u3742bm ATMEL Corporation, u3742bm Datasheet - Page 10

• Manufacturer Part Number: u3742bm
• Description: Uhf Ask/fsk Receiver
• Manufacturer: ATMEL Corporation

Receiving
Characteristics
Polling Circuit and
Control Logic
10
U3742BM
The RF receiver U3742BM can be operated with and without a SAW front-end filter. In a
typical automotive application, a SAW filter is used to achieve better selectivity. The
selectivity with and without a SAW front-end filter is illustrated in Figure 10. This exam-
ple relates to ASK mode. FSK mode exhibits similar behavior. Note that the mirror
frequency is reduced by 40 dB. The plots are printed relatively to the maximum sensitiv-
ity. If a SAW filter is used, an insertion loss of about 4 dB must be considered.
When designing the system in terms of receiving bandwidth, the LO deviation must be
considered as it also determines the IF center frequency. The total LO deviation is cal-
culated to be the sum of the deviation of the crystal and the XTO deviation of the
U3742BM. Low-cost crystals are specified to be within ±100 ppm. The XTO deviation of
the U3742BM is an additional deviation due to the XTO circuit. This deviation is speci-
fied to be ±30 ppm. If a crystal of ±100 ppm is used, the total deviation is ±130 ppm in
that case. Note that the receiving bandwidth and the IF-filter bandwidth are equivalent in
ASK mode but not in FSK mode.
Figure 10. Receiving Frequency Response
The receiver is designed to consume less than 1 mA while being sensitive to signals
from a corresponding transmitter. This is achieved via the polling circuit. This circuit
enables the signal path periodically for a short time. During this time the bit check logic
verifies the presence of a valid transmitter signal. Only if a valid signal is detected the
receiver remains active and transfers the data to the connected microcontroller. If there
is no valid signal present, the receiver is in sleep mode most of the time resulting in low
current consumption. This condition is called polling mode. A connected microcontroller
is disabled during that time.
All relevant parameters of the polling logic can be configured by the connected micro-
controller. This flexibility enables the user meets the specifications in terms of current
consumption, system response time, data rate etc.
Regarding the number of connection wires to the microcontroller, the receiver is very
flexible. It can be either operated by a single bi-directional line to save ports to the con-
nected microcontroller, or it can be operated by up to three uni-directional ports.
-100
-20
-30
-40
-60
-80
-90
-50
-70
-10
0
-6
-5
-4
-3
-2
-1
df (MHz)
0
with SAW
1
2
without SAW
3
4
5
6
4735A–RKE–11/03