TDA3845 Datasheet, PDF (8/18 Pages) NXP Semiconductors – Quasi split-sound circuit and AM demodulator

Philips Semiconductors
Quasi split-sound circuit and AM
demodulator
Objective specification
TDA3845; TDA3845T
Notes to the characteristics
1. Start of gain control (LOW IF input signal) at −3 dB intercarrier signal reduction at pin 12, AGC mode set to FM or
−3 dB AF signal reduction at pin 6, AGC mode set to AM.
2. End of gain control (HIGH IF input signal) at +1 dB intercarrier signal expansion at pin 12, AGC mode set to FM or
+1 dB AF signal expansion at pin 6, AGC mode set to AM.
3. Picture carrier (38.9 MHz) to sound carriers (33.4 MHz/33.158 MHz) ratio: 13/20 dB.
IF input signal (picture carrier at sync pulse); V_1-16 = 10 mV (RMS). Transmitter mode: DSB.
Reference for the (S + W)/W ratio (0 dB) corresponds to the sound modulation where f = 1 kHz and frequency
deviation ∆f = ± 50 kHz.
With reduced frequency deviation ∆f = ± 27 kHz and the (S + W)/W figures will decrease by 5.4 dB.
4. If the device is used only for the B/G standard (no AM), the capacitor at pin 5 can be omitted (pin 5 has to be
disconnected). In this event the AGC will always operate as a peak signal AGC and is independent of the voltage at
pin 7.
The AM mode can also be used for the B/G standard, consequently standard switching is not required. However, the
intercarrier level depends on the video modulation and the AF performance may decrease.
When the IC is operated from a 12 V power supply pin 7 can be connected to a 12 V logic level via a 2.2 kΩ resistor.
5. LC reference circuit for the picture carrier (pins 8 and 9); 68 pF/0.247 µH; in series with 27 pF:
Q loaded = 40 (Qo = 90); tuned to 38.9 MHz yields quadrature demodulation for the picture carrier which provides
optimum video suppression at the intercarrier output (e.g. black-to-white jump of the video modulation.The series
capacitor provides a notch at the sound carrier frequency in order to produce more attenuation for the sound carrier
in the PC reference channel. The ratio of parallel to series capacitance depends on the ratio of picture to sound
carrier frequency which has to be adapted to other TV transmission standards, if required, in accordance with the
formula:
C_S = C_P (f_PC ⁄ f_SC) ²â€“C_P
Where:
C_S = series capacitor
C = parallel capacitor
f_PC = picture carrier frequency
f_SC = sound carrier frequency
The result is an improved ‘intercarrier buzz’ in the stereo system B/G, particularly with 250 kHz video modulation
(up to 10 dB improvement in sound Channel 2), or to suppress 350 kHz video modulated beat in the digitally
modulated NICAM subcarrier
In order to optimize the AF signal performance, fine tuning to the optimum S/W at the sound Channel 2 may be
achieved by a 250 kHz video modulated square wave.
6. Residual video AM is defined as:
m = (A−B)/A
A = intercarrier level at sync pulse
B = intercarrier level at 100% white video modulation
7. If higher AC output current is required an external resistor must be connected between the output pin and ground in
order to increase the bias current of the emitter follower. The permissible maximum DC output current must not be
exceeded.
January 1993
8