REV. 0

AD9870

–12–

IF1 LNA/MIXER

The AD9870 contains a single-ended LNA followed by “Gilbert-

type” active mixer as shown in Figure 7a. The mixer’s differential

LO port is driven by an LO buffer stage which can be driven

single-ended or differential. The LO signal level can range from

0.3 V p-p to 1.0 V p-p with negligible effect on performance.

The input impedance at the IFIN pin is 360

Ω 2 pF (±20%)

and has no significant variation with respect to the programmable

bias settings. Figure 7b. shows the S11 parameters of the AD9870

with the following LNA/Mixer bias setting: LNAB = 3, MIXB = 3.

LNA GAIN = 15 dB

180

180

MIXER GAIN = –5 dB

LO INPUT =

0.3 TO 1.0 V p–p

DC SERVO

LOOP

MULTI-TANH

V–I STAGE

VCML

Figure 7a. Simplified Schematic of AD9870’s LNA/Mixer

10

25

50

100

200 400

J10

–J10

J25

–J25

J50

–J50

J100

–J100

J200

–J200

J400

–J400

0

323–J105

AT 73.35 MHz

255–J163

AT 140 MHz

166–J177

AT 240 MHz

Figure 7b. Input Impedance (i.e. S11) of the AD9870’s

IF1 Input

Both the LNA and mixer have four programmable bias settings

so that current consumption can be minimized for a given appli-

cation. Figures 7c, 7d, and 7e show how the LNA and mixer’s

noise figure (NF), linearity (IIP3), conversion gain, current

consumption and frequency response are all affected for a given

LNA/Mixer bias setting. The measurements were taken at an IF

= 73.35 MHz, an LO = 71.1 MHz, and supplies set to 3.0 V.

Note, since the current consumption of the LNA/Mixer portion

of the IC can be reduced by only 5 mA at most relative to the

nominal current consumption of the entire IC in the high IIP3

mode (i.e., 42 mA), most applications will benefit with the

AD9870’s LNA/Mixer configured for the high bias mode (i.e.,

LNAB = 3, MIXB = 3 for SPI Port Register 1).

LNA-MIXER POWER BIAS SETTING

12

0–0

11

10

9

8

7

6

0–10–20–31–01–11–21–32–02–12–22–33–03–13–23–3

CONVERSION

GAIN

NOISE

FIGURE

Figure 7c. LNA/Mixer Noise Figure and Conversion Gain

vs. Bias Setting

LNA-MIXER POWER BIAS SETTING

5

0–0

0

–5

–10

–15

–20

–25

0–10–20–31–01–11–21–32–02–12–22–33–03–13–23–3

LNA–MIXER

CURRENT

IIP3

9

8

7

6

5

4

3

2

1

0

Figure 7d. LNA/Mixer IIP3 and Current Consumption vs.

Bias Setting