MAX2 6 2 0 Eva lu a t io n Kit
Evluates:MAX620
out. Use the test port provided on the MAX2620 EV kit
to facilitate measurement by installing a 1000pF capac-
itor at C2 and removing C5. (Remove C2 and install C5
to use the MAX2620 as an oscillator.) When using the
test port, subtract an approximately 586ps electrical
delay from the S11 measurement (this delay can be
compensated for on most modern vector network ana-
lyzers) to account for the delay of the transmission line
from the test port to the MAX2620 TANK pin. The test
port should provide a negative input resistance and
thus return gain when S11 is measured on a vector net-
work analyzer. This return gain provides measurement
data that is outside the unit circle of the Smith chart.
The MAX2620 EV kit uses a low-voltage varactor. With
the coupling capacitor C17 kept small, the oscillator cir-
cuit is less affected by losses in the varactor. However,
keeping C17 small also reduces overall tuning range.
L1 on the MAX2620 is a ceramic coaxial resonator,
which provides the best phase-noise performance. For
cost-sensitive applications, the layout for L1 on the
MAX2620 EV kit is a dual pad that accepts either a
spring coil or a ceramic coaxial resonator. When prop-
erly specified, coaxial resonators provide tight toler-
ance inductance at very high Q for best circuit perfor-
mance. Spring coils, such as Coilcraft mini-spring coils,
provide a good cost/performance compromise for cost-
sensitive applications.
A useful technique is to configure the vector network ana-
lyzer to display 1/S11 for this measurement. The vector
network analyzer displays the information inside the unit
circle of the Smith chart. Most modern vector network
analyzers perform this conversion. Input-impedance data
presented in this format (1/S11) is the complement of the
input impedance, which is the impedance desired to pro-
vide the MAX2620 with feedback to oscillate at a particu-
lar frequency. The Typical Operating Characteristics sec-
tion of the MAX2620 data sheet contains a plot of 1/S11
for s p e c ific va lue s of C3 a nd C4 p rovid e d in the
MAX2620 EV kit. Also refer to the Tank Circuit Design
section in the MAX2620 data sheet.
_____________La yo u t Co n s id e ra t io n s
The MAX2620 EV kit can serve as a guide for your
board layout. To minimize the effects of parasitic ele-
ments, which may alter circuit performance, remove the
ground plane around and under the components that
make up the resonant circuit (C3–C6, C17, D1, and L1).
Keep PC board trace lengths as short as possible to
minimize parasitic inductance. Also keep decoupling
capacitors C1, C7, and C9 as close to the MAX2620 as
possible, with direct connection to the ground plane.
VCC
VCC
C8
1000pF
R1
10Ω
L3
10nH
R4
OPEN
C13
1.5pF
50Ω
SMA
C1
1000pF
1
C14
OPEN
OUT
8
U1
VCC1
OUT
C2
OPEN
J1 50Ω
MAX2620
7
6
5
2
3
TEST
PORT
SMA
VCC
TANK
FDBK
VCC2
GND
OUT
C9
1000pF
C5
1.5pF
C3
2.7pF
C12
1000pF
L4
OPEN
R5
51Ω
SMA
C4
1pF
50Ω
R2
1k
VCONT
C17
1.5pF
4
JU1
D1
L1
SMA
SHDN
C6
C11
OPEN
C10
1000pF
OUT
1pF
C7
1000pF
R3
10Ω
VCC
VCC
GND
SHDN
1
2
3
C15
10µF
25V
VCC
Figure 1. MAX2620 EV Kit Schematic
_______________________________________________________________________________________
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