LTC6946 datasheet(24/30 Pages) LINER

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LTC6946

6946fa

APPLICATIONS INFORMATION

The LTC6946 achieves an in-band normalized phase noise

floor of –226dBc/Hz (typical). To calculate its equiva-

lent input phase noise floor LM(IN), use the following

Equation 11:

LM(IN) = –226 + 10 • log10(fREF)

(11)

For example, using a 10MHz reference frequency gives

an input phase noise floor of –156dBc/Hz. The reference

frequency source’s phase noise must be at least 3dB better

than this to prevent limiting the overall system performance.

IN-BAND OUTPUT PHASE NOISE

The in-band phase noise produced at fRF may be calcu-

lated by using Equation 12.

M(OUT) = –226 + 10 log10 fPFD

()

+ 20 log

PFD

⎛

⎝⎜

⎞

⎠⎟

M(OUT) = –226 + 10 log10 fPFD

()

+ 20 log

⎛

⎝⎜

⎞

⎠⎟

(12)

As can be seen, for a given PFD frequency fPFD, the output

in-band phase noise increases at a 20dB-per-decade rate

with the N divider count. So, for a given output frequency

fRF, fPFD should be as large as possible (or N should be as

small as possible) while still satisfying the application’s

frequency step size requirements.

OUTPUT PHASE NOISE DUE TO 1/f NOISE

In-band phase noise at very low offset frequencies may

be influenced by the LTC6946’s 1/f noise, depending upon

fPFD. Use the normalized in-band 1/f noise of –274dBc/

Hz with Equation 13 to approximate the output 1/f phase

noise at a given frequency offset fOFFSET.

LM(OUT-1/f) fOFFSET

() = –274+20 • log10 fRF

()

–10 • log10 fOFFSET

()

(13)

Unlike the in-band noise floor LM(OUT), the 1/f noise

LM(OUT 1/f) does not change with fPFD, and is not constant

over offset frequency. See Figure 15 for an example of

in-band phase noise for fPFD equal to 3MHz and 100MHz.

The total phase noise will be the summation of LM(OUT)

and LM(OUT 1/f).

Figure 15. Theoretical In-Band Phase Noise, fRF = 2500MHz

OFFSET FREQUENCY (Hz)

–110

–100

100k

6945 F15

–120

–130

100

10k

–90

TOTAL NOISE

fPFD = 3MHz

TOTAL NOISE

fPFD = 100MHz

1/f NOISE

CONTRIBUTION

RF OUTPUT MATCHING

The RF± outputs may be used in either single-ended or dif-

ferential configurations. Using both RF outputs differentially

will result in approximately 3dB more output power than

single ended. Impedance matching to an external load in

both cases requires external chokes tied to VRF+. Measured

RF± s-parameters are shown below in Table 12 to aid in

the design of impedance matching networks.

Table 12. Single-Ended RF Output Impedance

FREQUENCY (MHZ)

IMPEDANCE (Ohms)

S11 (dB)

500

102.8 – j49.7

–6.90

1000

70.2 – j60.1

–6.53

1500

52.4 – j56.2

–6.35

2000

43.6 – j49.2

–6.58

2500

37.9 – j39.6

–7.34

3000

32.7 – j28.2

–8.44

3500

27.9 – j17.8

–8.99

4000

24.3 – j9.4

–8.72

4500

22.2 – j3.3

–8.26

5000

21.6 + j1.9

–8.02

5500

21.8 + j6.6

–7.91

6000

23.1 + j11.4

–8.09

6500

25.7 + j16.9

–8.38

7000

29.3 + j23.0

–8.53

7500

33.5 + j28.4

–8.56

8000

37.9 + j32.6

–8.64

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