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The Minispot 455kHz Modulated Oscillator

The Minispot produces a 455kHz carrier waveform which is amplitude-modulated with a 500Hz tone. You can use it to align the intermediate frequency (IF) stages of any AM broadcast or shortwave radio.

By Mauro Grassi

This project generates an amplitude modulated 455kHz RF signal. It can be used to accurately align the intermediate frequency stages of heterodyne AM receivers.

Click for larger image
Fig.1: the circuit consists of a multivibrator (transistors Q1 & Q2) running at 500Hz and this modulates a 455kHz oscillator based on transistor Q3 and a ceramic resonator.

If you are going to build the Aussie 3-Valve Radio described in this issue or if you are involved in restoring vintage radios, you will want this Minispot 455kHz modulated oscillator to accurately align the IF stages. For those readers with long memories, it is very similar to the Minispot circuit published in the February 1981 issue of "Electronics Australia" magazine.

The objectives of IF alignment are to ensure that all tuned circuits in the IF stages are tuned to the same frequency and that this frequency is the correct frequency, usually 455kHz.

If various parts of the IF stages are tuned to different frequencies, the sensitivity of the receiver will be poor. It may also be plagued with unwanted audible whistles appearing in the audio output. Therefore, correct IF alignment is essential to good performance.

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