Rainbow Electronics T5761 - Manual (Page 7)

7
T5760/T5761
4561B–RKE–10/02
The T5760/T5761 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 2 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 electrical characteristics. They should not be
exceeded to maintain full sensitivity of the receiver.
Receiving
Characteristics
The RF receiver T5760/T5761 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 and
large signal capability. The receiving frequency response without a SAW front-end filter
is illustrated in Figure 6 and Figure 7. This example relates to ASK mode. FSK mode
exhibits a similar behavior. The plots are printed relatively to the maximum sensitivity. If
a SAW filter is used, an insertion loss of about 3 dB must be considered, but the overall
selectivity is much better.
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
calculated, to be the sum of the deviation of the crystal and the XTO deviation of the
T5760/T5761. Low-cost crystals are specified to be within ±90 ppm over tolerance,
temperature and aging. The XTO deviation of the T5760/T5761 is an additional
deviation due to the XTO circuit. This deviation is specified to be ±30 ppm worst case for
a crystal with CM = 7 fF. If a crystal of ±90 ppm is used, the total deviation is ±120 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. Narrow Band Receiving Frequency Response
-60.0
-50.0
-40.0
-30.0
-20.0
-10.0
0.0
-4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0
df (MHz)
dP (dB)