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SW Converter for Digital AM Car Radio Schematic Circuit Diagram

Encouraging RF Experimentation

This circuit is intentionally left with several open possibilities (not literally, of course) to encourage experimentation with RF circuitry without substantial expenses. Upon examining the circuit diagram, you might recognize a modified version of the SW Converter for AM Radios detailed elsewhere in this publication. These modifications were essential to ensure compatibility with a digital AM car radio, diverging from the conventional analogue AM car radios.

The primary distinction between digital AM radios and their entirely analogue predecessors lies in their tuning method, specifically in 9 kHz steps (sometimes 4.5 kHz steps) to comply with the international frequency allocation standards for the band. While this step size is practical for MW, it presents a challenge when attempting to integrate a digital AM receiver with a frequency step-up converter for SW. In the realm of shortwave, characterized by its lack of fixed step sizes. The predetermined 9 kHz intervals become a limiting factor.

SW Converter for Digital AM Car Radio Schematic Circuit Diagram

Exploring Variable Oscillation Solutions

In the initial attempt, the objective was to create a crystal oscillator capable of a variable frequency range of approximately 5 kHz in both directions. Regrettably, despite earnest endeavors, the crystal proved unyielding, offering a pull of no more than 1 or 2 kHz. This setback necessitated the exploration of alternative solutions.

Opting for a Variable LC-Based Oscillator

Upon thorough examination of the NE/SA602/612 datasheet, a viable alternative emerged: a variable LC-based oscillator. The circuit successfully functioned after the implementation of a resonant LC circuit, complemented by a 0.1 µF series capacitor. This capacitor effectively blocked the DC component on pin 6 of the NE602 (612). To refine the tuning, a bandspread or fine-tuning feature was introduced. This was achieved by integrating a lightly loaded 365 pF tuning capacitor (C10) that shunted the LC resonant circuit. Both C10 and its main tuning counterpart, C8, were salvaged from an old transistor radio.

Designing the Tuning Coil and Secondary Details

The tuning coil, denoted as L1, was meticulously crafted with 8 to 10 turns of 0.6-0.8mm diameter enamelled copper wire (ECW) wound on a 6-8 mm diameter former, sans a core. With this particular coil configuration, the frequency coverage spanned approximately 4 MHz to 12 MHz. Notably, detailed information regarding Tr1 can be found in the relevant article. It’s noteworthy that no tuning capacitor was utilized on the secondary—instead, the inherent input stray capacitance of the NE602 (612) proved adequate for the task.

Enhancing Reception with a Beat Frequency Oscillator (BFO)

To enable the reception of Single Sideband (SSB) signals, a Beat Frequency Oscillator (BFO) was incorporated into the setup. T1-based BFO, simple yet powerful, offers substantial output and maintains stability to sustain an SSB signal without frequent adjustments. The BFO frequency is adjusted through C3. Tr2, a prefabricated 455 kHz Intermediate Frequency (IF) transformer, had its internal capacitor crushed and removed using pliers. When S2 is closed, the BFO output signal is superimposed onto the NE602 (612) IF output in the MW radio.

Shielding and Output Control for Optimal Performance

To optimize performance and minimize interference, it’s advisable to house the converter in a metal box for effective shielding. If the BFO output is excessive, a suggested solution from the circuit diagram involves disconnecting it. In such cases, allowing stray coupling to manage the process can be beneficial.

Impressive Sensitivity and Excitement of Reception

The sensitivity of the setup, even with a mere 1-meter length of a car radio aerial, is remarkably impressive. Considering that major international Shortwave (SW) broadcasting stations like Radio NHK Japan, Moscow, and BBC transmit with substantial power to ensure audibility, the thrill of capturing these signals for the first time on your car radio is undeniably exciting.


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