A grid oscillator uses a vacuum tube to generate an alternating current in the radio frequency range. A tank circuit controls the oscillation frequency, and a grid immersion meter can be used to check for energy transfer. An absorption wavemeter can be used to tune into the frequency of an RF source.
A grid oscillator is an electronic device that uses an active device, called a triode vacuum tube, to generate a periodic alternating current. The grid immersion oscillator output is typically in the radio frequency (RF) range, approximately 50,000 cycles per second (cps) and above. Oscillators are amplifiers with positive feedback, achieved by allowing some of the in-phase output to reach the input of the oscillator. Usually, a coil or inductor in parallel with a capacitor or capacitor forms a tank circuit that controls the oscillation frequency.
Vacuum tubes were the only active devices used in radio electronics until the 1940s. They consisted of a negatively charged electrode called a cathode which 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 reaching the plate. By adding a resistance in the cathode circuit, a negative voltage on the control grid can set the so-called duty 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 swing frequency of a pendulum. When the value of the capacitor or inductor is smaller, the resonant frequency is higher. If a given tank circuit, consisting 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 inductor at a rate of 100 kHz. At some point, all of the energy in a tank circuit will be at the capacitor as the peak voltage; half a cycle later, all the energy will be at the inductor as peak magnetic flux.
The Grid Immersion Meter uses a current meter to check for variations in the grid current. It can be connected to the grid immersion 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 immersion oscillator with a tunable tank circuit could be used. If the RF range of the tunable frequency grid immersion oscillator was between 200 and 300 kHz, you could tune the depth meter to approximately the center of the dial and find that the grid depth meter indicates maximum coupling to the circuit of the test tank.
An absorption wavemeter is a device that uses a calibrated dial that indicates resonant frequency. It can be a totally passive device using a tank circuit and a sense circuit with a capacitor filter. Given the presence of an RF source, the absorption wavemeter can be used to tune into the frequency of the RF source. If the resonance frequency of the absorption wavemeter tank circuit is within the range of the absorption wavemeter frequency range, the maximum DC output level can be tuned.
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