Power SuppliesSignal Generators

Video Demodulator Schematic Circuit Diagram

Advanced Demodulator Functionality with TDA8341 Chip

The demodulator utilized in this system is built around the Type TDA8341 chip, succeeding the well-known Types TDA2541 and TDA3541. Apart from serving as a demodulator, the chip houses an AGC (automatic gain control) section tailored for tuners, with the AGC voltage directly proportional to the gain, and an AFC (automatic frequency control) facility. In this particular design, the AFC functions primarily as a tuning indicator. The AGC control range spans 67 dB, and due to internal supply regulation, the IC boasts an input sensitivity of 40 uV.

Signal Processing and Standardization with IC Output

At pin 12, the IC produces a video signal with a level of 2.7V, directed through a TOKO low-pass filter, F11. This filter effectively eliminates any residual carrier frequencies from the video signal, offering 6 dB attenuation over its passband. To achieve a standard video signal of 1 Vpp at 75Ω, the filter may be succeeded by a video amplifier or buffer, such as the Type NE592. It’s crucial to note that the filter’s output impedance is 1 kΩ.

AGC Control and AFC Circuit Operation

The AGC operating point is set by P1, with the AGC control voltage extracted from pin 4 via R2. The AGC output current is limited to approximately 10 mA. The network composed of R7–C11–C12 forms an AGC detector, simultaneously providing pulses for the samples and holding circuits. This ensures the absence of video information in the AFC output. C6 serves as the reservoir capacitor for the S and H circuits. If pin 6 is directly connected to earth, the AFC is disabled, and the potential at pin 5 hovers around half the supply voltage. The AFC necessitates a synchronous demodulator with its tuned circuit L2–C8, taking into account any parasitic capacitive coupling with the tuned circuit for the reference amplifier (C9–C10).

Video demodulator Schematic diagram

Video demodulator Schematic diagram

Fine-Tuning the AFC Voltage for Optimal Indication

In the AFC circuit (L2–C8), where the skirt of AFC voltage is vital for tuning indication, a step characteristic is less than ideal. Therefore, it is imperative to keep C9 and C10 as small as possible. The tuning indicator takes the form of a center-zero meter, M1. Precision tuning of the reference circuit (to eliminate any carrier residue) is a delicate process and is most effectively achieved with the aid of an RF analyzer or modulated RF generator coupled with a frequency meter. However, if crystal-controlled PLL (phase-locked loop) tuning is employed. This adjustment can be performed without the need for such specialized test equipment.

Finalizing AFC Adjustment and Meter Reading

After ensuring the correctness of channel tuning and adjustment of L1–C7, the AFC circuit (L2–C8) can be tuned to achieve the center-zero reading on the meter, indicating half the supply voltage at the AFC output. Resistors R5 and R6 serve the dual purpose of maintaining the other terminal of M1 at half the supply voltage and limiting the current through the meter to approximately 100µA.

Addressing Cross-Talk Issues and Demodulator Current Draw

Potential cross-talk between the video output and IF input due to circuit layout can be mitigated by connecting a 6.8µH choke in series with pin 12 (as close to the IC as feasible). The demodulator exhibits a current draw of about 45 mA. For the input signal, a readily available SAW (surface acoustic wave) filter can be utilized, as most tuners—subject to confirmation—are typically capable of driving various SAW filters.

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