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参数资料
| 型号： | LTC6946IUFD-2#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/30页 |
| 文件大小： | 0K |
| 描述： | IC INTEGER-N PLL W/VCO 28-QFN |
| 软件下载： | PLLWizard™ PLLWizard™, with .NET 2.0 installer |
| 标准包装： | 73 |
| 类型： | 时钟/频率合成器（RF/IF），分数-N，整数-N， |
| PLL： | 是 |
| 输入： | 时钟 |
| 输出： | 时钟 |
| 电路数： | 1 |
| 比率 - 输入:输出： | 1:1 |
| 差分 - 输入:输出： | 是/是 |
| 频率 - 最大： | 4.91GHz |
| 除法器/乘法器： | 是/是 |
| 电源电压： | 3.15 V ~ 5.25 V |
| 工作温度： | -40°C ~ 105°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-WFQFN 裸露焊盘 |
| 供应商设备封装： | 28-QFN（4x5） |
| 包装： | 管件 |
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LTC6946
24
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.
L
M(OUT) = –226 + 10 log10 fPFD
()
+ 20 log
10
f
RF
f
PFD
or
L
M(OUT) = –226 + 10 log10 fPFD
()
+ 20 log
10
N
O
(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)
10
PHASE
NOISE
(dBc/Hz)
–110
–100
100k
6945 F15
–120
–130
100
1k
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 Table12 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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