Dead Zone

The phase comparator compares fp to a reference frequency (fr) as shown in Figure 4. Although the characteristics of

this circuit (see Figure 5) are such that the output voltage is proportional to the phase difference ø (line A), a region

(the dead zone) in which it is not possible to compare small phase differences occurs in actual ICs due to internal

circuit delays and other factors (line B). A dead zone as small as possible is desirable for products that must provide

a high S/N ratio.

However, since a larger dead zone makes this circuit easier to use, a larger dead zone is appropriate for popularly-

priced products. This is because it is possible for RF signals to leak from the mixer to the VCO and modulate the

VCO in popularly-priced products in the presence of strong RF inputs. When the dead zone is narrow, the circuit

outputs correction pulses and this output can further modulate the VCO and generate beat frequencies with the RF

signal.

Figure 4

Figure 5

2. Notes on the FMIN, AMIN, and IFIN Pins

Coupling capacitors must be placed as close as possible to their respective pin. A capacitance of about 100 pF is

desirable. In particular, if a capacitance of 1000 pF or over is used for the IF pin, the time to reach the bias level will

increase and incorrect counting may occur due to the relationship with the wait time.

3. Notes on IF Counting

→ SD must be used in conjunction with the IF counting time

When using IF counting, always implement IF counting by having the microprocessor determine the presence of the

IF-IC SD (station detect) signal and turn on the IF counter buffer only if the SD signal is present. Schemes in which

auto-searches are performed with only IF counting are not recommended, since they can stop at points where there is

no signal due to leakage output from the IF counter buffer.

4. DO Pin Usage Techniques

In addition to data output mode times, the DO pin can also be used to check for IF counter count completion and for

unlock detection output. Also, an input pin state can be output unchanged through the DO pin and input to the

controller.

5. Power Supply Pins

No. 4921-21/23

LC72131, 72131M