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Low Power Mixer/Limiter/RSSI 3 V Receiver IF Subsystem AD608 The RF and LO bandwidths both exceed 500 MHz. In a typical IF application, the AD608 will accept the output of a 240 MHz SAW filter and downconvert it to a nominal 10.7 MHz IF with a conversion gain of 24 dB (ZIF = 165 Ω). The AD608’s logarithmic/limiting amplifier section handles any IF from LF to as high as 30 MHz.

FEATURES Mixer –15 dBm 1 dB Compression Point –5 dBm IP3 24 dB Conversion Gain >500 MHz Input Bandwidth Logarithmic/Limiting Amplifier 80 dB RSSI Range ؎3؇ Phase Stability over 80 dB Range Low Power 21 mW at 3 V Power Consumption CMOS-Compatible Power-Down to 300 ␮W typ 200 ns Enable/Disable Time

The mixer is a doubly-balanced “Gilbert-Cell” type and operates linearly for RF inputs spanning –95 dBm to –15 dBm. It has a nominal –5 dBm third-order intercept. An onboard LO preamplifier requires only –16 dBm of LO drive. The mixer’s current output drives a reverse-terminated, industry-standard 10.7 MHz 330 Ω filter. The nominal logarithmic scaling is such that the output is +0.2 V for a sinusoidal input to the IF amplifier of –75 dBm and +1.8 V at an input of +5 dBm; over this range the logarithmic conformance is typically ± 1 dB. The logarithmic slope is proportional to the supply voltage. A feedback loop automatically nulls the input offset of the first stage down to the submicrovolt level.

APPLICATIONS PHS, GSM, TDMA, FM, or PM Receivers Battery-Powered Instrumentation Base Station RSSI Measurement

The AD608’s limiter output provides a hard-limited signal output at 400 mV p. The voltage gain of the limiting amplifier to this output is more than 100 dB. Transition times are 11 ns and the phase is stable to within ± 3° at 10.7 MHz for signals from –75 dBm to +5 dBm.

GENERAL DESCRIPTION

The AD608 provides both a low power, low distortion, low noise mixer and a complete, monolithic logarithmic/limiting amplifier using a “successive-detection” technique. It provides both a high speed RSSI (Received Signal Strength Indicator) output with 80 dB dynamic range and a hard-limited output. The RSSI output is from a two-pole post-demodulation lowpass filter and provides a loadable output voltage of +0.2 V to +1.8 V. The AD608 operates from a single 2.7 V to 5.5 V supply at a typical power level of 21 mW at 3 V.

The AD608 is enabled by a CMOS logic-level voltage input, with a response time of 200 ns. When disabled, the standby power is reduced to 300 µW within 400 ns. The AD608 is specified for the industrial temperature range of –25°C to +85°C for 2.7 V to 5.5 V supplies and –40°C to +85°C for 4.5 V to 5.5 V supplies. It comes in a 16-pin plastic SOIC.

FUNCTIONAL BLOCK DIAGRAM 24dB MIXER GAIN

IF INPUT –75dBm TO +15dBm2

±6mA MAX OUTPUT (±890mV INTO 165Ω) RFHI

5 MIXER

RF INPUT –95 TO –15dBm1 RFLO

MXOP 7 BPF DRIVER

6

VMID

LO PREAMP

10.7MHz BANDPASS FILTER 330Ω

9 5-STAGE IF AMPLIFIER (16dB PER STAGE)

330Ω

8

BIAS

1 +2.7V TO 5.5V

2

3 LO INPUT –16dBm

4

IFLO

100Ω

RSSI

RSSI OUTPUT 11 20mV/dB 0.2V TO 1.8V

COM3 12 VPS2

14 +2.7V TO 5.5V

LMOP FINAL LIMITER

10 100nF

≈ 2MHz LPF

10nF

MID-SUPPLY IF BIAS

COM2

7 FULL-WAVE RECTIFIER CELLS IFHI

15 LIMITER OUTPUT 400mVp

13

LOHI

VPS1 COM1

110dB LIMITER GAIN 90dB RSSI

3dB NOMINAL INSERTION LOSS

18nF

FDBK

AD608

±50µA

PRUP 16

CMOS LOGIC INPUT

REV. B Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

1

–15dBm = ±56mV MAX FOR LINEAR OPERATION

2

NOTES:

39.76µV RMS TO 397.6mV RMS FOR ±1dB RSSI ACCURACY

© Analog Devices, Inc., 1996 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 Fax: 617/326-8703


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