参数资料
型号: 73M1903-IGV
厂商: TERIDIAN SEMICONDUCTOR CORP
元件分类: 消费家电
英文描述: SPECIALTY CONSUMER CIRCUIT, PQFP32
封装: TQFP-32
文件页数: 16/45页
文件大小: 449K
代理商: 73M1903-IGV
73M1903
Modem Analog Front End
DATA SHEET
Page: 23 of 45
2005 TERIDIAN Semiconductor Corporation
Rev 1.4
Transmit Levels
The 16-bit transmit code word written by the DSP to the Digital Sigma-Delta Modulator (DSDM) (via TIF)
has a linear relationship with the analog output signal. So, decreasing a code word by a factor of 0.5 will
result in a 0.5 (-6dB) gain change in the analog output signal.
The following formula describes the relationship between the transmit code word and the output level at
the transmit pins (TXAP/TXAN):
Vout (V) = 2 * code/32,767 * DSDMgain * dacGAIN * Vref * TLPFgain * SMFLTgain * FreqFctr
Vout is the differential peak voltage at the TXAP and TXAN pins.
Code is the 16-bit, two’s compliment transmit code word written out by the DSP to the DSDM (via TIF).
The code word falls within a range of
± 32,767. For a sinusoidal waveform, the peak code word should
be used in the formula to obtain the peak output voltage.
DSDMgain is the scaling factor used on the transmit code word to reduce the possibility of saturating the
modulator. This value is set to 0.640625(–3.555821dB) at dc in the 48 tap transmit interpolation filter
(TIF) that precedes DSDM.
dacGAIN is the gain of the DAC. The value dacGAIN is calculated based on capacitor values inside
DAC1 and dacGAIN=8/9=0.8889. The number 32,767 refers to the code word that generates an 82%
“1’s” pulse density at the output of the DSDM. As one can see from the formula, the D to A conversion is
dependent on the level of Vref. Also when DTMFBST bit is set, Vref is increased from 1.36V to 1.586V
to allow higher transmit level or 16.6% increase in gain. This bit is intended for enhancing the DTMF
transmit level and should not be used in data mode.
TLPFgain is the gain of TLPF and nominally equals to 0.00dB or 1.0.
SMFLTgain is the gain of SMFLT and nominally equal to 1.445 or 3.2dB.
When TXBST0 bit is set, the gain is further increased by 1.65dB (1.21) for the total of 4.85 dB. This is to
accommodate greater hybrid insertion loss encountered in some applications.
FreqFctr shows dependency of the entire transmit path on frequency. See Figure 10.
With the transmit code word of +/- 32,767, the nominal differential swing at the transmit pins at dc is:
Vout (V) = 2 * code/32,767 * DSDMgain * dacGAIN * Vref * TLPFgain * SMFLTgain * FreqFctr
= 2 * 32,767/32767 * 0.6640625 * 0.8889 * 1.36 * 1.0 * 1.4454 * 1.0 = 2.31Vpk diff.
When DTMFBST bit is set, Vout (V) = 1.166 * 2.31= 2.693Vpk diff.
When TXBST0 bit is set, Vout (V) = 1.21 * 2.31= 2.795Vpk diff.
(1)
When both DTMFBST and TXBST0 are set to 1, Vout (V) = 2.795 * 1.166 = 3.259Vpk diff.
[1] If not limited by power supply or internal reference.
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