mc44bs374t1a Freescale Semiconductor, Inc, mc44bs374t1a Datasheet - Page 24

• Manufacturer Part Number: mc44bs374t1a
• Description: Pll Tuned Uhf And Vhf Audio/video High Integration Modulator
• Manufacturer: Freescale Semiconductor, Inc

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 ( 106 K Ohms
typical ).
The recommended capacitor value for BG standard (with a time constant of 50 S) is 470 pF.
The recommended capacitor value for M/N standard (with a time constant of 75 S) 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:
Sound PLL loop filter calculation
Loop filter equation :
C2, added to minimize glitches, is usually set to one tenth the size of C1.
Example :
Fref=31.25KHz Æ N=5.5MHz / 31.25KHz = 176
Assuming C1= 220 nF, then
Q depends on the desired frequency response ; Choosing Q = 0.7 as a starting point,
then R= 18 k . 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 k has been chosen.
ƒ
ƒ
ƒ
ƒ
ƒ
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
o = 3dB cut off frequency
Freescale Semiconductor, Inc.
Figure 9. Sound PLL Loop Filter Calculation
MC44BS374T1/374T1A Advance Information
For More Information On This Product,
o
o = 360 rad, and fo= 57Hz
Go to: www.freescale.com
Kpd
N
*
*
C
Kosc
1
,
Q
R
*
C
1
* 1
o
, with :