TDA7338_05 Datasheet, PDF (5/12 Pages) STMicroelectronics – STEREO DECODER

3. SCA (SUBSIDIARY COMMUNICATIONS AUTHORIZATION)
α 67
=
----^V-----^O----⁽--^s----ⁱ--^g---ⁿ----^a----^l--⁾--⁽---^a---^t--¹----^K-----^H-----^z---⁾----
V_O(spurious)(at9KHz)
; f_s = (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---⁾----
V_O(spurious)(at4KHz)
; f_s = 110KHz - (3 x 38KHz)
TDA7338
α 190
=
----^V-----^O----⁽--^s----ⁱ--^g---ⁿ----^a----^l--⁾--⁽---^a---^t---¹---^K-----^H-----^z---⁾----
V_O(spurious)(at4KHz)
; f_s = 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 T_R = 2µs, T_HIGH = 2µs, and T_F = 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.
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