STV0042 datasheet(21/24 Pages) STMICROELECTRONICS

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1st Step (Left) : SettingtheDemodulationParameters

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 by launching

afrequencysynthesis(theVCOisdrivento thewanted

frequency).This operationrequires 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, the

coded value is a number between 500 and 1000

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.

nd Step (Left) : VCO FrequencyChecking (VCO)

This secondstepis actually a waiting loop in which the

actual running frequencyof the VCO is measured.

To exit of this loop is allowed when : Subcarrier

Frequency - 10kHz

≤ Measured Frequency ≤ Sub-

carrier Frequency+10kHz (

± 10kHzis the maximum

dispersion of the frequencysynthesis function).

In practice,R8 B2 B3 and R9 are readand compared

to the value loaded in R6 B6 B7 and R7

±1 bit.

Note :

The duration of this step depends on how large is

frequency difference between the start frequency

and the targeted frequency. Typically :

- the rate of change of the VCO frequencyis about

3.75MHz/s (Cpump =10

µF)

- In addition to this settling time, 100ms must be

added to take into account the sampling period of

the watchdog.

rd Step (Left)

The FM demodulationcan be startedbyconnecting

the VCO to the phase locked loop (PLL).

In practice :

- SW3 closed

⇒ R6 B4 = H

- SW4 opened

⇒ R6 B2 = L

After this sequence of 3 steps for left channel,

a similar sequence is needed for the right channel.

Note :

In thesequenceforthe right,thereis no needto again

select the FM deviation (once is enoughfor the pair).

General Remark

Before to enable the demodulated signal to the

audio output, it is recommandedto keep the muting

and to check whether a subcarrier is present at the

wanted frequency. Such an informationis available

in R8 B0 and R8 B4 which can be read.

Two different strategies can be adoptedwhen ena-

bling the output :

- Eitherbothleftandrightdemodulatedsignalsare simul-

taneouslyauthorizedwhenbothchannelare ready.

- Or while the right channel sequence is running, the

already ready left signal is sent to the left and right

outputsandthereal stereosoundL/Ris outputwhen

both channels are ready. This second option gives

sound a few hundredsof ms before the first one.

Table 1 : Frequency Synthesis Register Setting for

the Most Common Subcarrier Frequencies

Subcarrier Freq. (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

FM DEMODULATION SOFTWARE ROUTINE (continued)

STV0042A

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