High Speed GMSK Modem 4k to 64kbps
Page 10 of 20
MX589
4.2.5 Rx S/N Detection
The RX S/N Detector system classifies the incoming zero-crossings as GOOD or BAD depending upon the
time when each crossing actually occurs with respect to its expected time as determined by the Clock
Extraction PLL. This information is then processed to provide a logic level output at the RX S/N pin. A high
level indicates a series of GOOD crossings; a low level indicates a BAD crossing.
By averaging this output, it is possible to derive a measure of the Signal-to-Noise-Ratio and hence the Bit-
Error-Rate of the received signal.
10-1
10-2
10-3
BER
MX589 BT = 0.3
10-4
BT = 1.0 (Theoretical)
10-5
MX589 BT = 0.5
10-6
5
6
7
8
9
10
13
15
16
18
19 20
11
12
14
17
S/N (dB) [Noise Bandwidth = Bit Rate]
Figure 6: Typical Bit-Error-Rate Performance
4.2.6 Rx Signal Quality
The effect of input Rx Signal quality on the Rx S/N output is shown in Figure 7.
% High Time
100
90
80
70
60
50
40
30
20
10
0
BT = 0.5
BT = 0.3
5
6
7
8
9
10
13
11
12
S/N (dB)
Figure 7: Typical Rx S/N Output High time (%) vs. Input S/N
2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 204800103.011
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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