AD8342ACPZ-R2 datasheet(17/25 Pages) AD

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Data Sheet

AD8342

Rev. C | Page 17 of 25

IF PORT

The IF port comprises open-collector differential outputs. The

NPN open-collectors can be modeled as current sources that

are shunted with resistances of ~10 kΩ in parallel with

capacitances of ~1 pF.

The specified performance numbers for the AD8342 were

measured with 100 Ω differential terminations. However,

different load impedances can be used where circumstances

dictate. In general, lower load impedances result in lower

conversion gain and lower output P1dB. Higher load imped-

ances result in higher conversion gain for small signals, but

lower IP3 values for both input and output.

If the IF signal is to be delivered to a remote load, more than a

few millimeters away at high output frequencies, avoid unintended

parasitic effects due to the intervening PCB traces. One approach is

to use an impedance transforming network or transformer located

close to the AD8342. If very wideband output is desired, a nearby

buffer amplifier may be a better choice, especially if IF response

to dc is required. An example of such a circuit is presented in

Figure 45, in which the AD8351 differential amplifier is used to

drive a pair of 75 Ω transmission lines. The gain of the buffer

can be independently set by appropriate choice of the value for

the gain resistor, RG.

0.5

FREQUENCY (Hz)

0.4

0.3

0.2

0.1

–0.1

–0.2

100M 200M 300M 400M 500M 600M 700M 800M 900M

Figure 44. IF Port Impedance

COMM

IFOP 7

IFOM 6

COMM

AD8342

AD8351

–

RFC

+VS

RFC

ZL = 100Ω

+VS

100Ω

Tx LINE ZO = 75Ω

Figure 45. AD8351 Used as Transmission Line Driver and Impedance Buffer

The high input impedance of the AD8351 allows a shunt

differential termination to provide the desired 100 Ω load to the

AD8342 IF output port.

It is necessary to bias the open-collector outputs using one of

the schemes presented in Figure 47 and Figure 48. Figure 47

illustrates the application of a center-tapped impedance transform-

er. The turn ratio of the transformer must be selected to provide

the desired impedance transformation. In the case of a 50 Ω

load impedance, use a 2-to-1 impedance ratio transformer to

transform the 50 Ω load into a 100 Ω differential load at the IF

output pins. Figure 48 illustrates a differential IF interface where

pull-up choke inductors are used to bias the open-collector outputs.

The shunting impedance of the choke inductors used to couple

dc current into the mixer core must be large enough at the IF

operating frequency so it does not load down the output current

before reaching the intended load. Additionally, the dc current

handling capability of the selected choke inductors must be at

least 45 mA. The self-resonant frequency of the selected choke

must be higher than the intended IF frequency. A variety of suitable

choke inductors is commercially available from manufacturers

such as Murata and Coilcraft®. Figure 46 shows the loading effects

when using nonideal inductors. An impedance transforming

network may be required to transform the final load impedance

to 100 Ω at the IF outputs. There are several good reference

books that explain general impedance matching procedures,

including:

•

Chris Bowick, RF Circuit Design, Newnes, Reprint Edition,

1997.

•

David M. Pozar, Microwave Engineering, Wiley,

Third Edition, 2004.

•

Guillermo Gonzalez, Microwave Transistor Amplifiers:

Analysis and Design, Prentice Hall, Second Edition, 1996.

180

330

50MHz

500MHz

270

300

120

240

150

210

REAL

CHOKES

IDEAL

CHOKES

Figure 46. IF Port Loading Effects Due to Finite Q Pull-Up Inductors

(Murata BLM18HD601SN1D Chokes)

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