DAC39RF10 datasheet(203/232 Pages) TI2

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Figure 9-6. Phase Noise of DAC Clock Examples with the Device Additive Phase Noise

9.2.4.1 Example 1: SWAP-C Optimized

The best SWAP-C sub-system leverages the high levels of integration offered by modern PLL+VCO devices,

such as the LMX2820. Figure 9-7 shows a block diagram of the clocking sub-system. An external reference

clock feeds the LMX2820 input, which is then used to lock the internal PLL+VCO before being fed to the output

buffers. Only external passives are required to construct the loop filter and complete the sub-system system.

PFD + CP

VCO

Loop

Filter

LMX2820

400 MHz

Ref In

/20

To DAC

Clock

Distribution

Figure 9-7. LMX2820 with Internal VCO

The LMX2820 is a flexible device and configuration can be overwhelming. For example, deciding how to

decipher configure dividers, set loop filter components, and so on. A few high-level guidelines can be taken into

consideration to optimize phase noise.

First,always operate the LMX2820 in integer mode when possible, as opposed to fractional mode. This implies

that relationship between reference clock and output clock follows the general form:

FOUT = (FREF/NREF) x NDIV/NOUT

(5)

where NREF, NDIV and NOUT are the reference, feedback and output dividers respectively. FREF is the input

reference frequency and FOUT is the output frequency used as the DAC clock. If this ratio cannot be found, then

fractional mode must be used at the expense of degraded overall phase noise.

Second, the best in-band phase noise is achieved when the phase detector frequency is maximized and the

feedback divider is minimized. The LMX2820 has a maximum phase detector frequency of 400MHz and an

optional reference doubler is available for reference inputs up to 200MHz. For the same output frequency, each

doubling of phase detector frequency (while halving the feedback divider) results in 3dB of in band phase noise

reduction.

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DAC39RF10-SP, DAC39RF10-SEP

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SBAS932A – MARCH 2024 – REVISED FEBRUARY 2026

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DAC39RF10 Datasheet(PDF) 203 Page - Texas Instruments