AD8342

Rev. 0 | Page 15 of 20

AC INTERFACES

The AD8342 is designed to downconvert radio frequencies (RF)

to lower intermediate frequencies (IF) using a high or low-side

local oscillator (LO). The LO is injected into the mixer core at a

frequency higher or lower than the desired input RF. The

to the desired RF signal, an RF image is downconverted to the

driven with a high side LO . When using a broadband load, the

conversion gain of the AD8342 is nearly constant over the

specified RF input band (see Figure 3).

The AD8342 is designed to operate over a broad frequency

range. It is essential to ac-couple RF and LO ports to prevent dc

offsets from skewing the mixer core in an asymmetrical man-

ner, potentially degrading noise figure and linearity.

The RF input of the AD8342 is high impedance, 1 kΩ across the

frequency range shown in Figure 41. The input capacitance

decreases with frequency due to package parasitics.

2.00

1.00

00

0

1G

FREQUENCY (Hz)

1.75

1.50

0.75

1.25

1.00

0.50

0.75

0.50

0.25

0.25

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

Figure 41. RF Input Impedance

The matching or termination used at the RF input of the

AD8342 has a direct effect on its dynamic range. The charac-

terization circuit, as well as the evaluation board, uses a 100 Ω

resistor to terminate the RF port. This termination resistor in

shunt with the input stage results in a return loss of better than

−10 dBm (relative to 50 Ω). Table 4 shows gain, IP3, P1dB, and

noise figure for four different input networks. This data was

measured at an RF frequency of 250 MHz and at an LO

frequency of 300 MHz.

Table 4. Dynamic Performance for Various Input Networks

Input

Network

50 Ω

Shunt

100 Ω

Shunt

500 Ω

Shunt

Matched

(Fig. 40)

Gain (dB)

0.66

3.5

5.3

9.3

IIP3 (dBm)

25.4

22.9

20. 6

18.5

P1dB (dBm)

10.8

8.4

6.3

2.3

NF (dB)

14

12.5

10.2

10.5

The RF port can also be matched using an LC circuit, as shown

in Figure 42.

1k

Ω

50

Ω

3.6pF

100nH

(1000 + j0)

Ω

Figure 42. Matching Circuit

Impedance transformations of greater than 10:1 result in a

higher Q circuit and thus a narrow RF input bandwidth. A 1 kΩ

resistor is placed across the RF input of the device in parallel

with the device internal input impedance, creating a 500 Ω load.

This impedance is matched to as close as possible to 50 Ω for

the source, with standard components using a shunt C, series L

matching circuit (see Figure 43).

25.0

10.0

10.0

25.0

50.0

100.0

200.0

500.0

500.0

200.0

100.0

50.0

Q = 3.0

1

2

3

4

Point 1(1000.0 + j0.0)Ω Q = 0.0 at 250.000 MHz

Point 2(500.0 + j0.0)Ω Q = 0.0 at 250.000 MHz

Point 3(55.6 − j157.2)Ω Q = 2.8 at 250.000 MHz

Point 4(55.6 − j0.1)Ω Q = 0.0 at 250.000 MHz

Figure 43. LC Matching Example