8

SL3522

A PRACTICAL APPLICATION FOR THE SL3522

AS A LOG/LIMITING AMPLIFIER

The SL3522, with the RF Output-Buffer ENABLED, has a

large limited RF Output level (0dBm on each of two RF Output

pins (9 and10)) and a wide RF bandwidth (450MHz) in a small

28 pin Miniature Ceramic S.O package. As a result, there is a

tendency for the device to become unstable unless care is

used in the application.

The PCB layout for a ”SL3522 DEMONSTRATION

BOARD” in Fig. 11 has proved reliably stable. The PCB is a

double layer Fibre epoxy board which uses SMDs where

possible. A circuit diagram for the Demonstration PCB appears

in Fig. 10.

The following points should be noted when this

application is realised practically:-

1) A wire needs to connect the two pads connected to pins

14 and 15 of the SL3522, to allow +5V to appear at both pins.

2) ALL the GND connections to the SL3522 are made

through the PCB to a Ground plane on the bottom side. It is

important to ensure that the impedance of each of these

connections is kept to an absolute minimum to prevent

instability. If these connections are achieved using through

plated holes, it is recommended that they are filled with solder

to lower their impedance.

3) The PCB is configured to accept SMA, SMB or SMC

connectors for the RF input, RF Output and Video Output

connections. These can be changed if necessary to an

alternative type, but it is vital to ensure that the ground plane

is solidly connected to the Guard Ring which surrounds the RF

Output tracks.

4) The PCB is configured to accept a small surface

mounting DC isolating BALUN transformer (e.g VANGUARD

VE43666, available from Vanguard Electronics Company Inc,

1480 West 178th St. GARDENA, C.A. 90248, U.S.A. Tel:-

U.S.A. (213) 323 – 4100) to couple a signal into the RF input

connections (pins 27 and 28). It is NOT recommended to

attempt operating the SL3522 with the RF Output Buffer

enabled, WITHOUT using an input BALUN, although it may be

possible, provided the input source impedance to both pins 27

and 28 remains balanced. The centre tap of the secondary

winding of the transformer should be soldered to the small

ground plane on the upper side of the PCB.

5) The RF Output connection to the PCB is from pin 9 of

the SL3522 only, with pin 10 being terminated on the PCB

using a 51

Ω resistor. It is important to ensure that both pin 9

and10 are terminated with equal impedances.

6) The RF Output Buffer can be enabled by soldering a

link (LK) between pin 8 of the SL3522 and the adjacent guard

track around the RF Output lines. Similarly, the buffer can be

disabled by removing the same link. When the buffer is

disabled, the following components can be omitted:-

1nF capacitors (C1, C2)

10nF capacitor (C8)

51

Ω resistor (R

7) The Slope (gain) and Offset of the Video Output can be

adjusted using two 1k

Ω trimmers, provision for which is

included in the PCB layout.

The plots in Fig. 12 to fig. 23 are typical of the performance

of SL3522 devices used with the PCB layout detailed in Fig.11.

Fig.10 SL3522 demonstration board circuit diagram

VIDEO

OUTPUT

C5

OFFSET

ADJUST

10n

GAIN

ADJUST

1n

1n

15

16

17

18

19

20

21

22

23

24

25

14

13

12

11

10

9

8

7

6

5

4

RF OUTPUT

10n

3

2

1

26

27

28

10n

RF INPUT

1k5

10n

VIDEO LOADING

R

200

C

20p

10n

10n

C4

C3

10n

LK

C1

C8

C2

C9

51

C7

C6

150

1k

1k

SL3522