AD8342

Rev. 0 | Page 17 of 20

AD8342

IF OUT

2:1

Figure 47. Biasing the IF Port Open-Collector Outputs

Using a Center-Tapped Impedance Transformer

COMM

8

IFOP

7

COMM

5

AD8342

RFC

RFC

IF OUT+

IF OUT–

IMPEDANCE

TRANSFORMING

NETWORK

Figure 48. Biasing the IF Port Open-Collector Outputs

Using Pull-Up Choke Inductors

The AD8342 is optimized for driving a 100 Ω load. Although

the device is capable of driving a wide variety of loads, to main-

tain optimum distortion and noise performance, it is advised

that the presented load at the IF outputs is close to 100 Ω. The

linear differential voltage conversion gain of the mixer can be

modeled as

LOAD

m

R

G

Av

×

=

where:

e

m

m

m

R

g

g

G

+

=

1

1

π

tion and dynamic range of the AD8342. Because the AD8342

has internal resistive degeneration, the conversion gain is pri-

marily determined by the load impedance and the on-chip

degeneration resistors. Figure 49 shows how gain varies with IF

most direct way to affect conversion gain is by varying the load

impedance. Small loads result in lower gains while larger loads

increase the conversion gain. If the IF load impedance is too

large it causes a decrease in linearity (P1dB, IP3). In order to

maintain positive conversion gain and preserve SFDR perform-

ance, the differential load presented at the IF port should

remain in the range of ~ 100 Ω to 250 Ω.

30

0

10

1000

IF LOAD (

Ω)

100

25

20

15

10

5

MEASURED

MODELED

Figure 49. Voltage Conversion Gain vs. IF Loading

LO CONSIDERATIONS

The LOIN port provides a 50 Ω load impedance with common-

mode decoupling on LOCM. Again, common-grade ceramic

capacitors provide sufficient signal coupling and bypassing of

the LO interface.

The LO signal needs to have adequate phase noise characteris-

tics and low second-harmonic content to prevent degradation

of the noise figure performance of the AD8342. An LO plagued

with poor phase noise can result in reciprocal mixing, a mecha-

nism that causes spectral spreading of the downconverted sig-

nal, limiting the sensitivity of the mixer at frequencies close-in

to any large input signals. The internal LO buffer provides

enough gain to hard-limit the input LO and provide fast switch-

ing of the mixer core. Odd harmonic content present on the LO

drive signal should not impact mixer performance; however,

even-order harmonics cause the mixer core to commutate in an

unbalanced manner, potentially degrading noise performance.

Simple lumped element low-pass filtering can be applied to help

reject the harmonic content of a given local oscillator, as shown

in Figure 50. The filter depicted is a common 3-pole Chebyshev,

designed to maintain a 1-to-1 source-to-load impedance ratio

with no more than 0.5 dB of ripple in the pass band. Other filter

structures can be effective as long as the second harmonic of the

LO is filtered to negligible levels, for example, ~30 dB below the

fundamental.

AD8342

LOIN

3

COMM

4

LOCM

2

C1

C3

LO

SOURCE

L2

C1 =

1.864

2

L

C3 =

1.834

2

L

L2 =

2

Figure 50. Using a Low-Pass Filter to Reduce LO Second Harmonic