Philips Semiconductors

Product specification

SA8025A

1.8GHz low-voltage Fractional-N synthesizer

1996 Oct 15

13

REF_IN

REFERENCE

DIVIDER

AUX/MAIN

DIVIDER

L

“1”

R

D

C

Q

R

D

C

Q

R

“1”

X

P

N

N-TYPE

CHARGE PUMP

PH

P-TYPE

CHARGE PUMP

VDDA

VSSA

REF_IN

L

R

X

P

N

IPH

SR00630

Figure 8. Phase Detector Structure with Timing

Phase Detectors

The auxiliary and main phase detectors are a two D-type flip-flop

phase and frequency detector shown in Figure 8. The flip-flops are

set by the negative edges of output signals of the dividers. The

rising edge of the signal, L, will reset the flip-flops after both flip-flops

have been set. Around zero phase error this has the effect of

delaying the reset for 1 reference input cycle. This avoids

non-linearity or deadband around zero phase error. The flip-flops

drive on-chip charge pumps. A source current from the charge

pump indicates the VCO frequency will be increased; a sink current

indicates the VCO frequency will be decreased.

Current Settings

The SA8025A has 3 current setting pins: RA, RN and RF. The

active charge pump currents and the fractional compensation

currents are linearly dependent on the current connected between

resistor) can be calculated with the formula:

R

+

V

I

R

I

R

The current can be set to zero by connecting the corresponding pin

Auxiliary Output Charge Pumps

The auxiliary charge pumps on pin PHA are driven by the auxiliary

phase detector and the current value is determined by the external

resistor RA at pin RA. The active charge pump current is typically:

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