STV0056AF View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'STV0056AF' PDF : 27 Pages View PDF
STV0056AF
FM DEMODULATION SOFTWARE ROUTINE
With the STV0056AFcircuit, for each channel,three
steps are required to acheive a FM demodulation :
- 1st step :To set the demodulation parameters :
• FM deviation selection,
• Subcarrier frequency selection.
- 2nd step : To implement a waiting loop to check
the actual VCO frequency.
- 3rd 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-
lowing example : stereo pair 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 :
Bw ≈ 2 (FM deviation + audio bandwidth)
Bw ≈ 2 (value given in table + audio bandwidth)
In the example :
R5Bits 7 6 5 4 3 2 1 0
XX1 1 0 11 0
B. The subcarrier frequency is selected by
launching a frequencysynthesis (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 (210 = 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)
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
Register 7
(Hex)
8B
90
91
94
9B
9D
A0
A2
A2
A5
A6
AA
AB
AF
B4
B5
B8
BD
C1
C4
C6
CD
D8
Register 6
Bit 7 Bit 6
1
0
0
0
0
0
1
0
0
0
1
0
0
0
0
0
1
0
0
0
0
1
0
0
0
1
1
0
0
0
0
1
1
0
0
0
1
0
0
1
0
0
0
0
0
1
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