DIY RF Inductors--The ABCs of inductors （7）

Published:2011/7/25 22:50:00 Author:Li xiao na From:SeekIC

By B. Kainka

Measurements

If you do not have a suitable signal generator or dip meter, the only way to measure the resonant frequency of a resonant circuit is to use an oscilloscope. The frequency can be measured by exciting the circuit so it oscillates freely (Figure 8).



This requires a steep-edged square-wave signal with a frequency well below the resonant frequency. Many oscilloscopes have a square-wave output for calibration purposes (usually 1 kHz). This should be coupled to the resonant circuit as loosely as possible via a small capacitor. With suitable adjustment of the time base, you will be able to observe free oscillations. Besides the resonant frequency, damping can also be measured in this manner.


Figure 9 shows a measurement made using the previously calculated resonant circuit. What is important is to affect the circuit as little as possible. This means you should use a 10:1 probe with an internal impedance of 10 MΩ. The figure shows the result of a measurement made with a horizontal deflection factor of 1 /μs/division. The measured frequency is slightly greater than 1100 kHz.

The Q factor can be determined from the number of oscillations required for the amplitude to drop to 0.37 (1/e) of the initial value. Here the Q factor is approximately 10.
The frequency and Q factor are affected by the measurement set-up. However, they will be affected even more strongly in an actual circuit.



A 10-MΩ coupling resistor can be used instead of a 10-pF coupling capacitor (Figure 10). In this case, the resulting oscillations will be somewhat weaker, so a more sensitive vertical scale must be selected. Using a resistor for coupling generates higher damping. However, the advantage of this approach is that it avoids any shift in the resonant frequency due to the coupling capacitor, so the frequency can be measured more accurately. In addition, measurements can also be made over a wide frequency range, from around 10 kHz to many megahertz.

This simple measurement technique can also be used to measure the values of unknown inductors. If you use a known capacitance value and measure the frequency, you can then determine the inductance. It is often necessary to try several different capacitors before you obtain readily measurable oscillations. With relatively large inductances, it is necessary to use correspondingly large capacitors. It is also possible to determine the value of an unknown capacitor using a known inductance.

The AL values of unknown cores can also be determined using this technique. To do this, wind a small test coil on the core and determine the resonant frequency with this coil connected to a known capacitor. From the number of turns and the inductance, you can then determine the approximate AL value.

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