A grid dip oscillator is an electronic device that uses an active device, called a triode vacuum tube, to generate a periodic alternating current. The output of the grid dip oscillator usually falls in the radio frequency (RF) range, approximately 50,000 cycles per second (cps) and higher. Oscillators are amplifiers with positive feedback, achieved by allowing a portion of the in-phase output to reach the input of the oscillator. Usually, a coil or inductor in parallel with a capacitor or condenser forms a tank circuit that controls the oscillation frequency.
Vacuum tubes were the only active devices used in radio electronics until the 1940s. They were made up of a negatively charged electrode called a cathode that emits electrons through a vacuum into a positively charged plate – the anode. A control grid between the cathode and the plate is able to control the amount of electrons that reach the plate. By adding a resistor in the cathode circuit, a negative voltage at the control grid can set the so-called operating point of the vacuum tube.
A tank circuit is added to the plate circuit of the vacuum tube. The center frequency of the tank circuit is analogous to the swinging frequency of a pendulum. When the capacitor value or the inductor value is lower, the resonant frequency is higher. If a given tank circuit, made of an inductor and a capacitor, is resonant at 100,000 cps or 100 kilohertz (kHz), the tank circuit will transfer energy between the capacitor and the inductor at a rate of 100 kHz. At one point, all the energy in a tank circuit will be at the capacitor as peak voltage; at half a cycle later, all the energy will be at the inductor as peak magnetic flux.
The grid dip meter uses a current meter to check for changes in grid current. It can be connected to the grid dip oscillator to indicate when there is a transfer of energy to a nearby test tank circuit. If the resonant frequency of an unknown tank circuit was 250 kHz, a grid dip oscillator can be used with a tunable tank circuit. If the RF range of the tunable frequency grid dip oscillator was 200 to 300 kHz, it is possible to tune the dip meter to about the middle of the dial and to find that the grid dip meter indicates maximum coupling to the test tank circuit.
An absorption wavemeter is a device that uses a calibrated dial that indicates resonant frequency. It may be a totally passive device that uses a tank circuit and a detection circuit with a capacitor filter. Given the presence of an RF source, the absorption wavemeter can be used to tune in to the frequency of the RF source. If the absorption wavemeter tank circuit’s resonance frequency is within the range of the absorption wavemeter frequency range, it is possible to tune for maximum direct current (DC) output level.