Philips Semiconductors

Product specification

SA647

Low-voltage digital IF receiver

1998 Aug 10

6

CIRCUIT DESCRIPTION

Mixer

The mixer has a single–ended input. The input impedance is 670

Ω

in parallel with a 3.0pF cap at 110.52MHz RF. The mixer output can

drive a 1500

Ω ceramic filter without any matching required.

Oscillator and Buffer

The on-board oscillator supplies the signal for the mixer

down-conversion. The internally biased transistor can be configured

as a Colpitts or Butler overtone crystal oscillator. The transistor’s

bias current can be increased if desired by adding a shunt resistor

from Pin 3 to ground. The oscillator’s buffered output (Pin 5) can be

used as a feedback signal to lock the oscillator to an appropriate

reference.

IF Amplifier and IF Limiter

The IF strip provides more than 96dB of power gain for the down

converted signal. Its bandwidth is 2MHz. The input and output

impedance of the IF amplifier and the input impedance of the IF

limiter are set to 1500

Ω. A second filter is connected between the IF

amplifier and the limiter for improved channel selectivity and

reduced instability. The overall gain can be reduced if desired by

adding an external attenuator after the IF amplifier The differential

limiter outputs (Pins 10 and 11) are available for demodulator

circuits.

RSSI

The received signal strength indicator provides a linear voltage

indication of the received signal strength in dB for a typical range of

85dB. The response time to a change in input signal is less than a

few microseconds and the delay is kept to a minimum because of

the use of a minimum phase shift circuit. Because of the speed of

the RSSI circuit, the RSSI rise and fall time will be dominated by the

bandwidth of the external bandpass filter that is placed between the

mixer and the IF. Since the RSSI function requires the signal to

propagate through the whole IF strip, and the rise and fall time of the

filters are inversely proportional to their bandwidth, there is a

trade-off between channel selectivity and RSSI response.

Therefore, it is recommended that all channel selection filters be

placed before the IF strip, just after the mixer. This will make the

delay of the RSSI independent of the mixer input signal amplitude.

Also, a 6dB insertion loss between the IF and limiter sections (Pins

16 and 14) will give optimum flatness of RSSI versus mixer input

signal.

The RSSI curve is temperature compensated and in addition is

designed for improved consistency from unit to unit.

DC Power Supply

The IC is designed for operation between 2.7 and 5.5V. A power

supply dependent biasing scheme is used in the mixers to benefit