LTC6945 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC6945' PDF : 28 Pages View PDF
LTC6945
APPLICATIONS INFORMATION
OUTPUT PHASE NOISE DUE TO 1/f NOISE
In-band phase noise at very low offset frequencies may
be influenced by the LTC6945’s 1/f noise, depending upon
fPFD. Use the normalized in-band 1/f noise of –274dBc/Hz
with Equation 12 to approximate the output 1/f phase
noise at a given frequency offset fOFFSET:
LM(OUT –1/f) (fOFFSET) = –274 + 20 • log10(fRF) (12)
– 10 • log10(fOFFSET)
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 17 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).
–90
–100
–110
TOTAL NOISE
fPFD = 3MHz
TOTAL NOISE
fPFD = 100MHz
–120
–130
10
1/f NOISE
CONTRIBUTION
100
1k
10k
OFFSET FREQUENCY (Hz)
100k
6945 F17
Figure 17. Theoretical In-Band Phase Noise, fRF = 2500MHz
VCO INPUT MATCHING
The VCO± inputs may be used differentially or single-ended.
Each input provides a single-ended 121Ω resistance to aid
in impedance matching at high frequencies. The inputs are
self-biased and must be AC-coupled using a 100pF capaci-
tors (or 270pF for VCO frequencies less than 500MHz).
The inputs may be used single-ended by applying the
AC-coupled VCO frequency at VCO+ and bypassing
VCO– to GND with a 100pF capacitor (270pF for frequen-
cies less than 500MHz). Measured VCO+ s-parameters
(with VCO– bypassed with 100pF to GND) are shown in
Table 10 to aid in the design of external impedance match-
ing networks.
Table 10. Single-Ended VCO+ Input Impedance
FREQUENCY (MHz) IMPEDANCE (Ω)
250
118 – j78
500
83.6 – j68.3
1000
52.8 – j56.1
1500
35.2 – j41.7
2000
25.7 – j30.2
2500
19.7 – j20.6
3000
17.6 – j11.2
3500
17.8 – j3.92
4000
19.8 + j4.74
4500
21.5 + j15.0
5000
21.1 + j19.4
5500
27.1 + j22.9
6000
38.3 + j33.7
6500
36.7 + j42.2
7000
46.2 + j40.9
7500
76.5 + j36.8
8000
84.1+ j52.2
S11 (dB)
–5.06
–5.90
–6.38
–6.63
–6.35
–5.94
–6.00
–6.41
–7.20
–7.12
–6.52
–7.91
–8.47
–6.76
–8.11
–9.25
–7.27
RF OUTPUT MATCHING
The RF± outputs may be used in either single-ended or
differential configurations. Using both RF outputs differen-
tially 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+.
6945f
22
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