u3741bm ATMEL Corporation, u3741bm Datasheet - Page 9

• Manufacturer Part Number: u3741bm
• Description: Uhf Ask Receiver Ic
• Manufacturer: ATMEL Corporation

Receiving
Characteristics
4662B–RKE–10/04
The RF receiver U3741BM 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 6. This example
relates to ASK mode and the 300-kHz bandwidth version of the U3741BM. FSK mode
and the 600-kHz version of the receiver exhibit similar behavior. Note that the mirror fre-
quency is reduced by 40 dB. The plots are printed relative to the maximum sensitivity. 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
U3741BM. Low-cost crystals are specified to be within ±100 ppm. The XTO deviation of
the U3741BM 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 6. Receiving Frequency Response
-100.0
-20.0
-30.0
-40.0
-60.0
-80.0
-90.0
-50.0
-70.0
-10.0
0.0
-6.0
-5.0 -4.0
-3.0
-2.0
-1.0
df (MHz)
0.0
with SAW
1.0
without SAW
2.0
3.0
4.0
5.0
U3741BM
6.0
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