Rainbow Electronics MAX2442 - Manual (Page 11)

Oscillator-Tank PC Board Layout
The parasitic PC board capacitance, as well as PCB
trace inductance and package inductance, can affect
oscillation frequency, so be careful in laying out the PC
board for the oscillator tank. Keep the tank layout as
symmetrical, tightly packed, and close to the device as
possible to minimize LO feedthrough. When using a PC
board with a ground plane, a cut-out in the ground
plane (and any other planes) below the oscillator tank
will reduce parasitic capacitance.
Using an External Oscillator
If an external 50
Ω LO signal source is available, it can
be used as an input to the TANK or TANK pin in place
of the on-chip oscillator (Figure 3). The oscillator signal
is AC coupled into the TANK pin and should have a
level of about 0dBm from a 50
Ω source. For proper
biasing of the oscillator input stage, TANK and TANK
must be pulled up to the V
CC supply via 50 Ω resistors.
If a differential LO source such as the MAX2620 is
available, AC couple the inverting output into TANK.
MAX2440/MAX2441/MAX2442
900MHz Image-Reject Receivers
______________________________________________________________________________________ 11
MAX2440/MAX2441/MAX2442
MAX2440
MAX2441
MAX2442
TANK
50 Ω
50 Ω
EXT LO
EXTERNAL LO LEVEL IS
0dBm FROM A 50
Ω
SOURCE.
V
CC
C BLOCK
0.01 µF
V CC
TANK
Figure 3. Using an External Local Oscillator
TRANSISTOR COUNT: 2802
MAX2440
MAX2441
MAX2442
TANK L1
L2
L3
L4
L5
R1
R2
R3
Ci
1/2 D1
1/2 D1
C1
V CC
VTUNE
TANK
L
T
L T
C2
Figure 4. Series Coupled Resonant Tank for Wide Tuning Range and Low Phase Noise
Chip Information