lc72134m Sanyo Semiconductor Corporation, lc72134m Datasheet - Page 24

• Manufacturer Part Number: lc72134m
• Description: Dual Pll Frequency Synthesizer For Fm Tuner Systems
• Manufacturer: Sanyo Semiconductor Corporation

Dead Zone
As shown in figure 1, the phase comparator compares a reference frequency (fr) with fp. As shown in figure 2, the phase
comparator’s characteristics consist of an output voltage (V) that is proportional to the phase difference ø. However, due
to internal circuit delay and other factors, an actual circuit has a region (the dead zone, B) where the circuit cannot
actually compare the phases. To implement a receiver with a high S/N ratio, it is desirable that this region be as small as
possible. However, it is often desirable to have the dead zone be slightly wider in popularly-priced models. This is
because in certain cases, such as when there is a strong RF input, popularly-priced models can suffer from mixer to VCO
RF leakage that modulates the VCO. When the dead zone is small, the circuit outputs signals to correct this modulation
and this output further modulates the VCO. This further modulation may then generate beats with the RF signal.
• Notes on the FMIN, AMIN, and IFIN pins
• Notes on IF counting
• DO pin usage
• Power supply pins
Coupling capacitors should be placed as close to their pin as possible. A capacitance of about 100 pF is desirable for
these capacitors. In particular, if the IFIN pin coupling capacitor is not held to under 100 pF, the time to reach the bias
level may become too long and incorrect counts may result due to the relationship with the wait time.
When counting the IF frequency, the microcontroller must determine the presence or absence of the IF IC SD (station
detect) signal and turn on the IF counter buffer output and execute the IF count only if there is an SD signal. Auto-
search techniques that only use the IF counter are subject to incorrect stopping at points where there is no station due to
IF buffer leakage.
The DO pin can be used for IF counter count completion checking and as an unlock detection output in addition to its
use in data output mode. It is also possible to have the DO pin reflect the state of an input pin to input that state to the
microcontroller.
A capacitor of at least 2000 pF must be inserted between the power supply V
capacitor must be placed as close as possible to the V
Use the SD signal in conjunction with IF counting
Figure 1
LC72134M
DD
and V
SS
pins.
DD
and V
SS
pins for noise exclusion. This
Figure 2
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