AD8342ACPZ-R2 datasheet(19/25 Pages) AD

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Data Sheet

AD8342

Rev. C | Page 19 of 25

HIGH FREQUENCY APPLICATIONS

The AD8342 is a broadband mixer capable of both up and

down conversion. Unlike other mixers that rely on on-chip

reactive circuitry to optimize performance over a specific band,

the AD8342 is a versatile general-purpose device that can be

used from arbitrarily low frequencies to several GHz. In general,

the following considerations help ensure optimum performance:

•

Minimize the ac loading impedance of IF port bias network.

•

Maximize the power transfer to the desired ac load.

•

For the maximum conversion gain and the lowest noise

performance, reactively match the input as described in the

IF Port section.

•

For the maximum input compression point and input

intercept points, resistively terminate the input as described

in the IF Port section.

As an example, Figure 51 shows the AD8342 as an up-

converting mixer for a W-CDMA single-carrier transmitter

design. For this application, it was desirable to achieve −65 dBc

adjacent channel power ratio (ACPR) at a −13 dBm output

power level. The ACPR is a measure of both distortion and

noise carried into an adjacent frequency channel due to the

finite intercept points and noise figure of an active device.

COMM

IFOP

IFOM

COMM

RFCM

RFIN

VPMX

VPDC

PWDN

EXRB

COMM

VPLO

LOCM

LOIN

COMM

AD8342

VPOS

34nH

100pF

1nF

ETC1-1-13

100pF

0.1µF

VPOS

1nF

1970MHz

OSC

1.82k

Ω

100pF

4.7pF

170MHz

INPUT

100nH

1nF

499

Ω

VPOS

100pF

0.1pF

2140MHz OUT

1nF

Figure 51. W-CDMA Tx Upconversion Application Circuit

Because a W-CDMA channel encompasses a bandwidth of

almost 5 MHz, it is necessary to keep the Q of the matching

circuit low enough so that phase and magnitude variations are

below an acceptable level over the 5 MHz band. It is possible to

use purely reactive matching to transform a 50 Ω source to match

the raw ~1 kΩ input impedance of the AD8342. However, the L

and C component variations can present production concerns

due to the sensitivity of the match. For this application, it is

advantageous to reduce the ~1 kΩ input impedance using an

external shunt termination resistor to allow a lower Q reactive

matching network. In Figure 52, the input is terminated across

the RFIN and RFCM pins using a 499 Ω termination. The

termination must be as close to the device as possible to

minimize standing wave concerns. The RFCM is bypassed to

ground using a 1 nF capacitor. A dc blocking capacitor of 1 nF

is used to isolate the dc input voltage present on the RFIN pin

from the source. A step-up impedance transformation is

realized using a series L shunt C reactive network. The actual

values used must accommodate for the series L and stray C

parasitics of the connecting transmission line segments. When

using the customer evaluation board with the components

specified in Figure 51, the return loss over a 5 MHz band

centered at 170 MHz was better than 10 dB.

External pull-up choke inductors feed dc bias into the open-

collector outputs. It is desirable to select pull-up choke

inductors that present high loading reactance at the output

frequency. Coilcraft 0302CS series inductors were selected due

to their very high self resonant frequency and Q. A 1:1 balun

was ac-coupled to the output to convert the differential output

to a single-ended signal and present the output with a 50 Ω ac

loading impedance.

The performance of the circuit is shown in Figure 52. The average

ACPR of the adjacent and alternate channels is presented vs.

output power. The circuit provides a 65 dBc ACPR at −13 dBm

output power. The optimum ACPR power level can be shifted

to the right or left by adjusting the output loading and the loss

of the input match.

–60

–70

–25

OUTPUT POWER (dBm)

–62

–64

–66

–68

–20

–15

–10

–5

ADJACENT

CHANNELS

ALTERNATE

CHANNELS

Figure 52. Single Carrier W-CDMA ACPR Performance of Tx

Upconversion Circuit (Test Model 1_64)

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