The SL531 C is a wide band amplifier designed for use in
logarithmic IF amplifiers of the true log type. The input and log
output of a true log amplifier are at the same frequency i e
detection does not occur. In successive detection log
amplifiers (using SL521 SL1521 types) the log output is
detected.
The small signal gain is 10dB and bandwidth is over
500MHz. At high signal levels the gain of a single stage
dropstounity. A cascade of such stages give a close
approximation to a log characteristic at centre frequencies
between 10and 200MHz.
An important feature of the device is that the phase shift is
nearly constant with signal level. Thus any phase information
on the input signal is preserved through the strip.
The device is also available as the 5962-92084 which has
guaranteed operation over the full Military Temperature
Range and is screened to MIL-STD-883 Class B. Data is
available separately
CM8
Fig.1 Pin connections
FEATURES
s
s
s
Low Phase Shift vs Amplitude
On-Chip Supply Decoupling
Low External Components Count
APPLICATIONS
s
s
s
True Log Strips with:-
Log Range
70dB
Centre frequencies
10 - 200MHz
Phase Shift
±
0.5 degrees / 10dB
Fig.2 Circuit diagram
ABSOLUTE MAXIMUM RATINGS
(Non-simultaneous)
Supply voltage
Storage temperature range
Operating temperature range
+12 volts
-65°C to +150°C
-55°C to +125°C
See operating notes
Max junction temperature
175°C
Junction - ambient thermal resistance
220°C/Watt
Junction - case thermal resistance
80°C/Watt
CIRCUIT DESCRIPTION
The SL531 transfer characteristic has two regions For small
input signals it has a nominal gain of 10 dB. at large signals the
gain falls to unity (see Fig 7) This is achieved by operating a
limiting amplifier and a unity gain amplifier in parallel (see Fig
3) Tr1 and Tr4 comprise the long tailed pair limiting amplifier,
the tail current being supplied byTrS, see Fig 2 Tr2 and Tr3
form the unity gain amplifier the gain of which is defined by the
emitter resistors The outputs of both stages are summed in the
300 ohm resistor and Tr7 acts as an emitter follower output
buffer. Important features are the amplitude and phase
linearity of the unity gain stage which is achieved by the use
of 5GHz transistors with carefully optimised geometries.
Fig.3 Block diagram
ORDERING INFORMATION
5962-92084 (SMD)
SL531 C CM
SL531
ELECTRICAL CHARACTERISTICS
These characteristics are guaranteed over the following condltions (unless otherwise stated)
Test circuit Fig (4)
Frequency 60MHz
Supply voltage 9 volts
Ambient temperature 22±2°C
Characteristic
Small signal voltage gain
High level slope gain
Upper cut off frequency
Lower cut off frequency
Supply current
Phase change with input amplitude
Input impedance
Output impedance
Value
Min.
8
-1
250
Typ.
10
0
500
3
17
1.1
Max.
12
+1
10
25
3
Units
dB
dB
MHz
MHz
mA
degress
Conditions
Vin = -30dBm
-3dB w.r.t.
±
60MHz
-Vin = 30dBm to +10dBm
f = 10 - 200MHz
2.pF parallel with 1kΩ
15Ω series with 25nH
OPERATING NOTES
1. Supply Voltage Options
An on chip resistor is provided which can be used to drop the
supply voltage instead of the external 180 ohms shown in the
test circuit. The extra dissipation in this resistor reduces the
maximum ambient operating temperature to 100°C. It is also
possible to use a 6 volt supply connected directly to pins 1 and
2 Problems with feedback on the supply line etc may occur in
this connection and RF chokes may be required in the supply
line between stages.
2. Layout P recautions
The internal decoupling capacitors help prevent high
frequency instability however normal high frequency layout
precautions are still necessary. Coupling capacitors should
be physically small and be connected with short leads. It is
most important that the ground connections are made with
short leads to a continuous ground plane.
3. Low Frequency Response
The LF response is determined by the on chip capacitors. It
can be extended by extra external decoupling on pins 5 and
1.
Fig.4 Test ciruit
Fig.5 Small signal frequency response
SL531
Fig.6 Phase v. input
TYPICAL APPLICATION—6 STAGE LOG STIP
Input log range OdBm to—70dBm
Low level gain 60dB (—70dBm in)
Output dynamic range 20dB
Phase shift (over log range)
±3°
Frequency range 10—200MHz
Fig.7 Transfer characteristics linear plot
The circuit shown in Fig 9 is designed to illustrate the use of
the SL531 in a complete strip The supply voltage is fed to each
stage via an external 180Ω resistor to allow operation to 125°C
ambient If the ambient can be limited to +100°C then the
internal resistor can be used to reduce the external
component count Interstage coupling is very simple with just
a capacitor to isolate bias levels being necessary. No
connection is necessary to pin 5 unless operation below
10MHz is required. It is important to provide extra decoupling
on pin 1 of the first stage to prevent positive feedback occuring
down the supply line. An SL560 is used as a unity gain buffer
the output of the log strip being attenuated before the SL560
to give a nominal 0dBm output into 50Ω.
Fig.8 Transfer characteristics logarithmic input scale
Fig.9 Circuit diagram 6 stage strip
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