ST7162 datasheet(12/17 Pages) STMICROELECTRONICS

12 / 17 page

PFD DESCRIPTION (pin 10 & 15)

Outputs PDTX or PDRX produce an output pulse

current, sourcing or sinking, whose width de-

pends on the delay between falling edges of ref-

erence frequency and RF frequency divided. Sim-

plified schematic of both PFD is described in fig-

ure 10. When the current output is off, PFD is in

high impedance state. The voltage at PFD out-

puts pins depends on the loop filter and VCO char-

acteristics (Fig. 9)

D93TL012

REF RX

(REF TX)

FIN RX / N1

(FIN TX / N2)

OUTPUT

CURRENT

TRI-STATE

Figure 9: PD Output Current vs. FIN / REF. Frequencies.

LOOP

FILTER

PDTX,

PDRX

OUTPUT

CURRENT

to VCO

VDD

Iup

I down

VSS

D93TL013

Figure 10: Simplified schematic of PFD outputs.

When the loop is locked, to prevent a dead area

in the PFD gain due to switching delays, a very

short phase offset is introduced in the loop, so the

PFD output current show the following waveform

Fig. 11.

T = 1/REF

OUTPUT

CURRENT

TRI-STATE

t1 = 1/OSC IN Freq.

D93TL014

Figure 11: PD output current in locked condition.

ST7162

12/17

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