GMSK Packet Data Modem and RF Transceiver
CMX990
6.4
Variable Bt
The CMX990 offers the ability to select the Bt factor in the transmit modulation. This allows
characteristics to be optimised for a particular bit/rate channel bandwidth. Standard options of B = 0.3
t
and Bt = 0.5 are available. Figure 27, 28 and 29 show the effect of changing Bt.
RBW
VBW
SWT
200 Hz
2 kHz
4.2 s
RF Att
Unit
30 dB
dBm
Ref Lvl
5 dBm
5
0
A
-10
-20
-30
-40
-50
-60
-70
-80
1VIEW
2VIEW
1RM
2RM
-90
-95
Center 800.0016238 MHz
3.35 kHz/
Span 33.5 kHz
Inner Trace is CMX990 with Bt = 0.3 Outer Trace is CMX990 with Bt = 0.5
Figure 27 Modulation spectrum of CMX990 at 8kbps with different Bt
ã 2004 CML Microsystems Plc
69
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
