lt5537 Linear Technology Corporation, lt5537 Datasheet - Page 10

• Manufacturer Part Number: lt5537
• Description: Wide Dynamic Range Rf/if Log Detector
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT5537
The 1:4 input transformer can also be replaced with a
narrow band discrete balun circuit using three compo-
nents as shown in Figure 6. Capacitors C11, C12 and
inductor L1 form a tank circuit having a transformer-like
function over a narrow bandwidth. The increased power-
to-voltage transformation and the narrower input passband
serve to improve the sensitivity of the logarithmic detector.
The resonant balun circuit using discrete components can
be custom designed for a range of different input imped-
ance or sensitivity requirements.
10
INPUT
R
J1
S
Figure 6. Input Matching Network
U
C11
C12
U
L1
R2
W
C1
C2
Figure 7. Measured Output with R
2.5
2.0
0.5
1.5
1.0
0
5537 F07
–100
IN
IN
2
3
T
200MHz
V
+
–
U
A
CC
R
= 25°C
IN
= ENBL = 3V
–80
INPUT POWER (dBm)
–60
BALUN
264Ω
–40
Table 2. Matching Network Component Values for 200MHz
Center Frequency
The examples given in Table 2 cover two different transfor-
mation ratios. The first one transforms single-ended 50Ω
to differential 264Ω. The V
Figure 7 indicate that the input power range for linear
logarithmic detection is shifted downward by 7dB with a
sensitivity improvement of 6dB compared with a simple
50Ω termination. The input return loss is 30dB at the
design frequency of 200MHz. Bandwidth for better than
10dB return loss is 55MHz. The second example has a
higher Q of 3.9 and a corresponding transformed imped-
ance of 828Ω. The input power range for linear operation
is shifted downward by 12dB with a sensitivity improve-
ment of 10dB compared with a simple 50Ω termination.
The input return loss is 25dB at the design frequency.
Bandwidth for better than 10dB return loss is 18MHz.
SINGLE ENDED
SENSITIVITY LOSS BW
–20
(dBm)
–82.4
–86.1
50Ω
0
IN
5537 F10
= 264Ω
RETURN
20
(MHz)
10dB
55
18
(nH)
120
L1
82
OUT
C11,
C12
(pF)
7.5
15
vs P
(Ω)
330
R2
2k
IN
transfer curves in
2.1
3.9
Q
RESISTANCE
EFFECTIVE
INPUT
264
828
(Ω)
5537fa