STV0042A
FM DEMODULATION SOFTWARE ROUTINE (continued)
1st Step (Left): SettingtheDemodulationParameters
the watchdog.
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 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
(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.
2nd Step (Left) : VCO FrequencyChecking (VCO)
This secondstepis actually a waiting loopin which the
actualrunning frequencyof the VCO is measured.
To exit of this loop is allowed when : Subcarrier
Frequency - 10kHz ≤ Measured Frequency ≤ Sub-
carrier Frequency+10kHz(± 10kHz is the maximum
dispersion of the frequencysynthesisfunction).
Inpractice,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 = 10µF)
- In addition to this settling time, 100ms must be
added to take into account the sampling period of
3rd 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,thereisnoneedtoagain
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
outputsand the real stereo sound L/R is 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)
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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