The demodulator is based on a Type TDA8341 chip, which is the successor of the well-known Types TDA2541 and TDA3541. A part from a demodulator, the chip contains AGC (automatic gain control) section for tuners whose AGC voltage is directly proportional to the gain and an AFC (automatic frequency control) facility. In the present design, the AFC is used merely as tuning indicator.
The control range of AGC is 67dB. Because of internal supply regulation, the input sensitivity of IC is 40uV.
The IC provides a video signal at a level of 2.7V at pin 12, from where it is passed to a TOKO low-pass filter,F11, which removes any residual carrier frequencies from a video signal. The filter has an attenuation of 6dB over its pass-band. To obtain standard video signal 1Vpp into 75Ω the filter may be followed by a video amplifier/buffer, for instance, a Type NE592. Note that output impedance of the filter is 1kΩ.
The AGC operating point is set with P1.The AGC control voltage is taken the pin 4 via R2. The level of current through the AGC output is limited to about 10mA. Network R7 – C11 –C12 forms an AGC detector, which also provides pulses for sample & holds circuit. That circuit ensures that no video information is present in the AFC output.
The reservoir capacitor for S & H circuit is C6. If the pin 6 is connected directed to earth, the AFC is disabled and the potential at pin 5 is then roughly half supply voltage.
The AFC requires a synchronous demodulator with its tuned circuit: L2 – C8.
Because of (parasitic) capacitive coupling (C9 – C10 ) with the tuned circuit for reference amplifier.
L1 – C7, the skirt of AFC voltage here provides a tuning indication, a step characteristic is not really desirable, so that C9 and C10 must be kept as small as possible. The tuning indicator is formed by center zero meter M1. Tuning the reference circuit (to remove any residue of the carrier) is tricky and can really be done properly only if an r.f analyzer or modulated r.f generator and frequency meter are available. If crystal controlled PLL (phase-locked loop) tuning is used, however, the adjustment can be carried out without this test equipment.
Once the channel tuning and adjustment of L1 – C7 is correct, the AFC circuit ,L2 – C8 may be tuned for the center-zero reading of the meter (half supply voltage at AFC output). Resistor R5 and R6 hold the other terminal of M1 at the half supply voltage and at the same time limit the current through the meter to about 100µA.
Because f the circuit layout, there may be cross-talk between video output and if. input. This may, however, be cured by connecting a 6.8µH choke in series with pin 12 (as close to IC as feasible). The demodulator draws a current of about 45mA.
The input signal may be taken from a SAW (surface acoustic wave) filter, which is readily available nowadays. Most tuners –although this must be checked– are capable of driving a variety of SAW filters.
