
AN93
Rev. 1.4
59
4.5.2. Audio Quality
The mulipole filter illustrated in this diagram is designed to shape the response for a pleasant sound and remove
interference, but note that, when PWM is demodulated in this way, it carries all the audio spectrum noise that is
present in the power supply of the modem minus 6 dB. This requires VCC to be as clean as one wants the call
progress audio to be. An alternative is for the AOUT signal to be buffered to a clean supply domain using a logic
gate or transistor buffer.
The 3-pole low-pass filter, with a 3 dB point at approximately 2 kHz, filters the 32 kHz square wave from AOUT and
allows only audio signals below 2 kHz to pass. See Figure 25 below. The amplifier provides differential speaker
drive, eliminating the need for a large coupling capacitor. Some additional design work and optimization must be
done to select the optimum gain and frequency response of this circuit, depending on speaker efficiency, final
product enclosure, and performance requirements. A two- or even one-pole filter may be adequate in some
applications.
Keep this audio circuitry well away from digital signals and use generous ground fill in the PCB layout.
Figure 25. Audio Filter Response
4.5.3. Power Dissipation in the Si3018 DAA
The power dissipated in the DAA and its support circuitry is dependent on the operational state. Table 44 shows
how power is distributed among the components in the BOM circuit. Two examples are given: one is the modem in
the default FCC termination when the loop current is at its maximum, 120 mA. The other shows the power when
the modem is attached to a TBR21 termination at the maximum loop current.
Table 44. Power Dissipation
Device
FCC Termination
Loop Current = 120 mA
TBR21 Mode
Loop Current = 56 mA
R1 14 mW
407 mW
R3
17 mW
244 mW
R4
26 mW
432 mW
R10
7 mW
203 mW
R11
468 mW
1 mW
Q1
110 mW
Q2
2 mW
Q3
70 mW
161 mW
Содержание Si2404
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Страница 27: ...AN93 Rev 1 4 27 Figure 7 Parallel Interface Read Timing Figure 8 Parallel Interface Write Timing ...
Страница 200: ...AN93 200 Rev 1 4 Figure 31 TAM Handset and Speakerphone Voice Paths ...
Страница 201: ...AN93 Rev 1 4 201 Figure 32 Si3000 Codec Gain and Signal Selection Options ...
Страница 290: ...AN93 290 Rev 1 4 Figure 57 256 Band Spectral Display Figure 58 2048 Band Spectral Display ...
Страница 305: ...AN93 Rev 1 4 305 Figure 76 Parallel or SPI Port Interrupt Service Flowchart ...