MC12206

MOTOROLA

HIPERCOMM

BR1334 — Rev 4

8

Figure 6. Detailed Phase Comparator Block Diagram

PHASE

FREQUENCY

DETECTOR

R

V

UP

DOWN

PHASE COMPARATOR

1

0

1

0

CHARGE

PUMP 1

Do

CHARGE

PUMP 2

BISW

LD

φR

φP

fr

fp

FC

LE

LOCK DETECT

The Lock Detect (LD) output pin provides a LOW pulse when fr and fp are not equal in phase or frequency. The output is normally

HIGH. LD is designed to be the logical NORing of the phase frequency detector’s outputs UP and DOWN. See Figure 6. In typical

applications the output signal drives external circuitry which provides a steady LOW signal when the loop is locked. See Figure 9.

OSCILLATOR INPUT

For best operation, an external reference oscillator is recommended. The signal should be AC–coupled to the OSCin pin through

a coupling capacitor. In this case, no connection to OSCout is required. The magnitude of the AC–coupled signal must be

between 500 and 2200 mV peak–to–peak. To optimize the phase noise of the PLL when used in this mode, the input signal

amplitude should be closer to the upper specification limit. This maximizes the slew rate of the signal as it switches against the

internal voltage reference.

The device incorporates an on–chip reference oscillator/buffer so that an external parallel–resonant fundamental crystal can be

connected between OSCin and OSCout. External capacitor C1 and C2 as shown in Figure 10 are required to set the proper

crystal load capacitance and oscillator frequency. The values of the capacitors are dependent on the crystal chosen (up to a

maximum of 30 pF each including parasitic and stray capacitance).

DUAL INTERNAL CHARGE PUMPS (“ANALOG SWITCH”)

Due to the pure Bipolar nature of the MC12206 design, the “analog switch” function is implemented with dual internal charge

pumps. The loop filter time constant can be decreased by bypassing the first stage of the loop filter with the charge pump output

BISW as shown in Figure 7 below. This enables the VCO to lock in a shorter amount of time.

When LE is HIGH or OPEN (“analog switch is ON”), the output of the second internal charge pump is connected to the BISW pin,

and the Do output is ON. The charge pump 2 output on BISW is essentially equal to the charge pump 1 output on Do. When LE is

LOW, BISW is in a high impedance state and Do output is active.

Figure 7. “Analog Switch” Block Diagram

CHARGE

PUMP 1

Do

CHARGE

PUMP 2

BISW

LE

LPF–1

LPF–2

VCO