參數(shù)資料
型號(hào)： LC72151V
元件分類： PLL合成/DDS/VCOs
英文描述： PLL FREQUENCY SYNTHESIZER, 40 MHz, PDSO30
封裝： SSOP-30
文件頁(yè)數(shù)： 17/29頁(yè)
文件大小： 326K
代理商： LC72151V
Other items
(1) Notes on the phase detector dead band
When the charge pump operates in ON/ON mode, the charge pump generates correction pulses even when the PLL is
locked. Here, it is easy for the loop to become unstable, and special care is required in designs that use this mode.
The following problems may occur in ON/ON mode.
Side bands may be generated due to reference frequency leakage.
Side bands may be generated due low-frequency leakage due to the envelope of the correction pulses.
No. 6976-24/29
LC72151V
DZ1
DZ0
Dead band mode
Charge pump
Dead band
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
When a dead band is present (OFF/OFF mode), the loop will be stable, but it will be harder to acquire a good C/N ratio.
On the other hand, with the mode that does not have a dead band (ON/ON mode), it will be easier to acquire a high C/N
ratio, but harder to acquire loop stability.
Therefore, the DZA and DZB modes, in which there is no dead band, can be effective if either a high signal-to-noise ratio
of 90 to 100 dB in FM reception or an increased pilot margin in AM stereo reception is required.
Inversely, if such a high FM signal-to-noise ratio is not required for FM reception, or an adequate pilot margin can be
acquired for AM stereo reception, then the DZC and DZD modes, in which a dead band is present, may be more
effective.
Dead zone (dead band) definition
The phase comparator compares fp with the reference frequency (fr) as shown in figure 1. This circuit outputs a voltage
V (A) that is proportional to the phase difference as shown in figure 2. However, due to internal delays and other
factors, the actual IC is unable to compare small phase differences, and thus a dead zone (B) appears in the output. To
achieve a high signal-to-noise ratio in the end product, the dead zone should be as small as possible.
However, in popularly-priced models, there are cases where a somewhat wider dead zone may be easier to work with.
This is because in some situations, such as when a powerful signal is applied to the RF input, in popularly-priced models
there may be RF leakage from the mixer to the VCC. When the dead zone is narrow, outputs to correct this leakage are
output, that output in turn modulates the VCO, and generates a beat signal with the RF.
Reference divider
Programmable divider
VCO
LPF
Phase
detector
MIX
RF
Leakage
fr
fp
(nsec)
(B)
(A)
V
Dead zone
Figure 1
Figure 2
(2) Notes on the FMIN, AMIN, HCTR/I-3, and LCTR/I-4 pins
The coupling capacitor must be located as close as possible to these pins. A capacitance of approximately 100 pF is
desirable.
In particular, if the HCTR/I-3 and LCTR/I-4 pin capacitor is over about 1000 pF, the time required to reach the bias level
may become excessive, and incorrect counting may occur due to the relationship with the wait time.
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PDF描述
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