ADRF6620ACPZ-R7 datasheet(35/52 Pages) AD

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Data Sheet

ADRF6620

Rev. 0 | Page 35 of 52

INTERSTAGE FILTERING REQUIREMENTS

Filtering at the mixer output may be necessary for improved

linearity performance. For applications where the frequency plan

requires low RF frequency inputs and IF outputs, the resulting sum

term at the mixer outputs, fRF + fLO, may fall within the band of

interest. The unwanted sum term may cause the IF DGA to

operate in its nonlinear region because of the unnecessary presence

of additional signal power. As a result, the linearity performance

degrades where OIP3 and OIP2 decrease substantially. For this

reason, a low-pass filter is necessary to attenuate the unwanted

signal while maintaining the integrity of the wanted signal within

the band of interest. In addition, the low-pass filter serves to

suppress the LO feedthrough. Because of the absence of blockers

in a typical DPD receive application, a lower order filter, such as

a third-order Chebyshev, is typically adequate.

The low-pass filter resides between the mixer outputs and the IF

DGA inputs, as shown in Figure 85. The signal flow starts with the

differential outputs of the mixer being dc biased to positive

supply (5 V) via a pair of pull-up inductors, L1 and L2. The

inductor value is determined by the low frequency cutoff of the

signal band of interest. Next, the third-order low-pass filter

attenuates the high frequency sum term. The combination of

the pull-up inductors and the low-pass filter results in a band-

pass filter profile. The outputs of the filter are then ac-coupled

through series capacitors and routed to the on-chip IF DGA via

the IFIN+ and IFIN− pins.

MXOUT+

MXOUT–

IFIN+

IFIN–

IFOUT1–

IFOUT1+

IFOUT2–

IFOUT2+

+5V

0.1µF

Figure 85. Low-Pass IF Filter

When designing the low-pass filter, it is important to consider

the output impedance of the mixer and the input impedance of

the IF DGA. The output impedance of the mixer has both a real

and reactive component, and its equivalent model is shown in

Figure 86. Correspondingly, Figure 87 shows the impedance vs.

frequency for the mixer output.

MXOUT+

MXOUT–

1.1pF

2.5pF

82.5Ω

90Ω

82.5Ω

Figure 86. Equivalent Model of the Mixer Output Impedance

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500

FREQUENCY (MHz)

600

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900

1000

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190

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290

PARALLEL CAPACITANCE

PARALLEL RESISTANCE

Figure 87. Mixer Output Impedance vs. Frequency

Likewise, Figure 88 shows the impedance vs. frequency for the

IF DGA. The four-port S parameter files for the IF DGA and

mixer are available on analog.com and can serve as a useful tool

to accurately capture the input and output impedance when

designing the interstage filter. As a first-order approximation

at low frequencies, the mixer output has a fixed impedance of

approximately 255 Ω, and the input impedance of the IF DAG is

approximately 150 Ω. Therefore, design the low-pass filter to

have an input impedance of 255 Ω and an output impedance of

150 Ω.

100

150

200

250

300

350

400

450

500

100

200

300

400

500

600

700

800

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1000

FREQUENCY (MHz)

OUTPUT CAPACITANCE

INPUT CAPACITANCE

OUTPUT RESISTANCE

INPUT RESISTANCE

PARALLEL CAPACITANCE

PARALLEL RESISTANCE

Figure 88. IF DGA Input/Output Impedance vs. Frequency

ADRF6620ACPZ-R7 Datasheet(PDF) 35 Page - Analog Devices

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