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参数资料
| 型号： | LC72144M |
| 厂商： | SANYO SEMICONDUCTOR CO LTD |
| 元件分类： | PLL合成/DDS/VCOs |
| 英文描述： | PLL FREQUENCY SYNTHESIZER, 40 MHz, PDSO24 |
| 封装： | MFP-24 |
| 文件页数： | 13/22页 |
| 文件大小： | 307K |
| 代理商： | LC72144M |
Others
1. Notes on the phase comparator dead zone
Cases where the charge pump is in the ON/ON state require special care during system design since the charge pump
outputs correction pulses even when the PLL is locked and it is easy for the loop to become unstable.
The following problems may occur in the ON/ON state.
Sidebands may be generated by reference frequency leakage.
y Sidebands may be generated by low frequency leakage due to the correction pulse envelope.
The settings that have a dead zone (the OFF/OFF settings) provide good loop stability, but it is hard to achieve a
good C/N ratio with these settings. Inversely, the settings with no dead zone (the ON/ON settings) allow a high C/N
ratio to be achieved but it is hard to achieve good loop stability with these settings.
Therefore, it can be effective to select either the DZA or DZB setting, i.e., a setting which has no dead zone, when an
S/N ratio of between 90 and 100 dB or higher is required in FM mode, or when the AM stereo pilot margin needs to
be increased. However, in cases where such a high FM S/N ratio is not required and where an adequate AM stereo
pilot margin can be achieved or AM stereo is not used, either the DZC or DZD setting, i.e., a setting which has a
dead zone, should be selected.
Dead Zone Definition
The phase comparator compares fp with a reference frequency (fr) as shown in Figure 1. Figure 2 shows the
characteristics of an ideal phase comparator, which outputs an output voltage (A) that is proportional to the phase
difference . However, in an actual IC, a region (dead zone) in which minute phase differences cannot be detected
occurs due to internal circuit delays and other factors (B). To implement an end product with a high S/N ratio, the
dead zone should be as small as possible.
However, there are cases where a larger dead zone can make a popularly-priced model easier to use. This is because
it is possible for RF leakage from the mixer to the VCO to modulate the VCO in popularly-priced models when a
strong RF input is applied. When the dead zone is small an output that compensates for this problem is generated,
and this output may itself modulate the VCO and generate beating with the RF frequency.
No. 5377-20/22
LC72144M
DZ1
DZ0
Dead zone mode
Charge pump
Dead zone
0
DZA
ON/ON
– –0 s
0
1
DZB
ON/ON
–0 s
1
0
DZC
OFF/OFF
+0 s
1
DZD
OFF/OFF
+ +0 s
Leakage
Figure 1
Figure 2
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相关代理商/技术参数 |
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