13

Applications Information

Typical application for FSK modulation

scheme

The HCPL- 810J is designed to

work with various transceivers

and can be used with a variety

of modulation methods

including ASK, FSK and BPSK.

Figure 17 shows a typical

application in a powerline

modem using Frequency Shift

Keying (FSK) modulation

scheme.

Transmitter

The analogue Tx input pin is

connected to the modulator via

an external coupling capacitor

C1 and a series resistor R3 (see

Figure 17). Optimal performance

is obtained with an input signal

of 250

µA

modulator with an output signal

capacitor of 100 nF, the optimal

series resistor R3 would be 2

k

Ω.

Tx AGC

To ensure a stable and constant

output voltage at Tx- PD- out, the

HCPL- 810J includes an

Automatic Gain Control (AGC)

circuit in the isolated transmit

signal path.

Figure 17. Schematic of HCPL-810J application for FSK modulation scheme.

100 nF

Filter

100 µF

GND2

24 k

Ω

C2

X2

GND2

N

C1

100 nF

GND2

Status

D1

R4

2

Ω

100 nF

100 nF

GND2

1 µF

R1

5 k

Ω

GND2

Rx-out

GND1

Filter

1 µF

GND1

Tx-en

Tx-in

L2

R3

2 k

Ω

R2

10 k

Ω

L

L1

330 µH

GND2

HCPL-810J

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Tx-en

Tx-in

Rx-PD-out

Rx-Amp-in

Status

Rx-out

GND1

Rx-in

Tx-LD-in

Tx-PD-out

Tx-out

GND2

This AGC circuit compensates

for variations in the input signal

level presented at Tx- in and

variations in the optical channel

over temperature and time. The

Tx- PD- out output signal is

effectively stabilized for input

Tx- in signals of between 150

µA

8). The AGC circuit starts to

function 10

µs after the Tx- en

signal is set to logic high. After

a period of 180

µs the Tx- PD-

out signal typically reaches 66%

of its steady state level (see

Figure 18). To ensure correct

operation of the internal

circuitry, an external 1

µF

capacitor needs to be connected

from pin 11 to GND2.

The optical signal coupling

technology used in the HCPL-

810J transmit path achieves

very good harmonic distortion,

which is usually significantly

better than the distortion

performance of the modulated

input signal.

However to meet

the requirements of some

international EMC regulations it

is often necessary to filter the

modulated input signal. The

optimal position for such a

filter is between pins 13 and 12

as shown in Figure 17. A

possible band- pass filter

topology is shown in Figure 19,

some typical values of the

components in this filter are

listed in Table 1.

Figure 18. Tx-PD-out AGC response time.

5 0

µs/Div

Tx-en 5 V/Div

Tx-PD-out 1 V/Div