AD608

REV. B

–9–

RSSI Output

The logarithmic amplifier uses a successive detection architec-

ture. Each of the five stages has a full-wave detector; two addi-

tional high level detectors are driven through attenuators at the

input to the limiting amplifiers, for a total of seven detector

stages. Because each detector is a full-wave rectifier, the ripple

component in the resulting dc is at twice the IF. The AD608’s

low-pass filter has a 2 MHz cutoff frequency, which is one

decade below the 21.4 MHz ripple that results from a 10.7 MHz

IF.

For operation at lower IFs such as 450 kHz or 455 kHz, the

AD608 requires an external low-pass filter with a single pole lo-

cated at 90 kHz, a decade below the 900 kHz ripple frequency

for these IFs. The RSSI range is from the noise level at approxi-

mately –80 dBm to overload at +15 dBm and is specified for

±1 dB accuracy from –75 dBm to +5 dBm. The +15 dBm

maximum IF input is provided to accommodate bandpass filters

of lower insertion loss than the nominal 4 dB for 10.7 MHz

ceramic filters.

Digitizing the RSSI

In typical cellular radio applications, the RSSI output of the

AD608 will be digitized by an A/D converter. The AD608’s

RSSI output is proportional to the power-supply voltage, which

not only allows the A/D converter to use the supply as a refer-

ence but also causes the RSSI output and the A/D converter’s

output to track over power supply variations, reducing system

errors and component costs.

Power Consumption

The total power-supply current of the AD608 is a nominal

7.3 mA. The power is signal-dependent, partly as the RSSI

output increases (the current is increased by 200

µA at an RSSI

output of +1.8 V) but mostly due to the IF BPF consumption

when being driven to

±891 mV assuming a 4 dB loss in this

filter and a peak input of +5 dBm to the log-IF amp, and tem-

perature dependent, as the biasing system used in the AD608 is

proportional to absolute temperature (PTAT).

Troubleshooting

The most common causes of problems with the AD608 are

incorrect component values for the offset feedback loop, poor

board layout, and pickup of RFI, which all cause the AD608 to

“lose” the low end (typically below –65 dBm) of its RSSI output

and cause the limiter to swing randomly. Both poor board lay-

out and incorrect component values in the offset feedback loop

can cause low level oscillations. Pickup of RFI can be caused by

improper layout and shielding of the circuit.