LT5537 データシートの表示（PDF） - Linear Technology
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LT5537 Datasheet PDF : 16 Pages
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LT5537
APPLICATIO S I FOR ATIO
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.
J1
INPUT
RS
C11
C1
IN+
2
L1 R2
RIN
C12
C2
IN–
3
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Figure 6. Input Matching Network
Table 2. Matching Network Component Values for 200MHz
Center Frequency
10dB
RETURN
C11,
SENSITIVITY LOSS BW L1 C12 R2
(dBm)
(MHz) (nH) (pF) (Ω)
EFFECTIVE
INPUT
RESISTANCE
Q
(Ω)
–82.4
55
82 15 330 2.1
264
–86.1
18
120 7.5 2k 3.9
828
The examples given in Table 2 cover two different transfor-
mation ratios. The first one transforms single-ended 50Ω
to differential 264Ω. The VOUT vs PIN transfer curves in
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.
2.5
TA = 25°C
200MHz
2.0 VCC = ENBL = 3V
BALUN
1.5
264Ω
SINGLE ENDED
1.0
50Ω
0.5
0
–100 –80 –60 –40 –20
INPUT POWER (dBm)
0 20
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Figure 7. Measured Output with RIN = 264Ω
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