two-frequency oscillator

Not so long ago, when semiconduc- tors were still quite expensive, it paid to make a transistor serve more than one function. Although this is no longer necessary because of cost con- siderations, it is still fun to do so - and it may even have its uses! The circuit presented here is an LC oscillator that changes frequency through reversal of the supply voltage. When the supply voltage is positive, D1conducts and short-circuits L1C1. Oscillations are then maintained by crystal XL2and L 2C2. The DC operating point is set by P1in a way which ensures a compromise between faultless starting of the oscillator and low distortion of the output signal. When the polarity of the supply voltage is reversed, transistor T1 operates in its inverted mode, i.e., the functions of emitter and collector are interchanged. This means that the amplification is reduced, but, of course, an oscillator needs an amplifi- cation of only just above unity to operate. Crystal XL2and L 2C2are effectively cut out...