TDA7338 Просмотр технического описания (PDF) - STMicroelectronics

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TDA7338

STEREO DECODER

STMicroelectronics 

3. SCA (SUBSIDIARY COMMUNICATIONS AUTHORIZATION)

α 67

=

----V-----O----(--s----i--g---n----a----l--)--(---a---t--1----K-----H-----z---)----

VO(spurious)(at9KHz)

; fs = (2 x 38KHz) - 67KHz

measured with : 81% mono signal; 9% pilot signal; fm=1KHz;

10% SCA - subcarrier (fs = 67KHz, unmodulated)

4. ACI (ADJACENT CHANNEL INTERFERENCE)

α 114

=

----V-----O----(--s----i--g---n----a----l--)--(---a---t---1---K-----H-----z---)----

VO(spurious)(at4KHz)

; fs = 110KHz - (3 x 38KHz)

TDA7338

α 190

=

----V-----O----(--s----i--g---n----a----l--)--(---a---t---1---K-----H-----z---)----

VO(spurious)(at4KHz)

; fs = 186KHz - (5 x 38KHz)

measured with : 90% mono signal; 9% pilot signal; fm=1KHz; 1% spurious signal

(fs = 110KHz or 186KHz, unmodulated)

5. Control range typ 11% of VR (see figure 5 and figure 6)

6. MEASUREMENT OF TRIGGER THRESHOLDS

All thresholds are measured by using a pulse with TR = 2µs, THIGH = 2µs, and TF = 10µs. The repetition

rate must not increase the PEAK voltage.

Figure 4.

Vin

VTR

DC

D95AU365

TR THIGH

TF

Time

4 FUNCTIONAL DESCRIPTION

4.1 Signal Path

The TDA7338 Stereodecoder contains all necessary functions for processing the MPX signal. Due to the

external input resistance (Pin 20) the circuit can be adapted to different MPX input levels. Behind a 80kHz

lowpass filter the adjustment free PLL for the pilot Tone is placed.

The only external component needed for the PLL is the ceramic resonator for the oscillator which runs at

456kHz.

The pilot detector output is designed as an open collector output, therefore an external pullup resistor is

needed. To force the decoder to "MONO" Pin 19 has to be clamped to a voltage below 0.8V.

The voltage level (signal strength from the IF part) applied to Pin 15 (VHCC) allows to control the time

constant of the deemphasis (nom. = 50µs, see fig. 5). If the RF-signal is weak, the corner frequency is

reduced down to 1kHz to improve the signal to noise ratio.

5/12

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