January 1993

8

Philips Semiconductors

Objective specification

Quasi split-sound circuit and AM

demodulator

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.

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 k

Ω 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:

Where:

C = parallel capacitor

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.

C

S

C

⁄

P

–

=