REV. B

–8–

AD608

IF Filter Terminations

The AD608 was designed to drive a parallel-terminated 10.7 MHz

bandpass filter with a 330

Ω impedance. With a 330 Ω parallel-

terminated filter, pin MXOP sees a 165

Ω termination and the

gain is nominally 24 dB. Other filter impedances and gains can

be accommodated by either accepting an increase or decrease in

gain in proportion to the filter impedance or by keeping the im-

pedance seen by MXOP a nominal 165

Ω (by using resistive di-

viders or matching networks). Figure 21 shows a simple resistive

voltage divider for matching an assortment of filter impedances,

and Table II lists component values.

The Logarithmic IF Amplifier

The logarithmic IF amplifier consists of five amplifier stages

of 16 dB gain each, plus a final limiter. The IF bandwidth is

30 MHz (–1 dB) and the limiting gain is 110 dB. The phase

skew is

± 3° from –75 dBm to +5 dBm (approximately 111 µV

p-p to 1.1 V p-p). The limiter output impedance is 200

Ω

and the limiter’s output drive is

± 200 mV (400 mV p-p) into a

5 k

Ω load. In the absence of an input signal, the limiter’s output

will limit on noise fluctuations, which produces an output that

continues to swing 400 mV p-p but with random zero crossings.

Offset Feedback Loop

Because the logarithmic amplifier is dc coupled and has more

than 110 dB of gain from the input to the limiter output, a dc

offset at its input of even a few

µV would cause the output to

saturate. Thus, the AD608 uses a low frequency feedback loop

to null out the input offset. Referring to Figure 21, the loop

consists of a current source driven by the limiter, which sends

50

µA current pulses to pin FDBK. The pulses are low pass

filtered by a

π-network consisting of C1, R4, and C5. The

smoothed dc voltage that results is subtracted from the input to

the IF amplifier at pin IFLO. Because this is a high gain ampli-

fier with a feedback loop, care should be taken in layout and

component values to prevent oscillation. Recommended values

for the common IFs of 450 kHz, 455 kHz, 6.5 MHz, and

10.7 MHz are listed in Table II.

24dB MIXER GAIN

110dB LIMITER GAIN

90dB RSSI

BIAS

MXOP

MIXER

BPF

DRIVER

VMID

LO

PREAMP

AD608

RFHI

RFLO

IFHI

IFLO

LMOP

VPS2

RSSI

FDBK

COM3

FINAL

LIMITER

100nF

C5

R1

±50µA

BANDPASS

FILTER

MID-SUPPLY

IF BIAS

6

5

7

8

10

9

13

14

12

15

11

1

2

3

4

16

PRUP

VPS1 COM1

COM2

LOHI

12dB NOMINAL

INSERTION LOSS

(ASSUMES 6dB IN FILTER)

5-STAGE IF AMPLIFIER

(16dB PER STAGE)

7 FULL-WAVE

RECTIFIER CELLS

≈

≈

R4

C1

+5V

C1

1

µF

LO INPUT

–16dBm

C2

100pF

CMOS

LOGIC

INPUT

R2

R3

2MHz

LPF

47k

Ω

Figure 21. Applications Diagram for Common IFs and Filter Impedances

Table II. AD608 Filter Termination and Offset-Null Feedback Loop Resistor and Capacitor Values for Common IFs

Filter

Filter Termination Resistor

Offset Null

IF

Impedance

Values

Feedback Loop Values

R1

R2

R3

R4

C1

C5

450 kHz

2

1500

Ω

174

Ω

1330

Ω

1500

Ω

1000

Ω

200 nF

100 nF

455 kHz

1500

Ω

174

Ω

1330

Ω

1500

Ω

1000

Ω

200 nF

100 nF

6.5 MHz

1000

Ω

178

Ω

825

Ω

1000

Ω

100

Ω

18 nF

10 nF

10.7 MHz

330

Ω

330

Ω

0

Ω

330

Ω

100

Ω

18 nF

10 nF

NOTES

at 900 kHz).