Preliminary Technical Data

ADL5350

PrC | Page 19 of 24

2.

Tune the LO port input network for optimum return loss.

Typically, a bandpass network is used to pass the LO signal to

the LOIN pin. It is desirable to block high frequency harmonics

of the LO from the mixer core. LO harmonics cause higher RF

frequency images to be down converted to the desired IF

frequency, and result in a sensitivity degradation. If the

intended LO source has poor harmonic distortion and spectral

purity, it may be necessary to employ a higher order bandpass

filter network. Figure 58 illustrates a simple L-C bandpass 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) presents a rather high input impedance alone.

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 shunt resistor to ground before the coupling capacitor (C3)

to present a lower loading impedance to the LO source .

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

cancel, 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 accommodate for the board parasitics, finite Q,

and self-resonant frequencies of the LC components when

designing the RF, IF, and LO filter networks. Table 9 provides

suggested values for initial prototyping.

Table 9. Suggested RF, IF, and LO Filter Networks for Low-Side LO Injection

RF Frequency

C1

L2

C2

L3

C3

450 MHz

8.3 nH

10 pF

10 nH

10 pF

10 nH

100 pF

850 MHz

6.8 nH

4.7 pF

4.7 nH

5.6 pF

8.2 nH

100 pF

1950 MHz

1.7 nH

1.5 pF

1.7 nH

1.2 pF

3.5 nH

100 pF

2400 MHz

0.67 nH

1 pF

1.5 nH

0.7pF

3.0 nH

100 pF

NF performance.