Attachment to the frequency meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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An attachment to the frequency meter is proposed to determine resonant frequency of the parallel oscillatory circuit. His circuit (Fig. 1) is well suited for this, because. for feedback, it is not necessary to make a tap from the loop coil.
Transistors T1 and T2 (BF244C) operate in an oscillator circuit similar to the well-known cathode-coupled lamp circuit. Cascade on T3 (2N708 or BC547V) - matching.
Ris.1
The above scheme works stably in the range of 1 ... 30 MHz. The values of the inductance in the drain circuit T2 and the capacitor Cr in the feedback circuit are important. By reducing the inductance, it is possible to provide a measurement in the 2-meter range. True, in this case, the lower limiting frequency of measurements greatly increases.
In order to measure on both HF and VHF, several switchable inductances can be introduced.
When determining the true resonance frequency on VHF, it is also necessary to take into account the input capacitance of the circuit.
Author: P. Schmidt (DJ9JFT); Publication: N. Bolshakov, rf.atnn.ru
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