Rainbow Electronics T5761 - Manual (Page 5)

5
T5760/T5761
4561B–RKE–10/02
To determine f
LO
, the construction of the IF filter must be considered at this point. The
nominal IF frequency is f
IF
= 950 kHz. To achieve a good accuracy of the filter corner
frequencies, the filter is tuned by the crystal frequency f
XTO
. This means that there is a
fixed relation between f
IF
and f
LO
.
f
IF
= f
LO
/915
The relation is designed to achieve the nominal IF frequency of f
IF
= 950 kHz for the
868.3 MHz version. For the 915 MHz version an IF frequency of f
IF
= 1.0 MHz results.
The RF input either from an antenna or from an RF generator must be transformed to
the RF input Pin LNA_IN. The input impedance of that pin is provided in the electrical
parameters. The parasitic board inductances and capacitances influence the input
matching. The RF receiver T5760/T5761 exhibits its highest sensitivity if the LNA is
power matched. This makes the matching to an SAW filter as well as to 50
W or an
antenna easier.
Figure 33 shows a typical input matching network for f
RF
= 868.3 MHz to 50 W . Figure 34
illustrates an according input matching for 868.3 MHz to an SAW. The input matching
network shown in Figure 33 is the reference network for the parameters given in the
electrical characteristics.
Analog Signal Processing
IF Filter The signals coming from the RF front-end are filtered by the fully integrated 4th-order IF
filter. The IF center frequency is f
IF
= 950 kHz for applications where f
RF
= 868.3 MHz
and f
IF
=1.0 MHz for f
RF
= 915 MHz. The nominal bandwidth is 600 kHz.
Limiting RSSI Amplifier The subsequent RSSI amplifier enhances the output signal of the IF amplifier before it is
fed into the demodulator. The dynamic range of this amplifier is DR
RSSI
= 60 dB. If the
RSSI amplifier is operated within its linear range, the best S/N ratio is maintained in ASK
mode. If the dynamic range is exceeded by the transmitter signal, the S/N ratio is
defined by the ratio of the maximum RSSI output voltage and the RSSI output voltage
due to a disturber. The dynamic range of the RSSI amplifier is exceeded if the RF input
signal is about 60 dB higher compared to the RF input signal at full sensitivity.
In FSK mode the S/N ratio is not affected by the dynamic range of the RSSI amplifier,
because only the hard limited signal from a high gain limiting amplifier is used by the
demodulator.
The output voltage of the RSSI amplifier is internally compared to a threshold voltage
V
Th_red
. V
Th_red
is determined by the value of the external resistor R
Sens
. R
Sens
is con-
nected between Pin SENS and GND or V
S
. The output of the comparator is fed into the
digital control logic. By this means it is possible to operate the receiver at a lower
sensitivity.
If R
Sens
is connected to GND, the receiver switches to full sensitivity. It is also possible to
connect the Pin SENS directly to GND to get the maximum sensitivity.
If R
Sens
is connected to V
S
, the receiver operates at a lower sensitivity. The reduced
sensitivity is defined by the value of R
Sens
, the maximum sensitivity by the signal-to-
noise ratio of the LNA input. The reduced sensitivity depends on the signal strength at
the output of the RSSI amplifier.