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 51Ω 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)
51Ω resistor (R
FO
)
7) The Slope (gain) and Offset of the Video Output can be
adjusted using two 1kΩ 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.
+5V V
CC
GAIN
ADJUST
VR
G
1k
C3
10n
28
27
26
25
24
23
22
21
C4
10n
20
19
18
17
OFFSET
ADJUST
VR
0
1k
C5
10n
16
15
RF INPUT
R
G
1k
R
T
1k5
R
O
1k
SL3522
1
2
3
C9
10n
LK
C1
1n
C2
1n
R
FO
51
4
5
6
7
8
9
10
11
C8
10n
12
13
14
R
VS
150
C7
10n
C6
10n
VIDEO
OUTPUT
–5V V
EE
VIDEO LOADING
R 200
C 20p
RF OUTPUT
Fig.10 SL3522 demonstration board circuit diagram
8
MITEL [ MITEL NETWORKS CORPORATION ]
