lc72135m Sanyo Semiconductor Corporation, lc72135m Datasheet - Page 21

• Manufacturer Part Number: lc72135m
• Description: Pll Frequency Synthesizer For Electronic Tuning
• Manufacturer: Sanyo Semiconductor Corporation

Clock Time Base Usage Notes
The pull-up resistor used on the clock time base output pin (BO1) should be at least 100 kΩ.
This is to prevent degrading the VCO C/N characteristics when a loop filter is formed using the built-in low-pass filter
transistor. Since the clock time base output pin and the low-pass filter have a common ground internal to the IC, it is
necessary to minimize the time base output pin current fluctuations and to suppress their influence on the low-pass filter.
Also, to prevent chattering we recommend using a Schmitt input at the controller (microprocessor) that receives this
signal.
Other Items
1. Notes on the Phase Comparator Dead Zone
Since correction pulses are output from the charge pump even if the PLL is locked when the charge pump is in the
ON/ON state, the loop can easily become unstable. This point requires special care when designing application
circuits.
The following problems may occur in the ON/ON state.
• Side band generation due to reference frequency leakage
• Side band generation due to both the correction pulse envelope and low frequency leakage
Schemes in which a dead zone is present (OFF/OFF) have good loop stability, but have the problem that acquiring a
high C/N ratio can be difficult. On the other hand, although it is easy to acquire a high C/N ratio with schemes in
which there is no dead zone, it is difficult to achieve high loop stability. Therefore, it can be effective to select DZA
or DZB, which have no dead zone, in applications which require an FM S/R ratio in excess of 90 to 100 dB, or in
which an increased AM stereo pilot margin is desired. On the other hand, we recommend selecting DZC or DZD,
which provide a dead zone, for applications which do not require such a high FM signal-to-noise ratio and in which
either AM stereo is not used or an adequate AM stereo pilot margin can be achieved.
DZ1
1
0
0
1
DZ0
0
1
0
1
Dead zone mode
DZC
DZD
DZA
DZB
Charge pump
OFF/OFF
OFF/OFF
ON/ON
ON/ON
LC72135M
Dead zone
+ +0 s
– –0 s
+0 s
–0 s
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