LC72121MA

No.A2009-21/24

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

φ.

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

and the RF signal.

Figure 1

Figure 2

(2) Notes on the FMIN, AMIN, and IFIN pins

Coupling capacitors should be placed as close to their pin as possible. A capacitance of about 100pF is desirable for

these capacitors. In particular, if the IFIN pin coupling capacitor is not held under 1000pF, the time to reach the bias

level may become excessive and incorrect counts may result due to the relationship with the wait time.

(3) Notes on IF counting

→ Use the SD signal in conjunction with IF counting

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.

Autosearch techniques that only use the IF counter are subject to incorrect stopping at points where there is no

station due to IF buffer leakage.

(4) DO pin usage

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.

(5) Power supply pins

Capacitors must be inserted between the power supply VDD and VSS pins for noise exclusion. These capacitors

must be placed as close as possible to the VDD and VSS pins.

(6) VCO setup

Applications must be designed so that the VCO (local oscillator) does not stop, even if the control voltage (Vtune)

goes to 0V. If it is possible for the oscillator to stop, the application must use the control data (DLC) to temporarily

force Vtune to VCC to prevent deadlock from occurring. (Deadlock clear circuit)

Reference divider

VCO

LPF

Programmable divider

Phase

Detector

MIX

RF

Leakage

fr

fp

φ(ns)

V

(A)

(B)

Dead zone