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STV0042/STV0056 APPLICATION NOTE

STMicroelectronics 

STV0042/STV0056 APPLICATION NOTE

3 - APPLICATION NOTES (continued)

a) Principle of the FM Demodulators

To properly operate the FM demodulators require

to go through different operating modes before to

produce the demodulated signal. The required se-

quence is described in the software part (Sec-

tion 3.2.2.1).

Because there is no selective filter (LC or ceramic)

which sets the operating frequency (ex : 10.7MHz),

the demodulation of the STV42/56 circuit directly

operates at the subcarrier frequency

(ex : fVCO = 7.02MHz when demodulating the

7.02MHz subcarrier). This first point explains why

each VCO can be driven by the Synthesizer.

- SW2 closed and SW5 to R to drive the right VCO

- SW4 closed and SW5 to L to drive the left VCO

During the Demodulation :

When the FM demodulator is in demodulation, it

operates like any PLL demodulator and its block

diagram can be simplified (as Figure 9).

Figure 9

Phase

Detector

VIN

(from AGC stage) KD

Loop

Filter

F(p)

VCO

Ko

K

Vo

(to audio processor)

F(p) : the transfert function of the external loop

filter which is connected at DET Pin.

KD : the phase detector gain (in V/radians).

KO : slope of the voltage controlled oscillator

(VCO) in (radians/volts or Hz/volts).

K : is a programmable coefficient related to the

register 05 bits 0 to 5.

In Annex 1 a more detailed study of this PLL

structure is provided. Out of this detailed study the

key results are :

KO : 460kHz/V ⇔ 2.89 ⋅ 106 radians/V (typ.)

KD

≈

DETH

⋅

R33

R36

(for left channel)

(0)

DETH is a parameter given in the specification.

Lock range = ωL = KO ⋅ K ⋅ KD

(1)

DC

gain =

VO PP

(deviation)PP

=

K

1

⋅ KO

(2)

Remarks about those relations :

- The relations (1) shows that a programmable K

coefficient corresponds to a programmable band-

width (FM deviation selection, table in Page 26 of

the specification dated June 1995).

- In the satellite receiver applications it is asked to

have a required audio level for a full deviation

signal (ex : 1VRMS on the scart output at full

deviation). Refering to relation (2), it means that

the K ⋅ KO products must remain constant for all

deviations.

Consequently to compensate the dispersion of

the VCO slope KO, the STV42/56 FM deviation

selection table offers many steps, so that there is

always a selection which guarantees :

K ⋅ KO = constant ± small dispersion

The method to select the optimum K factor is

described in Section 3.2.2.3

- Additionnally, in relation (1), it can be noticed that

a constant (K ⋅ KO) product gives a more stable

lock range.

b) Breaking Down the PLL Demodulator Section

(explanation for left channel and refered to Fig. 8)

- Phase Detector :

This part converts the phase difference between

the wanted subcarrier and the VCO signal

(with 90° phase log) into a current IPD. In the

calculation KD is given in Volt per Radians, be-

cause a current to voltage conversion is done by

the resistor R33 connected between Pin DET and

VREF.

- The Loop Filter :

It is made of external components connected

between DET Pin and VREF.

- The VCO :

The voltage controlled oscillator typically has a

460kHz/volt slope. It is driven by the SYNTHE-

SIZER during the frequency synthesis sequence

and by the demodulated signal during the de-

modulation.

The VCO part features a first output with 90°

phase lag compared with the subcarrier, this out-

put is used for phase detection ; the second

output is in phase with the subcarrier and is used

for synchronous amplitude detection (see AGC

stage).

- The Watchdog :

It is used to measure the VCO frequency. This

function is used to check if the VCO frequency

has reached the wanted subcarrier frequency

during the frequency synthesis. This operation is

also usefull to monitor the VCO frequency during

demodulation (preventing it from shifting away

when operating under abnormal conditions). The

watchdog is a 1/1000 divider (clocked by a 10kHz

reference frequency) consequently its averages

the VCO frequency over a 100ms period (equiva-

lent to 10Hz) ; this long period makes the results

independant from the audio modulating signal.

9/37

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