U3745BM Datasheet, PDF (8/29 Pages) ATMEL Corporation – UHF ASK RECEIVER IC

Analog Signal Processing
IF Amplifier
RSSI Amplifier
Demodulator and Data
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 = 1 MHz for applications where f_RF = 315 MHz or
f_RF = 433.92 MHz is used. For other RF input frequencies, refer to Table 1 to determine
the center frequency.
The receiver U3745BM employs an IF bandwidth of B_IF = 600 kHz. This IC can be used
together with the U2745B. SAW transmitters exhibit much higher transmit frequency tol-
erances compared to PLL transmitters. Generally, it is necessary to use B_IF = 600 kHz
together with such transmitters.
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.
Since different RF input networks may exhibit slightly different values for the LNA gain,
the sensitivity values given in the electrical characteristics refer to a specific input
matching. This matching is illustrated in Figure 4 and exhibits the best possible
sensitivity.
The signal coming from the RSSI amplifier is converted into the raw data signal by the
ASK demodulator.
In ASK mode, an automatic threshold control circuit (ATC) is employed to set the detec-
tion reference voltage to a value where a good signal-to-noise ratio is achieved. This
circuit also implies the effective suppression of any kind of in-band noise signals or com-
peting transmitters. If the S/N ratio exceeds 10 dB, the data signal can be detected
properly.
The output signal of the demodulator is filtered by the data filter before it is fed into the
digital signal processing circuit. The data filter improves the S/N ratio as its bandpass
can be adopted to the characteristics of the data signal. The data filter consists of a 1st-
order high-pass and a 1st-order low-pass filter.
The high-pass filter cut-off frequency is defined by an external capacitor connected to
pin CDEM. The cut-off frequency of the high-pass filter is defined by the following
formula:
f_{cu_DF}
=
-----------------------------¹-----------------------------
2 ´ p ´ 30 kW ´ CDEM
In self-polling mode, the data filter must settle very rapidly to achieve a low current con-
sumption. Therefore, CDEM cannot be increased to very high values if self polling is
used. On the other hand, CDEM must be large enough to meet the data filter require-
ments according to the data signal. Recommended values for CDEM are given in the
section “Electrical Characteristics”.
The cut-off frequency of the low-pass filter is defined by the selected baud rate range
(BR_Range). BR_Range is defined in the OPMODE register (refer to section “Configu-
ration of the Receiver”). BR_Range must be set in accordance to the used baud rate.
8 U3745BM
4663A–RKE–06/03