SI4711-B30-GM【PDF 23/40 页】IC TX FM RADIO 76-108MHZ 20UQFN

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Si4710/11-B30

Rev. 1.1

5.4. Line Input

The Si4710/11 provides left and right channel line inputs

(LIN and RIN). The inputs are high-impedance and low-

capacitance, suited to receiving line level signals from

external audio baseband processors. Both line inputs

are low-noise inputs with programmable attenuation.

Passive and active anti-aliasing filters are incorporated

to prevent high frequencies from aliasing into the audio

band and degrading performance.

To ensure optimal audio performance, the Si4710/11

has a TX_LINE_INPUT_LEVEL property that allows the

user to specify the peak amplitude of the analog input

(LILEVEL[9:0]) required to reach the maximum

deviation level programmed in the audio deviation

property, TX_AUDIO_DEVIATION. A corresponding line

input attenuation code, LIATTEN[1:0], is also selected

by the expected peak amplitude level. Table 14 shows

the line attenuation codes.

The line attenuation code is chosen by picking the

lowest Peak Input Voltage in Table 14 that is just above

the expected peak input voltage coming from the audio

baseband processor. For example, if the expected peak

input voltage from the audio baseband processor is

400 mV, the user chooses LIATTEN[1:0] = 10 since the

Peak Input Voltage of 416 mV associated with

LIATTEN[1:0] = 10 is just greater than the expected

peak input voltage of 400 mV. The user also enters

400 mV into the LILEVEL[9:0] to associate this input

level to the maximum frequency deviation level

programmed into the audio deviation property. Note that

selecting a particular value of LIATTEN[1:0] changes

the input resistance of the LIN and RIN pins. This

feature is used for cases where the expected peak input

level exceeds the maximum input level of the LIN and

RIN pins.

The maximum analog input level is 636 mVpK. If the

analog input level from the audio baseband processor

exceeds this voltage, series resistors must be inserted

in front of the LIN and RIN pins to attenuate the voltage

such that it is within the allowable operating range. For

example, if the audio baseband's expected peak

amplitude is 900 mV and the V

supply voltage is 1.8 V,

the designer can use 30 kW series resistors in front of

the LIN and RIN pins and select LIATTEN[1:0] = 11. The

resulting expected peak input voltage at the LIN/RIN

pins is 600 mV, since this is just a voltage divider

between the LIN/RIN input resistance (see Table 14,

60 kW for this example) and the external resistor. Note

that the Peak Input Voltage corresponding to the chosen

LIATTEN[1:0] code still needs to satisfy the condition of

being just greater than the attenuated voltage. In this

example, a line attenuation code of LIATTEN[1:0] = 11

has a Peak Input Voltage of 636 mV, which is just

greater than the expected peak attenuated voltage of

600 mV. Also, the expected peak attenuated voltage is

entered into the LILEVEL[9:0] parameter. Again, in this

example, 600 mV is entered into LILVEVEL[9:0]. This

example shows one possible solution, but many other

solutions exist. The optimal solution is to apply the

largest possible voltage to the LIN and RIN pins for

signal-to-noise considerations; however, practical

resistor values may limit the choices.

Note that the TX_LINE_INPUT_LEVEL parameter will

affect the high-pass filter characteristics of the ac-

coupling capacitors and the resistance of the audio

inputs.

The Si4710/11 has a programmable low audio level and

high audio level indicators that allows the user to

selectively enable and disable the carrier based on the

presence of audio content. The TX_ASQ_LEVEL_LOW

and TX_ASQ_LEVEL_HIGH parameters set the low

level and high level thresholds in dBFS, respectively.

The time required for the audio level to be below the low

threshold is set with the TX_ASQ_DURATION_LOW

parameter, and similarly, the time required for the audio

level to be above the high threshold is set with the

TX_ASQ_DURATION_HIGH parameter.

Table 14. Line Attenuation Codes

LIATTEN[1:0]

Peak Input

Voltage [mV]

RIN/LIN Input

Resistance [kW]

190

396

301

100

416

636

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