CMX868E2 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX868E2' PDF : 43 Pages View PDF
Low Power V.22bis Modem
30
CMX868 Advance Information
The Frequency values to be entered are calculated from the formula:
Value to be entered desired frequency (Hz) K 3.414
i.e. for 1kHz the value to be entered is 3414 (or 0D56 in Hex).
The Level values to be entered are calculated from the formula:
Value
to
be
entered
desired
VRMS
VDD
K
93780
i.e. for 0.5VRMS at VDD = 3.0V, the value to be entered is 15630 (3D0E in Hex)
Note that allowance should be made for the transmit signal filtering in the CMX868 which attenuates the
output signal for frequencies above 2kHz by 0.25dB at 2.5kHz, by 1dB at 3kHz and by 2.2dB at 3.4kHz.
On power-up or after a reset, the tone pairs TA-TC are set to notone, and TD is set to generate 2130Hz +
2750Hz at approximately –20dBm each.
Receive Tone Pair Programming
The programmable tone pair detector is implemented as shown in Figure 20. The filters are 4th order IIR
sections. The frequency detectors measure the time taken for a programmable number of complete input
signal cycles and compare this time against programmable upper and lower limits.
Rx signal
Detector 1
Filter
Detector 2
Filter
Frequency
measurement
enable
Level Detect
Frequency
measurement
enable
Level Detect
B6
B10
B7
Status
Register
Figure 20: Programmable Tone Detectors
input
b01 +
Z-1
b11 +
Z-1
b21
+
b02 +
+
-a11 Z-1
b12 +
-a21 Z-1
b22
+
output
-a12 Z-1
+
-a22 Z-1
H(z) =
b01.z + b11.z-1 + b21.z-2
1 + a11.z-1 + a21.z-2
b02.z + b12.z-1 + b22.z-2
1 + a12.z-1 + a22.z-2
Figure 21: Filter Implementation
Fsample = 9600Hz
¤2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 20480205.007
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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