Datasheet LT5537 (Analog Devices) - 10

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