TRU050, VCXO Based PLL

Vectron International, 267 Lowell Rd, Hudson NH 03051-4916

Tel: 1-88-VECTRON-1

Page 4 of 14

Rev: 26Aug2008

TRU050 Theory of Operation

Phase Detector

The phase detector has two buffered inputs, DATAIN and CLKIN, which are designed to switch at 1.4 volts.

DATAIN is designed to accept an NRZ data stream but may also be used for clock signals which have about

a 50% duty cycle. CLKIN is connected to OUT1 or OUT2, or a divided version of one of these outputs. CLKIN

and DATAIN and are protected by ESD diodes and should not exceed the power supply voltage or ground by

more than a few hundred millivolts.

The phase detector is basically a latched flip flop/exclusive-or gate/differential amplifier filter design to

produce a DC signal proportional to the phase between the CLKIN and DATAIN signals, see figure 4 for a

block diagram and figure 5 for a open loop transfer curve. This simplies the PLL design as the designer does

not have to filter narrow pulse signal to a DC level. Under locked conditions the rising edge CLKIN will be

centered in the middle of the DATAIN signal, see figure 6.

The phase detector gain is 0.53V/rad x data density for 5volt operation, and 0.35V/rad x data density for 3.3

volt operation. Data density = 1.0 for clock signals and is system dependent on coding and design for NRZ

signals, but 0.25 could be used as a starting point for data density.

The phase detector output is a DC signal for DATAIN frequencies greater than 1MHz but produces signficant

ripple when inputs are less than 200kHz. Additional filtering is required for low input frequency applications

such as 8kHz frequency translation, see figures 8 and 9.

Under closed loop conditions the active filter has a blocking capacitor which provides a very high DC gain, so

signifigantly with changes in input frequency (within lock range). The control (voltage pin 1) will vary

according to the input frequency offset, but PHO will remain relatively constant.

D

D

Gain = 2 / 3

Gain = 5 V / 2

π

30 k

Ω

20 k

Ω

PHO

(pin 6)

Clock In

(pin 9)

Data In

(pin 7)

Figure 4. Simplified Phase Detector Block Diagram