In between, let us have a look at Dip Meters which substitutes, conventional meters that measure inductance and capacitance. The main purpose of Dip Oscillators are finding the resonant frequency of a circuit. Dip Oscillator is a combination of a RF signal oscillator and a signal strength reading meter.
See fig. C-20/1. Here, in C-20/1A the coil you see out side the Dip Oscillator is part of the oscillator circuit inside the box. This coil 'plug in' facility is meant for using this equipment at multiple meter bands. The inner circuit remains the same and it responds more or less equally to coils that represents a limited spectrum of meter bands. The tuning disc in the picture is attached to the variable capacitor in the circuit. As we tune the disc, the oscillated frequency also changes accordingly. The frequency generated here is marked on the tuning disc, either verifying it with a frequency counter or scanning it in a radio. When the oscillator works, RF signal is generated and the meter shows the strength of the generated signal.
Now attach a circuit containing a coil and a capacitor (tuned circuit having a resonant frequency) at the out put of the Dip Oscillator as shown in C-20/1B. Further, tune the Dip Oscillator. When the Oscillation frequency and the Resonant Frequency of the attached circuitry match, the generated Oscillator signal will pass into that. This loss of energy will be shown in the strength meter as a dip. Dip Oscillators show the frequency to which a tuned circuit is actually tuned to.
Utilising this feature of the Dip Oscillator we can also acquire a close idea on the value unknown capacitors or inductors used in a circuit. Here, note that a value unknown capacitor is used together with value known inductor and vice versa only. For example, suppose the resonant frequency of a tuned circuit in which a coil with a value of 3.5 MH was used together with a value unknown capacitor was 7050 Khz according Dip Oscillator meter. There are formulas for finding out the resonant frequency of a circuit in which the value of both capacitor and inductor are given. There are formulas that let us know the inductive reactance and capacitive reactances required in a circuit for a particular frequency. If the resonant frequency and one of the reactances are given it is easy to find out the missing reactance and further the inductance of the inductor or the capacitance of the capacitor.
Fig. C-20/2 is thec ircuit of a model Dip Oscillator. Dip Oscilaltors can be used as frequency generators also. The Dip Oscilaltor illustrated in fig. C-20/2 can also be used as tone modulated Dip oscillator. The same circuit is enough for a Field Strength Meter (FSM) also. The only change is that S1 and S2 are to be put off. In this case, the meter records the strength of only that signal to which the Dip Oscillator is tuned. That is, it works as a tuned Field Strength Meter.
If anybody proposes to work this an untuned FSM, substitute the plug in coil with a piece of wire. Q1 in the picture shall be any transistor that works in the proposed frequency range at grounded configuration and Q2 shall be any audio amplifier transistor. Depending upon the active component selected, change the values of connected components too. Using this basic circuit anybody can easily build a 'multipurpose test meter'.
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