Only a handful of components are needed to build an instrument which is indispensable in any RF workshop: the grid dip meter or ‘dipper’. The main function of this clever piece of test equipment is to determine the resonance frequency of unknown tuned circuits within a certain range (typically, 1.5 to 80 MHz). Apart from this, the dipper doubles as an RF signal generator and an absorption frequency meter. Some amateurs even use it to repair radios or tune short-wave antennas!
The operation of the grid dip meter is based on the principle of energy absorption. An oscillator produces RF energy through an inductor. When this inductor is brought in the vicinity of another inductor with the same resonance frequency, the latter ‘draws’ energy via inductive coupling. Because the oscillator is equipped with an RF output level indicator, the energy loss is easily detected as a ‘dip’ when the instrument is tuned, and hits upon the resonance frequency of the unknown inductor. The name ‘grid dipper’ is historic, and a remnant of the days when this instrument was built using valves (electrically, the nearest equivalent of valve grid is the gate of a FET).
The oscillator in the grid dipper may be modulated with a fixed tone to make its signal easily identifiable on a short-wave tuning scale (where chaos may reign).
Basic Circuit and Operation
The heart of the circuit is an RF oscillator. The oscillator is usually a Colpitts design. In other words, the tuned circuit has a capacitive tap. Other oscillators, for example, the Hartley oscillator, use a tap on the inductive element. A capacitive tap is made by connecting two capacitors in series, and then in parallel with an inductor. The junction of the two capacitors forms the capacitive tap.
The capacitive part of the tuned circuit is formed by two series- connected halves of variable-capacitance diode (‘varicap’). Each coil gives a different frequency range, and is plugged into a socket. The tuning in each range is accomplished by varicap.
The adjustable voltage required for the tuning of the varicap is supplied by a potentiometer. So, turning this pot tunes the grid dipper across the relevant frequency range.