Low Power V.22 bis Modem
CMX868
Figure 12b Filter Implementation
Programming is done by writing a sequence of twenty-seven 16-bit words to the Programming Register.
The first word should be 32769 (8001 hex), the following twenty-six 16-bit words set the frequencies and
levels and are in the range 0 to 32767 (0000-7FFF hex).
Word Value written
Word Value written
1
2
3
4
5
6
7
8
9
10
11
12
13
14
32769
Filter #1 coefficient b2
Filter #1 coefficient b1
Filter #1 coefficient b0
Filter #1 coefficient a2
Filter #1 coefficient a1
Filter #1 coefficient b2
Filter #1 coefficient b1
Filter #1 coefficient b0
Filter #1 coefficient a2
Filter #1 coefficient a1
15
16
17
18
19
20
21
22
23
24
25
26
27
Filter #2 coefficient b2
1
1
1
1
1
2
2
2
2
2
1
1
1
1
1
2
2
2
2
2
Filter #2 coefficient b1
Filter #2 coefficient b0
Filter #2 coefficient a2
Filter #2 coefficient a1
Filter #2 coefficient b2
Filter #2 coefficient b1
Filter #2 coefficient b0
Filter #2 coefficient a2
Filter #2 coefficient a1
Freq measurement #1 ncycles
Freq measurement #1 mintime
Freq measurement #1 maxtime
Freq measurement #2 ncycles
Freq measurement #2 mintime
Freq measurement #2 maxtime
The coefficients are entered as 15-bit signed (two’s complement) integer values (the most significant bit
of the 16-bit word entered should be zero) calculated as 8192 * coefficient value from the user’s filter
design program (i.e. this allows for filter design values of -1.9999 to +1.9999).
The design of the IIR filters should make allowance for the fixed receive signal filtering in the CMX868
which has a low pass characteristic above 1.5kHz of 0.4dB at 2kHz, 1.2dB at 2.5kHz, 2.6dB at 3kHz and
4.1dB at 3.4kHz.
‘ncycles’ is the number of signal cycles for the frequency measurement.
‘mintime’ is the smallest acceptable time for ncycles of the input signal expressed as the number of
9.6kHz timer clocks. i.e. ‘mintime’ = 9600 * ncycles / high frequency limit
‘maxtime’ is the highest acceptable time for ncycles of the input signal expressed as the number of
9.6kHz timer clocks. i.e. ‘maxtime’ = 9600 * ncycles / low frequency limit
The level detectors include hysteresis. The threshold levels - measured at the 2 or 4-wire line with unity
gain filters, using the line interface circuits described in section 1.4.2, 1.0 dB line coupling transformer
loss and with the Rx Gain Control block set to 0dB - are nominally:
‘Off’ to ‘On’
‘On’ to ‘Off’
-44.5dBm
-47.0dBm
ã 2004 CML Microsystems Plc
34
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
