AD8342

Rev. 0 | Page 16 of 20

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, dif-

ferent load impedances may be used where circumstances dic-

tate. In general, lower load impedances result in lower conver-

sion gain and lower output P1dB. Higher load impedances

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 unin-

tended 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 out-

put 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 differen-

tial amplifier is used to drive a pair of 75 Ω transmission lines.

The gain of the buffer can be independently set by appropriate

50

0.5

0

0

1G

FREQUENCY (Hz)

45

0.4

40

0.3

35

0.2

30

0.1

25

0

20

–0.1

15

–0.2

10

5

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

Figure 44. IF Port Impedance

COMM

8

IFOP

7

COMM

5

AD8342

AD8351

+

–

RFC

RFC

100

Ω

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

The high input impedance of the AD8351 allows for 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 trans-

former. The turns ratio of the transformer should be selected to

provide the desired impedance transformation. In the case of a

50 Ω load impedance, a 2-to-1 impedance ratio transformer

should be used to transform the 50 Ω load into a 100 Ω differ-

ential load at the IF output pins. Figure 48 illustrates a differen-

tial 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

should 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 needs to be at least 45 mA. The self-

resonant frequency of the selected choke should be higher than

the intended IF frequency. A variety of suitable choke inductors

are 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 Text Books,

Second Edition, 1997.

• Guillermo Gonzalez, Microwave Transistor Amplifiers:

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

0

180

30

330

50MHz

50MHz

500MHz

500MHz

60

90

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)