8

LT1328

There is one drawback to the previous scheme. Lowering

the highpass frequency to 14Hz negates the normally

excellent ambient light rejection of the LT1328 to line-

operated light sources. By modulating the data in a very

simple and inexpensive way the ambient light rejection

can be maintained. The NRZ data is used to gate a 3MHz

square wave before driving the transmitter. The LT1328 in

FIR-4ppm mode (bandwidth of 1MHz to 4MHz) will re-

ceive the 3MHz signal. Thus, the low frequency response

is no longer required and the LT1328 can be configured as

a FIR receiver with no changes. Note that when in FIR/

4ppm mode, the LT1328 has excellent rejection to active-

ballast fluorescent lights (40kHz to 100kHz) as well as line

operated bulbs. The data is recovered with two stages of

retriggerable one-shots on the output of the LT1328. The

first one-shot pulse length is set to 80ns and acts as an

edge detector and the second one-shot pulse length is set

to 2.3

µs, just under a symbol period. With this modula-

tion, the range can be extended to over 30" and still have

Wireless Slave for Photographic Flash

1

2

3

4

8

7

6

5

LT1328

NC

NC

NC

D2

1N4148

Q1

MPS2907A

R2

1k

5V

TO HOT SHOE

OF SLAVE FLASH

R3

10k

C2

4.7

µF

R5

1k

R4

10k

Q2

MPSA42

1328 TA05

D3

MUR

160524

C5

0.1

µF

C4

330pF

D1

HAMAMATSU

S4797-01

LIGHT FROM

MASTER FLASH

R1

100

Ω

Figure 6. Simple Data Link with Improved Low Frequency Noise Rejection

5V

> 30"

R1

100

Ω

R2

10k

3MHz

R3

3.9

Ω

1/2W

Q3

2N7002

74LS00

2

1

3

BIT ERROR RATE

TRANSMITTER/RECEIVER

1328 F06

D2

HSDL 4220

OUT

IN

375bps NRZ DATA

1

2

3

15

NC

14

13

C7

10pF

1/2

74LS123

9

7

5V

6

5

10

11

R5

46.4k

1%

R4

5k

1%

C8

100pF

1/2

74LS123

1

2

3

4

8

7

6

5

C6

1000pF

LT1328

C4

100pF

C2

4.7

µF

5V

D1

TEMIC

BPV22NF

C5

4.7

µF