AD8342ACPZ-R2 datasheet(16/25 Pages) AD

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AD8342

Data Sheet

Rev. C | Page 16 of 25

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

frequency difference between the LO and the RF, fLO − fRF (high

side) or fRF − fLO (low side), is the intermediate frequency, fIF. In

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

to the desired IF frequency. The image frequency is at fLO + fIF

when 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

manner, 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

FREQUENCY (Hz)

1.75

1.50

0.75

1.25

1.00

0.50

0.75

0.50

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 characterization

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 6 shows gain, IP3, P1dB, and noise figure (NF) 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 6. Dynamic Performance for Various Input Networks

Input

Network

50 Ω

Shunt

100 Ω

Shunt

500 Ω

Shunt

Matched (see

Figure 42)

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)

12.5

10.2

10.5

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

in Figure 42.

1kΩ

ZO = 50Ω

fMAIN = 250MHz

50Ω

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).

100

200

500

200

100

Q = 3

POINT 1(1000 + j0) Ω

POINT 2(500 + j0) Ω

POINT 3(55.6 – j157.2) Ω

POINT 4(55.6 – j0.1) Ω

Figure 43. LC Matching Example

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