ADL5350

Rev. 0 | Page 18 of 24

2. Tune the LO port input network for optimum return

loss. Typically, a band-pass network is used to pass the

LO signal to the LOIN pin. It is recommended to block

high frequency harmonics of the LO from the mixer

core. LO harmonics cause higher RF frequency images

to be downconverted to the desired IF frequency and

result in sensitivity degradation. If the intended LO

source has poor harmonic distortion and spectral purity,

it may be necessary to employ a higher order band-pass

filter network. Figure 58 illustrates a simple LC band-

pass filter used to pass the fundamental frequency of the

LO source. Capacitor C3 is a simple dc block, while the

Series Inductor L3, along with the gate-to-source

capacitance of the buffer amplifier, form a low-pass

network. The native gate input of the LO buffer (FET)

alone presents a rather high input impedance. The gate

bias is generated internally using feedback that can result

in a positive return loss at the intended LO frequency.

If a better than −10 dB return loss is desired, it may be

necessary to add a shunt resistor to ground before the

coupling capacitor (C3) to present a lower loading

impedance to the LO source. In doing so, a slightly

greater LO drive level may be required.

3. Design the RF and IF filter networks. Figure 58 depicts

simple LC tank filter networks for the IF and RF port

interfaces. The RF port LC network is designed to pass

the RF input signal. The series LC tank has a resonant

frequency at 1/(2π√LC). At resonance, the series reactances

are canceled, which presents a series short to the RF

signal. A parallel LC tank is used on the IF port to reject

the RF and LO signals. At resonance, the parallel LC tank

presents an open circuit.

It is necessary to account for the board parasitics, finite

Q, and self-resonant frequencies of the LC components

when designing the RF, IF, and LO filter networks. Table 8

provides suggested values for initial prototyping.

Table 8. Suggested RF, IF, and LO Filter Networks for Low-Side LO Injection

RF Frequency (MHz)

L1 (nH)1

C1 (pF)

L2 (nH)

C2 (pF)

L3 (nH)

C3 (pF)

450

8.3

10

10

10

10

100

850

6.8

4.7

4.7

5.6

8.2

100

1950

1.7

1.5

1.7

1.2

3.5

100

2400

0.67

1

1.5

0.7

3.0

100