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SNAU210 – March 2017
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Changing Reference Oscillator and Setup
Appendix D
SNAU210 – March 2017
Changing Reference Oscillator and Setup
The reference can be single-ended or differential. To measure the performance of the PLL ONLY,
the reference should have at least this level of performance. We understand that this can be a
challenge at 100-Hz offset:
(1)
A noise source 10 dB down from the PLL noise will contribute to raise the noise by 0.4 dB.
Table 4. Reference Oscillator Requirements
100-MHz REFERENCE MINIMUM REQUIREMENTS FOR A 0.4-dB IMPACT ON PLL INBAND PN
(1)
Offset [Hz]
100
1k
10k
100k
Noise level [
dBc/Hz]
–139
–149
–159
–164
There are different options to provide a reference oscillator to LMX2594: Use on-board oscillator (default),
Enable LMK61xx from Reference Pro PCB, use external oscillator.
By default the onboard oscillator is enabled. To use external reference, onboard oscillator must be
disabled. Having multiple 100 MHz enabled or powered sharing V
CC
or Gnd will degrade the phase noise
performance of LMX2594.
For differential pair connection:
1. Switch R24b to R24.
2. Switch R29b to R29
3. Must remove R33 to remove power from oscillator
Figure 14. Single-Ended Reference Configuration
