RCVDL56DPFL/SP, RCV56DPFL/SP, and RCV336DPFL/SP Designer’s Guide
4-20
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Example:
A call-progress tone detector is required for the US telephone network to detect appropriate tones that exceed –35 dBm.
Solution:
The requirement can be met by detecting tones in the 245 Hz-650 Hz range. A bandpass filter with a passband of 245 Hz-
650 Hz must be designed. Any filter up to fourth order can be implemented and, normally, it is best to choose the highest
order available, especially for bandpass designs. A biquad filter design package could carry out this function by defining the
passband frequencies, the filter order, the filter gain (chose unity), and the filter sampling rate (7200 Hz). An example of
suitable coefficients is:
Function
A1
A2
A3
B1
B2
Biquad 1
0.1368
–0.2736
0.1368
1.8281
–0.8835
Biquad 2
0.1368
0.2736
0.1368
1.5716
–0.7920
These values should first be divided by two because coefficients greater than one are unrealized in the actual filter
implementation. This division should be done even if none of the coefficients in the design are greater than one. This is
because the biquad sections have been implemented as shown in Figure 4-7. The modified values are, therefore:
Function
A1'
A2'
A3'
B1'
B2'
Biquad 1
0.0684
–0.1368
0.0684
0.9140
–0.4418
Biquad 2
0.0684
0.1368
0.0684
0.7858
–0.3960
Next, convert the above numbers to fractional 2s complement numbers. In this case, the default coefficient values for
TONEA:
Function
A1'
A2'
A3'
B1'
B2'
Biquad 1
08C2
EE7C
08C2
74FE
C774
Biquad 2
08C2
1184
08C2
6495
CD4F
The second part of the requirement is to detect tones that exceed –35 dBm. The approximate values of THRESHU and the
corresponding tone level detected for TONEA at 500 Hz are:
THRESHU (Hex)
Tone Level Detected (dBm)
1100
–29
0C00
–32
0880
–35
0600
–38
THRESHU should be 0880h. If no hysteresis is required in the tone detector, then set THRESHL to 0880h (see Table 4-5). If
hysteresis is required, then make THRESHL < THRESHU. Threshold levels stated in the data sheets are measured at the
band edges. Other filter designs may require different values to those shown above. Note that changing threshold
coefficients may change the bandwidth response of tone detectors.
Table 4-6 shows the filter coefficient values for specific filters. Adjust THRESHL and THRESHU as necessary (see Table 4-
5).
Summary of Contents for RC336DPFL
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