[
]
307.2
ChBW
(DEC _ DIV 1) kHz
=
+
SWRS046H – NOVEMBER 2006 – REVISED MARCH 2015
In Manchester mode the maximum modulating signal occurs when transmitting a continuous sequence of
0s (or 1s). In NRZ mode the maximum modulating signal occurs when transmitting a 0-1-0 sequence. In
both Manchester and NRZ mode 2 × fm is then equal to the programmed baud rate. The equation for
SBW can then be rewritten as shown in
SBW = Baud rate + frequency separation
(14)
Furthermore, the frequency offset of the transmitter and receiver must also be considered. Assuming
equal frequency error in the transmitter and receiver (same type of crystal) the total frequency error is
shown in
.
f_error = ±2 × XTAL_ppm × f_RF
(15)
Where:
XTAL_ppm is the total accuracy of the crystal including initial tolerance, temperature drift, loading and
ageing.
F_RF is the RF operating frequency.
The minimum receiver channel filter bandwidth (ChBW) can then be estimated as shown in
ChBW > SBW + 2 × f_error
(16)
The DEC_DIV[4:0] bits in the FILTER register control the receiver channel filter bandwidth. The 6 dB
bandwidth is given by
.
(17)
Where:
the IF frequency is set to 307.2 kHz.
In SmartRF Studio the user specifies the channel spacing and the channel filter bandwidth is set
according to
.
For narrowband systems with channel spacings of 12.5 and 25 kHz the channel filter bandwidth is 12.288
kHz and 19.2 kHz respectively to comply with ARIB STD-T67 and EN 300 220.
For wideband systems (channel spacing of 50 kHz and above) it is possible to use different channel filter
bandwidths than given in
.
There is a trade-off between selectivity as well as sensitivity and accepted frequency tolerance. In
applications where larger frequency drift is expected, the filter bandwidth can be increased, but with
reduced adjacent channel rejection (ACR) and sensitivity.
Table 5-5. Channel Filter Bandwidths Used for the Channel Spacings Defined in SmartRF Studio
FILTER.DEC_DIV
CHANNEL SPACING
FILTER BANDWIDTH
[4:0]
[kHz]
[kHz]
[decimal(binary)]
12.5
12.288
24 (11000b)
25
19.2
15 (01111b)
50
25.6
11 (01011b)
100
51.2
5 (00101b)
150
102.4
2 (00010b)
200
153.6
1 (00001b)
500
307.2
0 (00000b)
28
Detailed Description
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