U3745BM Datasheet, PDF (9/29 Pages) ATMEL Corporation – UHF ASK RECEIVER IC

Receiving
Characteristics
U3745BM
The U3745BM is designed to operate with data coding where the DC level of the data
signal is 50%. This is valid for Manchester and Bi-phase coding. If other modulation
schemes are used, the DC level should always remain within the range of V_{DC_min} = 33%
and V_{DC_max} = 66%. The sensitivity may be reduced by up to 1.5 dB in that condition.
Each BR_Range is also defined by a minimum and a maximum edge-to-edge time
(t_{ee_sig}). These limits are defined in the section “Electrical Characteristics”. They should
not be exceeded to maintain full sensitivity of the receiver.
The RF receiver U3745BM can be operated with and without a SAW front end filter. The
selectivity with and without a SAW front-end filter is illustrated in Figure 5. This example
relates to ASK mode of the U3745BM. Note that the mirror frequency is reduced by
40 dB. The plots are printed relatively 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
U3745BM. Low-cost crystals are specified to be within ±100 ppm. The XTO deviation of
the U3745BM is an additional deviation due to the XTO circuit. This deviation is speci-
fied to be ±50 ppm. If a crystal of ±100 ppm is used, the total deviation is ±150 ppm in
that case. Note that the receiving bandwidth and the IF-filter bandwidth are equivalent in
ASK mode.
Figure 5. Receiving Frequency Response
0.0
-10.0
-20.0
-30.0
without SAW
-40.0
-50.0
-60.0
-70.0
-80.0
-90.0
with SAW
-100.0
-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0
df (MHz)
9
4663A–RKE–06/03