Easy setup of complex LC filters. Scheme, description

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Easy setup of complex LC filters. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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High-frequency LC filters are usually tuned by observing its frequency response on the screen of a curve tracer or oscilloscope equipped with a special attachment - a sweeping frequency generator, and achieving its shape to match the required one. But what if there are no mentioned devices in the radio amateur's laboratory?

As you know, LC filters, even the most complex ones, are combinations of ordinary series and parallel oscillatory circuits. If the values of the resonant frequency of each circuit are known from the description of the filter or found by calculation, then to adjust the filter as a whole, each circuit should be tuned to resonance separately. And for this it is enough to have a signal generator tunable in the desired frequency range, and a high-frequency voltmeter.

Easy setup of complex LC filters
Fig. 1

For tuning, each contour is temporarily separated from other filter elements. For example, the filter cell shown in Fig. 1 can be divided into two series (L1C1 and L3C3) and one parallel (L2C2) circuits. The resonant frequency of the circuit can be calculated using the well-known formula

where f0 - resonant frequency, Hz; L - inductance, H; C - capacity, F.

Considering that at its resonant frequency, the parallel oscillatory circuit has the maximum resistance, and the series circuit has the minimum, in the first case it is connected to the measuring instruments - the generator G1 and the voltmeter PV1 - according to the circuit shown in Fig. 2, and in the second - in Fig. 3. L1 and C1 - elements of a custom circuit.

Easy setup of complex LC filters
Fig. 2

Easy setup of complex LC filters
Fig. 3

The value of the resistor R1 is chosen ten or more times greater than the reactance of the loop capacitor or coil at the resonant frequency, equal to

otherwise, the minimum reading of the voltmeter, according to which the adjustment is made, will not be sharp enough. I most often used a resistor with a nominal value of 2 kOhm.

The generator is required with a low output impedance (almost all measuring signal generators satisfy this requirement) and the voltmeter with a large input impedance. In the absence of a sufficiently high-frequency voltmeter, it can be replaced by an oscilloscope. The input capacitance of the device, taking into account the connecting cable, must be many times less than the capacitance of the loop capacitor, otherwise it will noticeably change the resonant frequency of the loop.

To reduce the input capacitance, you can use an external probe or a 1:10 voltage divider, which is included with many high-frequency voltmeters and oscilloscopes. one high resistance resistor.

By tuning the generator to a frequency that is obviously different from the expected resonant frequency of the circuit, by changing the amplitude of the generator signal, by choosing the measurement limit of the voltmeter, the readings of the latter are obtained as large as possible, not exceeding, however, several hundred millivolts. Then, by changing the frequency of the generator, they find the one at which the readings of the PV1 device are minimal. This is the resonant frequency of the circuit.

Initially, of course, it differs from the required one. If to a greater side, you will have to increase the capacitance of the capacitor or the inductance of the coil (or the values of both values at the same time), if to a smaller one, reduce them. In cases where it is possible to smoothly change the capacitance or inductance (for example, the coil has a trimmer), it is more convenient to install on the generator a frequency equal to the required resonant one, and achieve a minimum voltmeter reading by rotating the trimmer with an insulating screwdriver.

In the range from audio frequencies to several megahertz, it is possible to tune LC filters of any complexity in the described way, and the accuracy of their tuning will be no worse than using a curve tracer or an oscilloscope with an attachment. At a higher frequency, parasitic capacitance and inductance of both connecting wires and and measuring instruments. Here, by adjusting the contours separately, you can only perform a preliminary adjustment of the filter, which will later have to be refined.

Author: S. Ilyenko, Mariupol, Ukraine; Publication: radioradar.net

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