to zero. The spectrum analyzer span is set to
view the characteristic rising, peaking, and falling of the phase noise. To measure the loop
bandwidth is rather complex. It is simpler to measure the 0 dB bandwidth. Although, not exactly
the same, the 0 dB bandwidth is a sufficient estimate of the loop filter bandwidth. The 0 dB
bandwidth is defined as the frequency where the phase noise falls back to the level of the close-in
value after rising to its peak value. The value measured is typically greater than the true loop
filter bandwidth. For this evaluation, the 0 dB bandwidth is measured.
LMX2430SLEFPEB
Phase Noise Measurement Using A Spectrum Analyzer
Loop Filter Bandwidth Measurement Using A Spectrum Analyzer
10
[Resolution Bandwidth] is equal to the phase noise in dBc/Hz. A 30 Hz resolution
LMX2430SLE EVALUATION BOARD OPERATING INSTRUCTIONS
February 3,2006
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