AD608

Rev. C | Page 9 of 16

THEORY OF OPERATION

The AD608 consists of a mixer followed by a logarithmic IF

strip with RSSI and hard-limited outputs (see Figure 22).

MIXER

The mixer is a doubly balanced, modified gilbert-cell mixer. Its

maximum input level for linear operation is either ±56.2 mV,

regardless of the impedance across the mixer inputs, or −15 dBm

for a 50 Ω input termination. The input impedance of the mixer

can be modeled as a simple parallel RC network; the resistance

and capacitance values vs. frequency are listed in Table 5. The

bandwidth from the RF input to the IF output at the MXOP pin

is −1 dB at 30 MHz and then rapidly decreases as frequency

increases (see Figure 4).

MIXER GAIN

The conversion gain of the mixer is the product of its trans-

conductance and the impedance seen at Pin MXOP. For a 330 Ω

parallel-terminated filter at 10.7 MHz, the load impedance is

165 Ω, the gain is 24 dB, and the output is 15.85 × 56.2 mV (or

±891 mV) centered on the midpoint of the supply voltage. For

other load impedances, the expression for the gain in decibels is

where:

The gain of the mixer can be increased or decreased by changing

current at MXOP and the maximum allowable voltage swing at

Pin MXOP, which is ±1.0 V for a 3 V supply or 5 V supply.

24dB MIXER GAIN

110dB LIMITER GAIN

90dB RSSI

BIAS

MXOP

MIXER

BPF

DRIVER

VMID

LO

PREAMP

AD608

RFHI

RFLO

IF INPUT

–75dBm TO

IFHI

IFLO

LMOP

VPS2

RSSI

FDBK

COM3

FINAL

LIMITER

100nF

10nF

MIDSUPPLY

IF BIAS

LIMITER

OUTPUT

400mV p-p

PRUP

RF INPUT

–95dBm TO

VPS1 COM1

COM2

LOHI

RSSI OUTPUT

20mV/dB

0.2V TO 1.8V

3dB NOMINAL

INSERTION LOSS

2.7V TO 5.5V

5-STAGE IF AMPLIFIER

(16dB PER STAGE)

7 FULL-WAVE

RECTIFIER CELLS

2.7V TO

5.5V

LO INPUT

–16dBm

CMOS LOGIC

INPUT

±6mA MAX OUTPUT

(±890mV INTO 165Ω)

18nF

2MHz

LPF

10.7MHz

BAND-PASS

FILTER

5

6

1

2

3

4

16

8

7

10

13

9

15

14

12

11

+

Figure 22. Functional Block Diagram

Table 5. Mixer Input Impedance vs. Frequency

Frequency (MHz)

Resistance (Ω)

Capacitance (pF)

45

2800

3.1

70

2600

3.1

100

1900

3.0

200

1200

3.1

300

760

3.2

400

520

3.4

500

330

3.6