2 Terminal Test Oscillator

Posted on Feb 6, 2014

Anyone who is interested in home construction will at some time or other have the need to measure the inductance of small coils. This can be a difficult problem, as commercially made equipment of sufficient accuracy is often far too expensive for occasional use in an amateur workshop. The simple LCR bridges that are affordable generally only Work

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at audio frequency and are notoriously inaccurate on their Low inductance ranges. The result of this is that coil construction is often so hit-and-miss that for many applications the home constructor has to use expensive ready made coils, which could have been wound at home from materials costing a few pence if test facilities had been available. The method of measurement described here is a inexpensive solution to the problem that I have been using for over 40 years. It uses the coil under test in parallel with a known 1% capacitor to form the tuned circuit for a 2-terminal test oscillator. After making allowance for circuit strays, the total circuit capacity and the oscillating frequency can be entered in the standard resonant circuit formula to calculate the inductance. of the coil accurately enough for all normal purposes. Using this method has the advantages that a) coils can be tested in a practical circuit at the operating frequency and b) measurements are just as easy to make on toroids and sealed coils such as IF transformers as they are on open coils. An additional bonus is that the circuit can be used for a variety of other purposes such as a signal generator, a dip meter or to measure small capacitors. Fig. la shows the basic circuit of an FET source coupled 2terminal oscillator, which is the semiconductor version of a valve circuit that was popular in the 1950s. This circuit, which was derived from an early...

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