APPLICATION NOTE 2241

MAX2640 LNA Operates at 315MHz for Automotive Keyless Entry Applications


Abstract: The MAX2640 low-cost, ultra low-noise amplifier (LNA) was originally characterized over the 400MHz to 2500MHz range. The device has also proven useful at 315MHz for keyless entry applications. Testing has shown that the part can typically achieve 16dB gain and 1.3dB noise figure at 315MHz, while consuming only 3.5mA from a single 2.7 to 5V supply.

General

The MAX2640 low cost, ultra-low noise amplifier was designed for operation in the cellular, PCS, GPS, and 2.4GHz ISM Bands. It operates from a single 2.7 to 5.5V supply and typically consumes only 3.5mA of current. Offered in a 6-pin SOT23 package, it is ideal for portable devices where PCB space is at a premium.

Originally characterized over the 400MHz to 2500MHz range, the device has also proven useful at 315MHz for keyless entry applications. Maxim has recently tested the IC for this extended frequency range with promising results. The data presented here will demonstrate the IC's ability to function well at 315MHz, with typical gain of approx. 16dB, and a 1.3dB noise figure.

Objective

Optimize the MAX2640 EVKit for 315MHz operation, and demonstrate selected operating characteristics over a statistical sample.

Results

A standard evaluation kit was obtained and modified as shown in the schematic in Figure 1. The modifications require positioning of Z1a and Z1b on the pads for Z1. See Figure 2 for detailed component placement information.

Figure 1.
Figure 1.

Figure 2.
Figure 2.

Key parameters were measured, and the results are shown in Table 1 (Board 'A'). A second board (Board 'B') was modified to the same schematic and displayed similar results. A plot of return-loss and gain is shown in Figure 3.

Figure 3.
Figure 3.

Table 1.
Vcc = 3.0Vdc
Fin = 315MHz
Ta = 25°C

Parameter Measured
(Board A) (Board B)
S11 (dB) -9.2 -10.0
S21 (dB) 17.2 16.5
S12 (dB) >50 >50
S22 (dB) -13.1 -13.4
Supply Current (mA) 3.6 3.3
Input 1dB Compression Point (dBm) -27 -28
Input IP3 (dBm) -17.4 -18.1
Noise Figure (dB) 1.8 1.7

After the initial tuning measurements, the same parameters were tested over an additional 30 IC's. The actual and averaged data from these tests is shown in Tables 2 and 3 respectively. This data demonstrates that the IC is very capable of operating at 315MHz with only slight application circuit adjustments.

Table 2
Vcc = 5V
Fin = 315MHz
Ta = 25°C

Parameter Icc(mA) S11(dB) S21(dB) S12(dB) S22(dB) NF(dB) IIP3(dBm) P1dB(dBm)
IC#:
01 3.38 -9.06 15.60 -70.0 -13.56 1.32 -17.48 -28.80
02 3.62 -8.54 16.26 -59.70 -13.30 1.28 -17.79 -28.60
03 3.57 -9.30 15.98 -60.00 -13.00 1.36 -17.53 -28.80
04 3.54 -8.91 15.42 -59.50 -14.50 1.28 -17.74 -28.80
05 4.00 -8.32 17.14 -59.27 -13.10 1.29 -17.00 -28.20
06 3.52 -9.01 15.96 -65.00 -13.71 1.30 -17.24 -28.70
07 3.59 -8.60 16.08 -65.00 -13.83 1.30 -17.16 -28.60
08 3.67 -9.23 16.44 -64.89 -13.89 1.30 -17.44 -28.80
09 3.50 -8.89 15.76 -65.00 -13.70 1.31 -16.85 -28.80
10 3.45 -9.58 15.86 -65.00 -13.83 1.30 -17.31 -28.80
11 3.55 -9.09 15.95 -66.00 -13.68 1.31 -17.28 -28.80
12 3.63 -9.10 16.25 -67.00 -13.98 1.32 -17.08 -28.60
13 3.63 -9.48 16.30 -67.90 -13.65 1.33 -17.25 -28.70
14 3.69 -9.52 16.38 -68.90 -13.70 1.34 -17.07 -28.60
15 3.54 -8.97 15.94 -67.40 -13.64 1.29 -17.20 -28.70
16 3.69 -8.98 16.40 -69.90 -13.71 1.28 -16.99 -28.70
17 3.49 -9.27 15.91 -70.00 -13.85 1.32 -17.25 -28.70
18 3.47 -9.31 15.99 -68.00 -13.73 1.32 -17.37 -28.80
19 3.50 -9.23 15.98 -68.70 -13.78 1.30 -17.44 -28.70
20 3.50 -8.81 15.98 -71.50 -13.80 1.29 -17.35 -28.70
21 3.68 -9.03 16.35 -70.00 -13.63 1.30 -17.08 -28.60
22 3.50 -9.10 15.97 -71.00 -13.94 1.30 -17.28 -28.70
23 3.56 -9.32 16.19 -70.00 -13.78 1.31 -17.33 -28.70
24 3.63 -9.26 16.30 -70.00 -13.82 1.32 -17.22 -28.70
25 3.55 -8.59 16.10 -68.70 -13.65 1.26 -17.45 -28.60
26 3.42 -8.97 15.76 -71.40 -13.65 1.32 -17.41 -28.80
27 3.72 -9.34 16.44 -70.50 -13.96 1.32 -16.83 -28.60
28 3.58 -9.52 16.20 -71.20 -13.69 1.34 -17.23 -28.70
29 3.58 -9.40 16.17 -70.00 -13.77 1.32 -17.23 -28.60
30 3.49 -8.95 15.84 -70.23 -13.67 1.31 -17.29 -28.70

Table 3
Vcc = 5V
Fin = 315MHz
Ta = 25°C
Parameter Average (30pcs.) Standard Deviation
S11 (dB) -9.08 0.31
S21 (dB) 16.10 0.30
S12 (dB) -67.52 3.60
S22 (dB) 13.72 0.25
Noise Figure (dB) 1.31 0.02
Input IP3 (dBm) -17.24 0.23
Input P1dB (dBm) -28.68 0.11


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APP 2241:
APPLICATION NOTE 2241,AN2241, AN 2241, APP2241, Appnote2241, Appnote 2241