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Tuesday, October 5, 2010

Frequency converter

This is a 35.3 to 10.7 MHz converter circuit. It converts the 35.3 MHz signal coming from a VHF/UHF tuner down to an FM tuner to decode the TV audio in FM quality. The IC being used (TBA120) is one of the most common components in radio and TV circuits. Although it was designed as an RF amplifier and FM demodulator, it can also be used for many other applications.
The circuit works according to a simple principle: a converter has always a mixer and an oscillator. The TBA120 can be used as the mixer. Its amplifier component can then be used as the oscillator by adding an LC network to it (L1/C1).
Using the values given on the diagram, the circuit oscillates at 46 MHz. This is then mixed with the input signal of 35.3 MHz rezulting to a 10.7 MHz output signal. Following this example, one can modify the circuit to convert other frequencies. The components that need to be redimensioned are the L1/C1 oscillator network and the output filter L2/C5. If the desired oscillator frequency is far below 46 MHz, the R1/C3 must also be adapted as well as the L1/C1. Both values must be increased for lower oscillator frequencies but their exact values are not very critical and can be derived experimentally.The pcb design must be very simple due to the simplicity of the circuit. However, it is highly recommended to provide lots of ground copper plating. Also, keep the copper lines short and narrow. Most specially, keep the copper lines of the decoupling capacitors to ground as short as possible.