STV0056AF datasheet(23/27 Pages) STMICROELECTRONICS

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FM DEMODULATION SOFTWARE ROUTINE

With the STV0056AFcircuit, for each channel,three

steps are required to acheive a FM demodulation :

-1

st step :To set the demodulationparameters :

• FM deviation selection,

• Subcarrier frequency selection.

-2

nd step : To implement a waiting loop to check

the actual VCO frequency.

-3

rd step :To close the demodulationphase locked

loop (PLL).

Refering to the FM demodulation block diagram

(page 12), the frequency synthesis block is com-

mon to both channels (left and right) ; consequently

two completesequenceshave to be done one after

the other when demodulating stereo pairs.

Detailed Description

Conventions :

- R = Stands for Register

- B = Stands for Bit

Example : R05 B2 = Register 05, Bit 2

For clarity, the explanations are based on the fol-

l ow ing example : st ereo pai r 7.02MHz /L

7.20MHz/R, deviation

±50kHz max.

1st STEP (LEFT) : SETTING THE DEMODULATION PARAMETERS

A. The FM deviation is selected by loading R5 with

the appropriate value. (see R5 truth table).

NB : Very wide deviations (up to

±592kHz) can be

accomodated when R5 B5 is low.

Corresponding bandwidth can be calculated as

follows :

≈ 2 (FM deviation + audio bandwidth)

≈ 2 (value given in table + audio bandwidth)

In the example :

R5Bits 7654

XX1

B. The subcarrier

frequency

is selected

launching a frequency synthesis (the VCO is driven

to the wanted frequency). This operation requires

two actions :

- To connect the VCO to the frequency synthesis

loop. Refering to the FM block diagram (page 12):

• SW4 closed ⇒ R6 B2 = H

• SW3 to bias ⇒ R6 B4 = L

• SW2 to bias ⇒ R6 B3 = L

• SW1 opened ⇒ R6 B5 = L

- To load R7 and R6 B6 B7 with the value corre-

sponding to the left channel frequency. This 10

bits value is calculated as follows :

Subcarrier frequency = coded value x 10kHz

(10kHz is the minimum step of the frequency

synthesis function)

Considering that the tunning range is comprised

between 5 to 10MHz, thecoded value is a number

between 500 and 1000 (2

10 = 1024) then 10 bits

are required.

Example :

7.02MHz = 702 x 10kHz

702

⇒ 1010 1111 10 ⇒ AF + 10

R7 is loaded with AF and R6 B6 : L, R6 B7 : H.

The Table 1 gives the setting for the most common

subcarrier frequencies.

Table 1 : Frequency Synthesis Register Setting

for the Most Common Subcarrier Frequencies

Subcarrier Frequency

(MHz)

(Hex)

Bit 7

Bit 6

5.58

5.76

5.8

5.94

6.2

6.3

6.4

6.48

6.5

6.6

6.65

6.8

6.85

7.02

7.20

7.25

7.38

7.56

7.74

7.85

7.92

8.2

8.65

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