Attachment to NWT for testing LC circuits. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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NWT frequency response meters are widely used by radio amateurs. The desire to improve the accuracy of measuring the quality factor of the circuits with its help (compared to the simplest circuit solutions) led me to the idea of ​​making an attachment to the NWT in the form of a compact probe. Moreover, such that it would be possible to measure the resonant frequency, quality factor and frequency response of circuits with a sufficiently high accuracy - both separately taken and installed directly in the structures. Of course, in this case, it is necessary to ensure that the signal voltage on the circuit under study does not exceed the level of -20 dB on the frequency response graph so that silicon pn junctions do not open.

The appearance of the probe is shown in fig. 1, and its diagram is in Fig. 2. A high-resistance buffer amplifier with an input resistance of 1 MΩ and an input capacitance of approximately 2 pF is assembled on transistors VT1, VT3. The use of such a probe and design features are described in sufficient detail in B. Stepanov's article "A Simple Resonance Indicator", published in the collection "Radio Yearbook 1985". Compared with the device described there, the proposed probe version has better characteristics. The use of a more sensitive NWT detector made it possible to significantly (almost four times) reduce the capacitance of the coupling capacitors, which significantly reduced the influence of measuring circuits on the quality factor of the circuit under study. Due to this, the error in measuring the quality factor of the circuit (up to 400 ... 500) does not exceed 5 ... 10% at frequencies from hundreds of kilohertz to 30 MHz. The probe is connected to the investigated LC circuit, for example, using crocodile clips (see Fig. 1).

Rice. 1. Appearance of the probe

Rice. 2. Diagram of the probe

The input capacitance of such a probe can be about 2 pF, but in practice, with such values, the parasitic capacitance of the installation already noticeably affects. The high input impedance of the test probe made it necessary to shield it. On fig. 3 shows that without an external screen, at certain low levels, noise appears on the frequency response. Installing the probe in a shielding case almost completely removes interference and improves the "input - output" decoupling, but at the same time, the input capacitance increases to 4,9 ... 5 pF. With the input contacts of the probe closed, the isolation will be at least 62 dB at a frequency of 20 MHz.

Rice. 3. Frequency response graph

To improve the accuracy of measuring the real resonant frequency of circuits f (this is important, for example, when checking or adjusting the coupling of circuits), it is necessary to introduce a correction according to the formula given in the article by B. Stepanov, but instead of the number 3,5, substitute the number 2,5 into it. For this probe, it looks like this:

f=f_р(1+2,5/C),

where f_p - measured value of the resonant frequency of the circuit; C is the capacitance of the circuit capacitor in picofarads.

A photo of the probe design is shown in fig. 4. In order to exclude direct signal penetration to the detector input bypassing the circuit under test, double-sided foil fiberglass is used, and mounting is carried out on "patch" on both sides of the board.

Rice. 4. Probe design

Both sides of the common wire-screen are interconnected by jumpers in four to five places (evenly over the entire area of ​​the board). The connection points of the coupling capacitors are separated - the input of the high-resistance probe is on one side, and on the opposite side of the board there is a solid screen ("ground"). The soldering point of the NWT R1 output load resistor is located on the other side of the board, and opposite it on the opposite side is a solid screen ("ground"). Between the coupling capacitors, a screen made of thin tin is installed for almost their entire length. It is soldered to the board and covered with black electrical tape. When repeating the design, instead of this additional screen, I recommend simply making the board 10 ... 15 mm longer.

The high output stage current of the high-resistance probe buffer amplifier (approximately 30 mA) provides an output signal amplitude of up to 1,4 V into a low-resistance load (50 Ω). This allows you to maximize the dynamic range of the NWT detector. Setting up the amplifier comes down to installing a constant voltage of +2 ... 4 V on the collector of the transistor VT5. This is achieved by selecting the resistor R3. The current consumed by the probe from the power source is about 40 mA.

The real load for the circuit is created by the NWT generator with an output impedance of 50 ohms and a load resistor R1 with a resistance of 51 ohms connected in parallel to it (as a result, about 25 ohms). They are connected to the circuit under test through a 1 pF coupling capacitor C1.

You can estimate the degree of influence of this circuit on the quality factor of the circuit using the formulas given in the article by B. Stepanov. Anyone who wants to can look, for example, V. Popov's book "Fundamentals of the Theory of Circuits" (M.: Vysshaya Shkola, 1985), but the formulas given there are somewhat difficult to analyze and understand the physical meaning of what is happening.

It will be easier to understand the essence of what is happening if we use the concept of loss resistance. Total loop loss resistance R_п can be determined by the formula

R_п=X_L/Q_н,

where X_L - inductive resistance of its coil; Q_н - her kindness.

Loss resistance of the loaded circuit R_п equal to the sum of the resistances of own losses of an unloaded circuit R_к and losses introduced by the load R_н. The last for our case of turning on the resistance of a low-resistance signal source R_east through a capacitive current divider is equal to

R_н = R_east (C_St./(FROM_к+ C_vh))².

If the contour capacity C_к significantly larger than the input capacitance C_vh, this formula simplifies to

R_н = R_east (C_St./FROM_к)²,

the resistance introduced into the circuit decreases in proportion to the square of the ratio of the capacitances of the coupling and circuit capacitors.

Rice. 5. Frequency response graph

Consider a real example of measuring the parameters of an oscillatory circuit, which consists of a high-quality inductor wound on an Amidon T50-6 ring and a 38 pF capacitor.

1. Full circuit capacity

С_m = C_к+ C_vh\u43d XNUMX pF.

2. According to the frequency response graph (Fig. 5), we determine the resonant frequency f = 18,189 MHz and the quality factor Q_н\u237,76d XNUMX (albeit weakly, but still a loaded circuit).

3. Go to the "Radio Engineering Calculations" tab of the NWT program, enter the circuit capacitance and its resonant frequency into the cells of the table and find the coil inductance L = 1,78 μH. Its inductive reactance X_L= = 203,5 Ohm.

Thus, the loss resistance of the loaded circuit, calculated by the formula R_п = X_L/Q_н will be 0,86 ohm. Introduced by the load, the signal source, the loss resistance is found by the formula

R_н = R_east (C_St./(FROM_к+ C_vh))².

Substituting into it the known values ​​of the parameters of the elements, we obtain the value R_н\u0,0135d XNUMX Ohm. From here we find the loss resistance of the actual unloaded circuit R_к\u0,847d XNUMX Ohm and quality factor of an unloaded circuit Q_к= 240.

The directly measured quality factor, without these recalculations, is 237,76. As you can see, the measurement error due to the influence of a low-resistance signal source in our device is negligible and will be the smaller, the greater the capacitance of the circuit or the higher its characteristic impedance.

Author: Sergey Belenetsky (US5MSQ)

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