MC44BS374T1/374T1A Advance Information

Modes of Operation

16.10 Sound Section

The oscillator is fully integrated and does not require any external components. An internal low-pass filter

and matched structure provide very low harmonics levels.

The sound modulator system consists of an FM modulator incorporating the sound subcarrier oscillator.

The audio input signal is AC-coupled into the amplifier, which then drives the modulator. The audio pre-

emphasis circuit is a high-pass filter with an external capacitor and an internal resistor ( 106KOhms

typical).

The recommended capacitor value for BG standard (with a time constant of 50

µS) is 470 pF.

The recommended capacitor value for M/N standard (with a time constant of 75

µS) is 750 pF.

To increase the audio bandwidth at low frequencies it is possible to change the sound PLL loop filter. The

recommended values are 220 nF and 22 nF. To increase the bandwidth at low frequencies (lower than 50

Hz) it is possible to use larger loop filter capacitor values. This will give a lower cut off frequency. In this

case it is necessary to calculate the new values of the loop filter according to the required Q factor and loop

bandwidth. This is described in the following figure:

Figure 9. Sound PLL Loop Filter Calculation

Sound PLL loop filter calculation

Loop filter equation :

1

*

*

C

N

Kosc

Kpd

o

=

ω

,

o

C

R

Q

ω

*

1

*

1

=

, with :

ω

o = 3dB cut off frequency

Kpd = 1uA : Phase detector current

Kosc = 5MHz/V : sound oscillator slope (in fact Kosc=40MHz/prescaler divider

=8) This gives 5 MHz/V

N = sound divider ratio

Q = quality factor

C2, added to minimize glitches, is usually set to one tenth the size of C1.

Example :

Fref=31.25KHz

N=5.5MHz / 31.25KHz = 176

Assuming C1= 220 nF, then ωo = 360 rad, and fo= 57Hz

Q depends on the desired frequency response ; Choosing Q = 0.7 as a starting point,

then R= 18 kΩ. The resistance acts directly on the factor quality and can be adjusted to

create peaking on the low frequency range. It is recommended to adjust the value of R

experimentally depending on the application and requirements. In the proposed

application schematic a value of 15 kΩ has been chosen.

Freescale Semiconductor, Inc.

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