ADF4377 datasheet(33/79 Pages) AD

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Data Sheet

ADF4377

APPLICATIONS INFORMATION

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Rev. 0 | 33 of 79

Figure 85. Differential CML

Reference Phase Noise

The ADF4377 achieves an in-band normalized phase noise floor

of LNORM = −239 dBc/Hz typical. To calculate the equivalent input

phase noise floor (LIN), use the following Equation 17. Equation 17

is plotted in Figure 86.

LIN=LNORM+10×log10fREF

(17)

Figure 86. Equivalent Input Reference Noise Floor vs. Reference Input

Frequency

For example, a 100 MHz reference input frequency gives an LIN

of −159 dBc/Hz. The phase noise of the reference frequency

source must be at least 6 dB less than LIN to avoid impacting and

increasing the overall system phase noise.

To maintain typical LNORM performance, Table 7 provides criteria

for selecting the optimal REF_SEL setting based on the input

reference signal type and amplitude.

OUTPUT PHASE NOISE CHARACTERISTICS

In-Band Output Phase Noise

The in-band phase noise floor (LOUT) produced at fOUT may be

calculated by Equation 18 and Equation 19.

LOUT=LNORM+10×log10fPFD +20×log10

fOUTfPFD

(18)

LOUT=LNORM+10×log10fPFD +20×log10

(19)

where LNORM = −239 dBc/Hz. See Figure 36 to Figure 41, which

show LNORM variation vs. ICP, N_DEL, R_DEL, BLEED_I bit fields,

Bits[9:0], and reference slew rate.

As shown from Equation 18 and Equation 19 for a given PFD

frequency (fPFD), the output in-band phase noise increases at a 20

dB per decade rate with the N divider count. Therefore, for a given

output frequency (fOUT), fPFD must be as large as possible (or N

must be as small as possible) while still satisfying the frequency

step size requirements of the application.

Output Phase Noise Due to 1/f Noise

In-band phase noise at very low offset frequencies may be influ-

enced by the 1/f noise of the ADF4377, depending on fPFD. Use the

normalized in-band 1/f noise (L1/f ) of −287 dBc/Hz with Equation

20 to approximate the output 1/f phase noise at a given frequency

offset (fOFFSET).

LOUT1/f =L1/f+20×log10fOUT −10

×log10fOFFSET

(20)

Often L1/f is normalized to a 1 GHz signal at a 10 kHz offset, as

shown in Equation 21.

L1/f_1G_10k=L1/f+20×log101 GHz −10

×log1010 kHz =L1/f+140 dB

(21)

Unlike the in-band noise floor (LOUT), the 1/f noise (LOUT(1/f)) does

not change with fPFD and is not constant over offset frequency. See

Figure 87 for an example of in-band phase noise for fPFD equal to

100 MHz and 500 MHz. The total phase noise is the summation of

LOUT and LOUT(1/f), calculated by Equation 22.

LOUTTOTAL fOFFSET =10×log10

10LOUT10+10LOUT1/f fOFFSET10

(22)

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