CD74ACT297

DIGITAL PHASE-LOCKED LOOP

SCHS297D – AUGUST 1998 – REVISED JUNE 2002

5

POST OFFICE BOX 655303

detailed description

The phase detector generates an error-signal waveform that, at zero phase error, is a 50% duty-cycle square wave.

At the limits of linear operation, the phase-detector output is either high or low all of the time, depending on the

direction of the phase error (

The phase-detector output can be varied between

±1 according to the relation:

Phase-detector output

+

%high – %low

100

XORPD and ECPD are commonly used digital types. The ECPD is more complex than the XORPD, but can be

because its output remains high (PD output = 1) for a phase error of one-fourth cycle. Similarly, for the ECPD,

the zero-phase-error point, i.e., the phase separation of the phase-detector inputs for

the basic DPLL system of Figure 2,

one-fourth cycle out of phase for zero phase error. For the ECPD,

phase.

D/U

φB

φA1

Divide-by-N

Counter

Divide-by-K

Counter

fin,

φin

Mfc

I/D Circuit

I/D OUT

I/D CLK

Carry

Borrow

K CLK

XORPD OUT

fout,

φout

2Nfc

Figure 2. DPLL Using Exclusive-OR Phase Detection

The phase-detector output controls the up/down input to the K counter. The counter is clocked by input frequency

Recycling while counting down generates a borrow pulse. If the carry and borrow outputs are conceptually combined

into one output that is positive for a carry and negative for a borrow, and if the K counter is considered as a frequency

The carry and borrow pulses go to the increment/decrement (I/D) circuit, which, in the absence of any carry or borrow

pulse, has an output that is one-half of the input clock (I/D CLK). The input clock is just a multiple (2N) of the loop

center frequency. In response to a carry or borrow pulse, the I/D circuit either adds or deletes a pulse at I/D OUT. Thus,

The output of the N counter (or the output of the phase-locked loop) is:

fo + fc ) (kdfeMfc) 2KN

When this result is compared to the equation for a first-order analog phase-locked loop, the digital equivalent of the

(1)

(2)