Voltage controlled TTL oscillator - simple construction using common ICs

The problem of not having the right IC to hand is an well known stumbling- block for constructors: When a VCO is required urgently, the ideal IC is invariably not available and those that are will probably not suit the purpose. It is therefore very handy to be able to have something "home-made" for emergencies. This circuit will make sure that your hair will turn grey because of age and not because of this particular problem. Whenever an oscillator with an adjustable frequency is required, it is desirable to use one that is voltage controlled, because this is as versatile as it is possible to get. Whereas a potentiometer is fine for manual setting, a control voltage is far more useful for automatic frequency control purposes. The circuit must have a wide frequency and supply range in order for it to be suitable for the majority of applications. This particular circuit has a frequency range of more than 1 : 1000 and can be used from AF up to 50 MHz. The basis of the circuit is the well-