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19-1493; Rev 0; 7/99
MAX2685 Evaluation Kit
General Description
The MAX2685 evaluation kit (EV kit) simplifies testing of
the MAX2685. This EV kit allows evaluation of the low-
noise amplifier (LNA) as well as the downconverter
mixer.
o
50Ω SMA Inputs and Outputs
o
Fully Assembled and Tested
o
Allows Testing of All Device Functions
Features
o
+2.7V to +5.5V Single-Supply Operation
Evaluates: MAX2685
Component List
DESIGNATION QTY
C1, C3, C5,
C17
C2, C14, C15
C4, C8
C6, C7, C10
C9
C11, C18
C12
C13
C16
L1
L2
L3
L4
R1, R2
R3
R4
R5
R6, R7, R8
IFOUT+,
IFOUT-, LNAIN,
LNAOUT, LO,
MIXIN
JU1, JU2
GND, V
CC
U1
None
4
0
2
2
1
2
1
1
1
1
1
1
1
2
1
1
1
3
DESCRIPTION
0.1µF ceramic capacitors
Ordering Information
PART
TEMP. RANGE
-40°C to +85°C
IC PACKAGE
16 QSOP
MAX2685EVKIT
Not installed
47pF ceramic capacitors
330pF ceramic capacitors
3.3pF ceramic capacitor
6pF ±0.1pF ceramic capacitors
1000pF ceramic capacitor
12pF ±0.1pF ceramic capacitor
10µF tantalum capacitor
AVX TAJC106K016
12nH inductor
Toko LL1608-SH12NK
680nH ±5% inductor
Coilcraft 1008CS-681XJBC 5%
820nH ±5% inductor
Coilcraft 1008CS-821XJBC 5%
Not installed
1kΩ resistors
75Ω resistor
15kΩ resistor
0Ω resistor
Not installed
_________________________Quick Start
The MAX2685 EV kit is fully assembled and factory
tested. Follow the instructions in the
Connections and
Setup
section for proper device evaluation.
Test Equipment Required
This section lists the equipment recommended for veri-
fying the MAX2685’s operation. It is intended as a
guide only; some substitutions are possible.
•
Two RF signal generators capable of delivering at
least 0dBm of output power at frequencies to 2GHz
(HP 8648C or equivalent). One generator is required
for the local oscillator (LO) source; the other is
required for the mixer and LNA RF input. Only one
generator is required to operate the LNA.
•
An RF spectrum analyzer that covers the MAX2685’s
operating frequency range (the HP 8561E, for exam-
ple).
•
A power supply that can provide up to 100mA at
+2.7V to +5.5V.
•
An ammeter for measuring the supply current
(optional).
•
Several 50Ω SMA cables.
6
SMA connectors (PC edge mount)
Connections and Setup
2
2
1
1
3-pin headers
Eyelets
MAX2685EEE (16-pin QSOP)
MAX2685 PC board
This step-by-step guide explains how to get the EV kit
operational and how to evaluate both the LNA and the
downconverter mixer.
________________________________________________________________
Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
MAX2685 Evaluation Kit
1)
2)
3)
4)
5)
6)
7)
8)
9)
Low-Noise Amplifier (LNA)
Connect the
SHDN
jumper (JU2) on the EV kit to the
“ON” position,
SHDN
= V
CC
. This enables the
MAX2685.
Connect the GAIN jumper (JU1) on the EV kit to the
“HI” position, GAIN = V
CC
. This places the LNA in
high-gain mode.
Connect a 3V DC supply (through an ammeter, if
desired) to the V
CC
and GND terminals on the EV kit.
Connect one RF signal generator to the LNAIN SMA
connector. Do not turn on the generator’s output. Set
the generator for an output frequency of 880MHz
and a power level of -30dBm.
Connect a spectrum analyzer to the LNAOUT SMA
connector on the EV kit. Set the spectrum analyzer
to a center frequency of 880MHz, a total span of
200MHz, and a reference level of 0dBm.
Turn on the DC supply. The supply current should
read approximately 8.5mA (if using an ammeter).
Activate the RF generator’s output. An 880MHz sig-
nal should show on the spectrum analyzer’s display
and should indicate a typical gain of 15dB after
accounting for cable losses.
Test the MAX2685’s low gain mode by changing the
output power on the signal generator to -10dBm and
setting the GAIN jumper (JU1) on the EV kit to the
“LO” position, GAIN = GND. The supply current
should read approximately 4mA (if using an amme-
ter). An 880MHz signal should show on the spectrum
analyzer’s display and should indicate an attenuation
of 12dB after accounting for cable losses.
If desired, test the shutdown feature by moving the
SHDN
jumper (JU1) into the “OFF” position. This dis-
ables the part and reduces supply current below
1.0µA.
Evaluates: MAX2685
4) Connect the other RF signal generator (with the out-
put disabled) to the MIXIN SMA connector. Set the
frequency to 880MHz and the amplitude to -25dBm.
5) Connect the spectrum analyzer to the IFOUT+ SMA
connector. Set the spectrum analyzer to an 80MHz
center frequency, a 1MHz span, and a 0dBm refer-
ence level.
6) Turn on the LO and RF signal generators.
7) An 80MHz signal should show on the spectrum ana-
lyzer’s display and should indicate a typical gain of
6.1dB after accounting for cable losses.
8) If desired, connect the GAIN jumper (JU1) on the EV
kit to the “LO” position, GAIN = GND. This places
the mixer in low-gain mode. An 80MHz signal should
appear on the spectrum analyzer’s display and
should indicate a typical gain of 4.6dB after
accounting for cable losses.
Detailed Description
The MAX2685 EV kit’s circuitry is described in this sec-
tion. For more detailed information about the operation of
the device itself, refer to the MAX2685 data sheet.
Low-Noise Amplifier
The LNA circuitry consists of matching networks and
DC-blocking capacitors at the input and output.
Downconverter Mixer RF Input
The downconverter mixer’s RF input, MIXIN, requires a
simple matching network. C10 provides DC blocking,
and C9 is used to match the input to 50Ω at 880MHz.
Downconverter Mixer LO Input
The downconverter mixer’s LO input is terminated with
a 75Ω resistor, R3, at the SMA input. C8 provides DC
blocking.
Downconverter Mixer
1) Connect the GAIN jumper (JU1) on the EV kit to the
“HI” position, GAIN = V
CC
. This places the mixer in
high-gain mode.
2) Remove the RF signal generator and spectrum ana-
lyzer from the LNAIN and LNAOUT connections. The
DC supply connections needed for testing the mixer
are the same as in the LNA section.
3) Connect one RF signal generator (with the output
disabled) to the LO SMA connector. Set the frequen-
cy to 960MHz and the output power to -8dBm.
Downconverter Mixer
Differential IF Output
The mixer output can be evaluated in either a single-
ended or differential configuration. The MAX2685 EV kit
is shipped with a differential to single-ended converter
for single-ended testing. See the
IF Output Configuration
section.
2
_______________________________________________________________________________________
MAX2685 Evaluation Kit
Gain Control and LNA Bypass Switch
Jumper JU1 controls the LNA bypass switch and the
overall gain of the MAX2685. Enable the LNA and
switch the device to high-gain mode by placing the
GAIN jumper (JU1) into the “HI” position, GAIN = V
CC
.
Bypass the LNA and reduce the overall gain by placing
the GAIN jumper (JU1) into the “LO” position, GAIN =
GND.
the 50Ω circle at the desired IF frequency, 80MHz.
Choose the final value of the series capacitor, C13, to
transform the impedance to the center of the Smith
chart. This is a narrowband match. To reduce the Q of
this match and make it more broadband, add R4. This
is required to make the match manufacturable consid-
ering variations in component tolerance. In initially
choosing the component values for the resonant net-
work (L2, C11, and C18), note that increasing the value
of L2 makes the match more broadband. If a higher
impedance output is desired to interface with a filter,
C13 may not be required and you may set the output
impedance with R4.
For a differential output, footprints for L4 and C15 are
available on the EV board to establish a differential out-
put on the board edge. In addition, footprints labeled
as R5–R8 are available for other experimentation.
Evaluates: MAX2685
Shutdown
Jumper JU2 controls the IC’s operating modes. Enable
the MAX2685 by connecting the
SHDN
pin to V
CC
. Do
this by placing the shunt across pins 1 and 2 of jumper
JU2. Shut down the MAX2685 by connecting the
SHDN
pin to GND. Do this by placing the shunt across pins 2
and 3 of jumper JU2.
IF Output Configuration
The EV board contains a circuit that converts the
MAX2685 mixer’s differential output to a single-ended
output. This is done with components L2, L3, C11, C13,
C18, and R4. The resonant network composed of L2,
C11, and C18 transforms the IFOUT- signal by 180° at
IFOUT+. L3 is primarily a choke.
The 50Ω match on the EV board was obtained by initial-
ly using a large value for C13 so it does not affect the
impedance. Using a network analyzer calibrated
between 70MHz and 90MHz, monitor S22 of the IF out-
put. It is a high-resistance output that swings near the
high-impedance end of the Smith chart. Adjust L2, C11,
and C18 so that the network analyzer sweep crosses
PC Board Layout
Good PC board layout is an essential aspect of RF cir-
cuit design. The EV kit’s PC board can serve as a guide
for laying out a board using the MAX2685.
Each V
CC
node on the PC board has its own decou-
pling capacitor. This minimizes supply coupling from
one section of the MAX2685 to another. A star topology
for the supply layout, in which each V
CC
node on the
MAX2685 circuit has a separate connection to a central
V
CC
node, can further minimize coupling between the
LNA and mixer sections of the MAX2685.
_______________________________________________________________________________________
3
MAX2685 Evaluation Kit
Evaluates: MAX2685
C1
0.1µF
C2
OPEN
R1
1k
VCC
1
2
JU1
GAIN
3
C3
0.1µF
L1
12nH
1
16
C7
330pF
J5
LNAOUT
J1
LNAIN
GND
LNAOUT
2
3
MAX2685
15
V
CC
LNAIN
GND
GND
14
C8
47pF
C10
330pF
13
C9
3.3pF
VCC
4
J6
MIXIN
VCC
C12
1000pF
L3
820nH
C18
6pF
C14
OPEN
L4
OPEN
J8
IFOUT-
C15
OPEN
R7
OPEN
R8
OPEN
C13
12pF
R5
0Ω
R6
OPEN
GAIN
MIXIN
VCC
5
C4
47pF
R2
1k
6
V
CC
SHDN
GND
12
11
10
L2
680nH
R4
15k
GND
IFOUT+
7
LO
C5
0.1µF
J7
IFOUT+
VCC
1
2
JU2
SHDN
3
8
GND
IFOUT-
9
C11
6pF
VCC
J4
LO
R3
75Ω
J9
VCC
1
J10
C16
2 10µF
C17
0.1µF
C6
330pF
Figure 1. MAX2685 EV Kit Schematic
4
_______________________________________________________________________________________
MAX2685 Evaluation Kit
Evaluates: MAX2685
1.0"
Figure 2. MAX2685 EV Kit Component Placement Guide—
Component Side
1.0"
Figure 3. MAX2685 EV Kit Pad Placement
1.0"
Figure 4. MAX2685 EV Kit PC Board Layout—Component Side
_______________________________________________________________________________________
5

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