Other Items

• Notes on the phase comparator dead zone

When the charge pump is used with one of the ON/ON modes, correction pulses are generated from the charge pump

even if the PLL is locked. As a result, it is easy for the loop to become unstable, and special care is required in

application design. The following problems can occur if an ON/ON mode is used.

— Sidebands may be created by reference frequency leakage.

— Sidebands may be created by low-frequency leakage due to the correction pulse envelope.

Although the loop is more stable when a dead zone is present (i.e. when an OFF/OFF mode is used), a dead zone

makes it more difficult to achieve excellent C/N characteristics. On the other hand, while it is easy to achieve good C/N

characteristics when there is no dead zone, achieving good loop stability is difficult. Accordingly, the DZA and DZB

settings, in which there is no dead zone, can be effective in situations where a signal-to-noise ratio of 90 to 100 dB or

higher is required in FM reception, or where it is desirable to increase the pilot margin in AM stereo reception.

However, if such a high signal-to-noise ratio is not required for FM reception, if an adequate pilot margin can be

acquired in AM stereo reception, or if AM stereo is not required, then either DZC or DZD, in which there is a dead

zone, should be chosen.

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 and the RF signal.

• Notes on the FMIN, AMIN, and IFIN 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 under 1000 pF, the time to reach the bias

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

• 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. 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.

No. 5815-21/23

LC72121, 72121M, 72121V

DZ1

DZ0

Dead zone mode

Charge pump

Dead zone

0

0

DZA

ON/ON

– –0s

0

1

DZB

ON/ON

–0s

1

0

DZC

OFF/OFF

+0s

1

1

DZD

OFF/OFF

+ +0s

Figure 1

Figure 2