MAX2685_00 Datasheet, PDF (7/10 Pages) Maxim Integrated Products – Low-Cost, 900MHz, Low-Noise Amplifier and Downconverter Mixer

Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
Detailed Description
The MAX2685 consists of five major components: a
low-noise amplifier (LNA), an LNA bypass switch, a
downconverter mixer, a local-oscillator (LO) buffer, and
a power-management block.
Low-Noise Amplifier (LNA)
The LNA is a wideband, single-ended cascode amplifi-
er that operates over a wide range of frequencies. The
input of the LNA (LNAIN) requires an appropriate
matching network and a DC-blocking capacitor. The
typical operating circuit shown in Figure 1 is optimized
for frequencies around 880MHz, requiring only a 0.1µF
capacitor in series with a 12nH inductor. See Table 1
for the LNA “S” parameters for matching to other fre-
quencies.
The output of the LNA (LNAOUT) is internally biased to
V_CC. It is internally matched to 50Ω and incorporates
an internal DC-blocking capacitor.
LNA Bypass Switch
and Gain Control
When a large input signal is present, enable the LNA
bypass function to increase linearity and reduce supply
current. Set GAIN low to enable the LNA bypass func-
tion.
Receive Mixer
The downconverter mixer is a wideband, single-bal-
anced design with a low noise figure and high linearity.
The RF signal at the MIXIN port is mixed with the signal
at the LO port, and is downconverted to an IF frequen-
cy at the differential IF port.
RF Input
The MIXIN input requires a simple external matching
network and a series DC-blocking capacitor. See
Figure 1 for a matching network example, optimized for
880MHz operation. Table 2 lists mixer “S” parameters
for matching to other frequencies.
Table 1. LNA Typical S-Parameters (V_CC = +3V, T_A = +25°C)
FREQUENCY
(MHz)
|^S11|
MAG
S11
PHASE
(degrees)
High-Gain Mode (GAIN = VCC)
800
0.761
840
0.753
880
0.747
920
0.733
960
0.719
1000
0.693
Low-Gain Mode (GAIN = GND)
800
0.625
840
0.621
880
0.619
920
0.611
960
0.608
1000
0.607
-64.5
-68.6
-73.2
-78.0
-82.8
-87.5
-45.6
-48.1
-50.9
-53.3
-55.5
-57.5
|^S21|
MAG
4.98
5.06
5.07
4.91
4.68
4.40
0.188
0.195
0.199
0.200
0.200
0.200
S21
PHASE
(degrees)
177.9
167.2
156.6
146.6
137.7
130.3
73.0
65.5
58.1
51.6
46.1
41.2
|^S12|
MAG
0.018
0.022
0.026
0.030
0.035
0.039
0.191
0.198
0.201
0.202
0.201
0.200
S12
PHASE
(degrees)
-163.7
-167.1
-171.3
-175.7
178.0
171.0
71.9
64.2
56.7
50.3
44.7
40.0
|^S22|
MAG
0.376
0.264
0.172
0.149
0.200
0.263
0.483
0.423
0.370
0.337
0.322
0.317
S22
PHASE
(degrees)
-107.3
-107.0
-94.6
-62.9
-42.4
-38.8
-91.3
-91.3
-89.9
-86.1
-80.9
-76.7
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