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