Philips Semiconductors Product specification
SA647Low-voltage digital IF receiver
1998 Aug 10
6
CIRCUIT DESCRIPTION
Mixer
The mixer has a single–ended input. The input impedance is 670Ω
in parallel with a 3.0pF cap at 110.52MHz RF. The mixer output can
drive a 1500Ω 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 1500Ω. 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
from the large headroom available at higher V
CC
s.
