10

520 - 99 - 05

Fig. 14 GS9015A Typical Test Circuit Using -5V Supply

All resistors in ohms, all capacitors in microfarads unless otherwise stated.

VCO Frequency Setting Resistors

There are two modes of VCO operation available in the

GS9015A depending on the state of the

÷2 block. The ÷2

block is enabled according to :

÷2 ENABLE = ƒ/2 • SS1. When

the ƒ/2 ENABLE (pin 10) is LOW, any of the four VCO

frequency setting resistors, RVCO0 through RVCO3, (pins

13, 14, 15 and 17) may be used for any data rate from 100

Mb/s t o 400 Mb/s. For example, for 143 Mb/s data

rate, the value of the total RVCO resistance is approximately

6k8 and for 270 Mb/s operation, the value is

approximately 3k5. The 5k potentiometers will then tune

the desired data rate near their mid-points.

Jitter performance at the lower data rates (143, 177 Mb/s) is

improved by operating the VCOat twice the normal frequency.

This is accomplished by enabling the divide by two block in

the PLL section of the GS9015A.

When ƒ/2 (Pin 10) is HIGH, two of the RVCO pins are assigned

to data rates below 200 Mb/s and two are assigned to data

rates over 200 Mb/s. The selection is dependent upon the

level of STANDARD SELECT BIT, SS1 (pin 21). When SS1

is LOW, RVCO0 and RVCO1 (pins 13 and 14) are used for the

higher data rates. When SS1 is HIGH, the VCO frequency is

now twice the bit rate and its frequency is set by RVCO2 and

RVCO3 (pins 15 and 17).

For 143 Mb/s and 270 Mb/s operation, (the VCO is at

286 MHz and 270 MHz respectively) the total resistance

required is approximately the same for both data rates. This

also applies for 177 Mb/s and 360 Mb/s operation (the VCO

is tuned to 354 MHz and 360 MHz respectively). This means

that one potentiometer may be used for each frequency pair

with only a small variation of the fixed resistor value. This

halves the number of adjustments required.

ECL

DATA

INPUTS

-5V

-5V

-5V

100

100

100

100

390

390

390

390

0.1

µ

25

24

23

22

21

20

19

5

6

7

8

9

10

11

4

3

2

1 28 27 26

12 13 14 15 16 17 18

910

5.6p

10n

-5V

÷1

÷2

0.1

µ

10k

DATA

CARRIER

DETECT

OUTPUT

DATA

CLOCK

CLOCK

GS9015A

DD

I

DD

I

VCC1

VEE1

VEE1

ƒ/2

VEE3

SDO

SDO

SCO

SCO

SS1

SS0

CD

STAR

ROUTED

LOOP

VOLTAGE

See Figure 15

-5V

-5V

-5V

0.1

µ