For applications that require a wide tuning range and
low phase noise, a series-coupled resonant tank may
be required as shown in Figure 5. This tank will use the
package inductance in series with inductors L1, L2,
and capacitance of varactor D1 to set the net equiva-
lent inductance which resonates in parallel with the
internal oscillator capacitance. Inductors L1 and L2
may be implemented as microstrip inductors, saving
component cost. Bias is provided to the tank port
through chokes L3 and L5. R1 and R3 should be cho-
sen large enough to de-Q the parasitic resonance due
to L3 and L5 but small enough to minimize the voltage
drop across them due to bias current. Values for R1
and R3 should be kept between 0 and 50Ω. Proper
high frequency bypassing (C1) should be used for the
bias voltage to eliminate power supply noise from
entering the tank.
Oscillator Tank PC Board Layout
The parasitic PC board capacitance, as well as PCB
trace inductance and package inductance, affect oscil-
lation 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
reduces parasitic capacitance.
Using an External Oscillator
If an external 50Ω 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 6). The oscillator signal
is AC coupled into the TANK pin and should have a
level of about 0dBm from a 50Ω source. For proper
biasing of the oscillator input stage, pull up the TANK
and TANK pins to the V
CC
supply via 50Ω resistors.
If a differential LO source such as the MAX2620 is
available, AC-couple the inverting output into TANK.
MAX2424/MAX2426
900MHz Image-Reject Receiver
with Transmit Mixer
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