Rainbow Electronics MAX2685 - Manual (Page 7)

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 50 Ω 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.
MAX2685
Low-Cost, 900MHz, Low-Noise Amplifier
and Downconverter Mixer
_______________________________________________________________________________________ 7
Table 1. LNA Typical S-Parameters (V
CC
= +3V, T
A
= +25°C)
| S11 |
MAG
FREQUENCY
(MHz)
800 0.761
S11
PHASE
(degrees)
-64.5
| S21 |
MAG
S21
PHASE
(degrees)
177.9 4.98
| S12 |
MAG
S12
PHASE
(degrees)
-163.7
| S22 |
MAG
S22
PHASE
(degrees)
-107.3 0.376 0.018
-68.6 167.2 5.06 -167.1 840 0.753 -107.0 0.264 0.022
-73.2 156.6 5.07 -171.3 880 0.747 -94.6 0.172 0.026
-78.0 146.6 4.91 -175.7 920 0.733 -62.9 0.149 0.030
-82.8 137.7 4.68 178.0 960 0.719 -42.4 0.200 0.035
-87.5 130.3 4.40 171.0 1000 0.693 -38.8 0.263 0.039
-45.6 73.0 0.188 71.9 800 0.625 -91.3 0.483 0.191
-48.1 65.5 0.195 64.2 840 0.621 -91.3 0.423 0.198
-50.9 58.1 0.199 56.7 880 0.619 -89.9 0.370 0.201
-53.3 51.6 0.200 50.3 920 0.611 -86.1 0.337 0.202
-55.5 46.1 0.200 44.7 960 0.608 -80.9 0.322 0.201
-57.5 41.2 0.200 40.0 1000 0.607 -76.7 0.317 0.200
High-Gain Mode (GAIN = VCC)
Low-Gain Mode (GAIN = GND)