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LTC6948 View Datasheet(PDF) - Linear Technology

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LTC6948 Datasheet PDF : 36 Pages
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LTC6948
Applications Information
In-Band Output Phase Noise
The in-band phase noise floor LOUT produced at fRF may
be calculated by using Equation 17.
LOUT = LNORM + 10 • log10 (fPFD)
(17)
+ 20 • log10 (fRF/fPFD)
or
LOUT ≈ LNORM + 10 • log10 (fPFD)
+ 20 • log10 (N/O)
where LNORM is –226dBc/Hz for integer mode and
–225dBc/Hz for fractional mode. See the Typical Perfor-
mance Characteristics section for graphs showing LNORM
variation versus ICP and fVCO.
As can be seen from Equation 17 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 LTC6948’s 1/f noise, depending upon
fPFD. Use the normalized in-band 1/f noise L1/f of –274dBc/
Hz with Equation 18 to approximate the output 1/f phase
noise at a given frequency offset fOFFSET.
LOUT(1/f) (fOFFSET) = L1/f + 20 • log10 (fRF)
(18)
– 10 • log10 (fOFFSET)
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 14 for an example of integer mode
in-band phase noise for fPFD equal to 3MHz and 100MHz.
The total phase noise will be the summation of LOUT and
LOUT(1/f).
–90
–100
TOTAL NOISE
fPFD = 3MHz
–110
TOTAL NOISE
fPFD = 100MHz
–120
1/f NOISE
CONTRIBUTION
–130
10
100
1k
10k
OFFSET FREQUENCY (Hz)
100k
6948 F14
Figure 14. Theoretical Integer Mode In-Band
Phase Noise, fRF = 2500MHz
Integer Boundary Spurs
Integer boundary spurs are caused by intermodulation
between harmonics of the PFD frequency fPFD and the
VCO frequency fVCO. The coupling between the frequency
source harmonics can occur either on- or off-chip. The
spurs are located at offset frequencies defined by the beat
frequency between the reference harmonics and the VCO
frequency, and are attenuated by the loop filter. The spurs
only occur while in fractional mode.
Integer boundary spurs are most commonly seen when
the fractional value F approaches either zero or one such
that the VCO frequency offset from an integer frequency
is within the loop bandwidth:
fPFD F ≤ BW
or
fPFD • (1 – F) ≤ BW
The spur will have a relatively constant power in-band,
and be attenuated by the loop out-of-band. An example
integer boundary spur measurement is shown in Figure 15.
RF Output Matching
The RF± outputs may be used in either single-ended or dif-
ferential configurations. Using both RF outputs differentially
6948f
30
For more information www.linear.com/LTC6948
 

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