This is a very accurate inductance/capacitance meter based on the PIC16F628A microcontroller. The idea is implemented on an example accurate inductance/capacitance meter .The design of the device is slightly different from similar devices found on the Internet. The goal of my hard work was to provide a simple solution that is easy to put together on the first try. Most designs of this type of device do not work as described in the documentation, or there is simply not enough reference information on them. The most difficult part of the project was to program all the floating point math code into the 2k program memory of the 16F628A microcontroller.
Typically, an inductance/capacitance meter is a frequency meter that includes an oscillator that oscillates and measures L or C, after which the final result is calculated. The frequency error is 1Hz. For more information on measuring frequency with timing devices, please refer to my article on digital frequency counter.
Theoretical information: Look carefully at the diagram; I didn't use a reed relay because I couldn't find one in my local radio market. So I decided to use a MOSFET first instead of a reed relay. But I got the best result with a regular NPN transistor like the BC547. If you don't trust transistors, then you can add a reed relay yourself. I used the controller's internal comparator for the oscillator and connected it to Timer1's external clock source to calculate the frequency. Thanks to this, it was not necessary to use an external operational amplifier Lm311. Relay RL1 was used to select the measurement mode L and C. The meter operates on the basis of four basic equations, which are presented below:
For both unknown quantities L and C, the equation 1 and 2 is usually applied. We average the values of F1 with an LC resonant circuit, then connect C cal in parallel to the resonant circuit and obtain the value F2.
Right after that,
- The capacitance requires F3 (Equation 3), leaving Cx parallel to the tank circuit, then calculate Cx from Equation 4
- The inductance requires F3 (equation 7), leaving Lx in series with the tank circuit, and c then calculates Lx from equation 8
Therefore, for both inductance and capacitance, equations 1, 2, and equations 5, 6 are the same.
After obtaining the approximate values of the inductance or capacitance, the program will automatically convert the values to engineering units, which will display on the LCD display with a resolution of 16x2.
If you find it difficult to master all the mathematical calculations, then it is better to leave them for a while and move on to the hardware. To begin, follow the calibration process which is explained in the next chapter.
Design:
The measurement accuracy depends on the condition of your components. The two 33pF capacitors in the generator should be tantalum (for low series resistances/inductances). Use C4, C5 (C cal) polystyrene type because the green capacitors have too much value deviation. Avoid using ceramic capacitors. Some of them have large attenuation.
- First, check that all components fit perfectly into their places on the board.
- Program the chip (16F628A) using the Hex file below on this page. If you don't have a programmer/loader then refer to my schematic. It is very easy to assemble it yourself.
- First apply power to the circuit without a chip, then check the voltage at pin 5, 14 of the IC pad with a voltmeter. If the voltage is 5V, then everything is fine.
- Place the chip in the IC socket and apply power. If the liquid crystal display has an increased contrast, then increase the value of the resistor R11 by a few kilo-ohms.
Calibration:
- Short the two test leads and apply power to the circuit. This will perform an automatic calibration. The device will enter the default mode - inductive mode. Allow a few minutes to "warm up", then press the "zero" button to perform a forced recalibration. The display should now show ind = 0.00 uH (uH)
- Now open the two test leads and connect a known inductance, such as 10 uH or 100 uH. The inductance/capacitance meter should read approximately the same value (error up to +/- 10% is allowed).
- After that, you need to adjust the meter to display the result with an error of about +/- 1%. To do this, check that there are 4 jumpers Jp1 ~ Jp4 in the circuit. Jumpers Jp1 and Jp2 are designed to increase (+) and decrease (-) values. To increase the value, first set Jp1 and follow steps 1-2, to decrease the value, set Jp2 and follow steps 1-2.
- If the display shows the required values, then remove the jumpers. After that, the chip will remember the calibration until you go to make changes again.
- If you still can't get the desired value, set the Jp3 jumper to see the value of F1. The display will show a value of about 503292 with an inductance of 100µH and a capacitance of 1nF. Or set the Jp4 jumper to see the value of F2. If nothing appears on the display, this means that your generator is not working properly. Check your board again.
List of radio elements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| U1 | Linear Regulator | LM7805 | 1 | To notepad | ||
| U3 | MK PIC 8-bit | PIC16F628A | 1 | To notepad | ||
| Q1, Q2 | bipolar transistor | BC547B | 2 | To notepad | ||
| D1, D3 | rectifier diode | 1N4001 | 2 | To notepad | ||
| C1, C2, C6, C7 | electrolytic capacitor | 10uF | 4 | To notepad | ||
| C3, C10 | Capacitor | 0.1uF | 2 | To notepad | ||
| C4, C5 | Capacitor | 1000 pF | 2 | To notepad | ||
| C8, C9 | Capacitor | 33 pF | 2 | To notepad | ||
| R1, R3, R4 | Resistor | 100 kOhm | 3 | To notepad | ||
| R2, R14, R15 | Resistor | 10 kOhm | 3 | To notepad | ||
| R5 | Resistor | 47 kOhm | 1 | To notepad | ||
| R6 | Resistor | 1.5 kOhm | 1 | To notepad | ||
| R7, R9-R12 | Resistor | 1 kOhm | 5 | To notepad | ||
| R8, R13 | Resistor | 560 ohm | 2 | To notepad | ||
| LCD1 | LCD display | 16x2 LCD | 1 | To notepad | ||
| X1 | Quartz resonator | 16 MHz | 1 | To notepad | ||
| RL1 | Relay | 5 V | 1 |
Devices for direct evaluation and comparison
Measuring instruments for direct assessment of the value of the measured capacitance include microfaradmeters, the action of which is based on the dependence of the current or voltage in the alternating current circuit on the value included in it. The capacitance value is determined on the scale of the pointer meter.
More widely used for measurement and inductances balanced AC bridges, allowing to obtain a small measurement error (up to 1%). The bridge is powered by generators operating at a fixed frequency of 400-1000 Hz. As indicators, rectifier or electronic millivoltmeters, as well as oscilloscope indicators, are used.
The measurement is made by balancing the bridge as a result of alternating adjustment of its two arms. The readings are taken from the limbs of the handles of those shoulders with which the bridge is balanced.
As an example, consider measuring bridges, which are the basis of the EZ-3 inductance meter (Fig. 1) and the E8-3 capacitance meter (Fig. 2).
Rice. 1. Diagram of a bridge for measuring inductance
Rice. Fig. 2. Bridge circuit for measuring capacitance with small (a) and large (b) losses
When the bridge is balanced (Fig. 1), the inductance of the coil and its quality factor are determined by the formulas Lx = R1R2C2; Qx = wR1C1.
When balancing bridges (Fig. 2), the measured capacitance and loss resistance are determined by the formulas
Measurement of capacitance and inductance using the ammeter-voltmeter method
To measure small capacitances (not more than 0.01 - 0.05 μF) and high-frequency inductors in the range of their operating frequencies, resonant methods are widely used. The resonant circuit usually includes a high-frequency generator, inductively or through a capacitance connected to the measuring LC circuit. As resonance indicators, sensitive high-frequency devices are used that respond to current or voltage.
Using the ammeter-voltmeter method, relatively large capacitances and inductances are measured when the measuring circuit is powered from a low-frequency source of 50 - 1000 Hz.
For measurements, you can use the diagrams in Fig. 3.
Figure 3. Schemes for measuring large (a) and small (b) resistances to alternating current
According to the readings of the instruments, the impedance
Where
from these expressions it is possible to determine
When active losses in a capacitor or inductor can be neglected, the circuit of Fig. 4. In this case
Rice. 4. Schemes for measuring large (a) and small (b) resistances using the ammeter - voltmeter method
Measuring the mutual inductance of two coils
The proposed prefix to the frequency meter for calculating the inductance in the range of 0.2 μH ... 4 H differs from the prototypes by a reduced voltage on the measured inductance (amplitude not more than 100 mV), which reduces the measurement error for coils on small-sized ring and closed magnetic circuits and makes it possible to measure with sufficient accuracy for practice, the initial magnetic permeability of the magnetic circuits. In addition, the low value of the voltage on the circuit allows you to evaluate the inductance of the coil directly in the structure, without dismantling.
For many novice radio amateurs, the manufacture and evaluation of the inductance of coils, chokes, transformers becomes a "stumbling block". Industrial meters are inaccessible, self-made finished designs, as a rule, are difficult to repeat, and industrial devices are required when setting them up. Therefore, simple attachments to a frequency meter or oscilloscope are especially popular.
Descriptions and diagrams of similar devices have been published in periodicals. They are easy to repeat and easy to use. But the information in the articles regarding the declared errors and measurement limits often leads to erroneous conclusions and distorted results. So it is indicated that the attachment allows you to measure the inductance of more than 0.1 μH, and the measurement error depends on the selection of the capacitor, which in the author's design has a permissible deviation of the nominal capacitance of no more than ±1%. And this despite the fact that on the transistors indicated in the diagram, stable generation begins with an inductance of the oscillatory circuit of 0.15 ... .14 uH. Another article indicates an error of up to 1.5% of the upper limit (by the way, note that the lower limit is 0.5 μH with an error of 0.9 μH - and this is true, in other words, the measurement of such quantities is an estimate) for both small and and large values of inductance, without taking into account the self-capacitance of the coils. And such a capacitance can reach a value commensurate with the contour value and introduce an additional error of up to 10 ... 20%.
This article attempts to fill in some way the noted gap and show methods for estimating measurement error and how to apply a really simple and useful design in the laboratory of every radio amateur.
The proposed prefix to the frequency meter is designed to evaluate and measure inductance with sufficient accuracy for practice in the range of 0.2 μH ... 4 H. It differs from the prototypes by a reduced voltage on the measured inductance (amplitude not more than 100 mV), which reduces the error in measuring the inductance on small-sized ring and closed magnetic cores and makes it possible to measure the initial magnetic permeability of the magnetic cores. In addition, the low value of the voltage on the circuit allows you to evaluate the inductance of the coil directly in the structure, without dismantling. This opportunity will be appreciated by those who often have to deal with the repair and adjustment of equipment in the absence of diagrams and descriptions.
To work with the prefix, any home-made or industrial frequency meters are suitable, allowing you to measure the frequency up to 3 MHz with an accuracy of at least 3 digits. If you don't have a frequency counter, an oscilloscope will do. The accuracy of measuring the time parameters of the latter, as a rule, is of the order of 7 ... 10%, which will determine the measurement error of the inductance.
Principle of inductance measurement is based on a well-known relationship that connects the parameters of the elements of the oscillatory circuit with the frequency of its resonance (Thomson's formula)
Here and below, in all formulas, the frequency is indicated in megahertz, the capacitance is in picofarads, and the inductance is in microhenries.
With a circuit capacitance Sk = 25330 pF, the formula is simplified
, where T is the period in microseconds.
In the console (its diagram is shown in rice. 1) an emitter-coupled oscillator is used in
a two-stage amplifier, the frequency of harmonic oscillations of which is determined by the capacitance of the capacitor C1 and the measured inductance Lx, connected to the spring clamps X1. Since the base of the transistor VT1 is directly connected to the collector VT2, the loop gain of the generator is high, which ensures stable generation when the L / C ratio changes over a wide range. The loop gain is proportional to the steepness of the transistors used and can be effectively controlled by changing the emitter current, for which a rectifier based on diodes VD1, VD2 and a control transistor VT3 are used. The introduction of an amplifier based on a VT4 transistor with KU = 8 ... 9 made it possible to reduce the voltage amplitude on the circuit to the level of 80 ... 90 mV at an output amplitude of 0.7 V. The emitter follower provides operation for a low-resistance load.
The device is operational when the supply voltage changes in the range of 5 ... 15 V, while the variations in the output voltage level do not exceed 20%, and the frequency drift is F = 168.5 kHz (with a high quality coil wound on a 50 VCh core with an inductance L = 35 μH) no more than 40 Hz!
In design you can use in positions VT1, VT2 transistors KT361B, KT361G, KT 3107 with any letter index, although somewhat better results are achieved with KT326B, KT363; in position VT3 - silicon transistors of p-n-p structure, for example, KT209V, KT361B, KT361G, KT3107 with any letter index. For the buffer amplifier (VT4, VT5), most high-frequency transistors are suitable. The parameter h21Э for the transistor VT4 is more than 150, for the rest it is not less than 50.
Diodes VD, VD2 - any high-frequency silicon, for example, series KD503, KD509, KD521, KD522.
Resistors - MLT-0.125 or similar. Capacitors, except for C1, are small-sized ceramic and electrolytic, respectively, with a spread of 1.5 ... 2 times.
Capacitor C1 with a capacity of 25330 pF determines the measurement accuracy, therefore it is advisable to choose its value with a deviation of no more than ±1% (it can be made up of several thermostable capacitors, for example 10000 + 10000 + 5100 + 220pF from the KSO, K31 group. If it is not possible to accurately select the capacitance, you can use the method described below.
It is convenient to use spring clips for "acoustic" cables as the X1 connector. Connector X3 for connection with a frequency meter - SR-50-73F.
Parts are mounted on a printed circuit board ( rice. 2) from one-sided foil fiberglass. 
A drawing of a printed circuit board in lay format developed by P. Semin can be
It is permissible to use hanging mounting. As a case for the set-top box, you can use any suitable-sized box made of any material. It is necessary to place the X1 connector in such a way as to ensure the minimum length of the conductors connecting it to the board. The photo, for example, shows a neatly executed design from Pavel Semin. 
After checking the correct installation, apply 12 V power supply without connecting the coils to the X1 connector. The voltage at the emitter VT5 should be approximately equal to half the supply voltage; if the deviation is greater, the selection of the resistor R4 will be required. The current consumption will be close to 20 mA. Connect an Lx coil with an inductance in the range of tens to hundreds of microhenries (the exact value is not critical) to connector X1, and an oscilloscope or a high-frequency voltmeter to connector X3. The output of the set-top box should have an alternating voltage of 0.45 ... 0.5 V eff (peak value 0.65 ... 0.7 V). If necessary, its level can be set in the range of 0.25 ... 0.7 Veff by selecting the resistor R8.
Now you can start set-top box calibration by connecting it to the frequency meter.
This can be done in several ways.
If it is possible to measure with an accuracy of at least 1% a coil on an open magnetic circuit with an inductance of the order of tens or hundreds of μH, then using it as a model, select the capacitance of the capacitors C1 so that the readings of the prefix coincide with the required value.
In the second case, you will need one thermally stable reference capacitor, the capacitance of which is at least 1000 pF and is known with high accuracy. In the extreme case, if it is not possible to accurately measure the capacitance, capacitors KSO, K31 can be used with a tolerance of ± 2–5%, resigned to the probable increase in the error. The author used a K31-17 capacitor with a nominal capacitance of 5970 pF ± 0.5%. First, using the frequency meter, we fix the frequency F1 for the Lx coil without an additional external capacitor. Then we connect the reference capacitor Cet in parallel with the coil and fix the frequency F2. Now we can determine the real input capacitance of the assembled set-top box and the inductance of the coil Lx using the formulas
In order to be able to use the simplified formulas given at the beginning of the article, it is necessary, by selecting a group of capacitors C1, to set the capacitance Cv equal to 25330 ± 250 pF. After the final adjustment of the capacitance of the capacitors C1, take a control measurement according to the above method to make sure that the capacitance C in corresponds to the required one. It takes a long time to manually make multiple recalculations, so the author uses a successful calculation program MIX10
developed by A. Bespalchik.
After that, the prefix is ready to work. Let's try to evaluate its capabilities; To do this, we will conduct several experiments.
- When measuring small inductance values, a large error is introduced by the set-top box's own inductance, which consists of the inductance of the conductors connecting the X1 connector to the board, and the mounting inductance. Let's try to measure it. First, we close the contacts of connector X1 with a straight short conductor. The twisted wires going to connector X1, 30 mm long, and the jumper, 30 mm long, form one turn of the coil. If there are KT326B transistors in the generator, oscillations occur only during shock excitation of the circuit by periodically turning on the power; in this case, the frequency F1 = 2.675 ... 2.73 MHz, which corresponds to an inductance of 0.14 μH (no generation occurs at all with KT3107B transistors). Now we will make a ring with a diameter of 3 from a wire with a diameter of 0.5 mm with a calculated inductance of about 0.08 μH and connect it to X1. For a generator based on KT326B transistors, the frequency meter showed a value of 2.310 MHz, which corresponds to an inductance of 0.19 μH. The variant on KT3107B transistors generated only with shock excitation of the circuit. Thus, the own inductance of the attachment turned out to be in the range of 0.1 ... 0.14 μH.
Conclusions: high measurement accuracy is provided for inductances over 5 µH. With values in the range of 0.5 ... 5 μH, it is necessary to take into account the intrinsic inductance of 0.1 ... 0.14 μH. With an inductance of less than 0.5 μH, the measurements are estimated. Confidently recorded minimum inductance value of 0.2 μH.
- Measurement of unknown inductance. Let's say for it the frequency F1 \u003d 0.16803 MHz, which, according to the simplified formula for calculating the inductance, gives 35.42 μH.
When checking with a reference capacitor, the frequency F2 = 0.15129 MHz corresponds to an inductance of 35.09 μH. The error is less than 1%.
- Using the measured inductance as a reference, you can estimate the input capacitance of the generator. The capacitance of the circuit consists of the capacitance of a group of capacitors C1 and the capacitance Cgen, consisting of the sum of the mounting capacitance and the capacitance introduced by the transistors VT1, VT2, i.e. Cv = C1 + C gene.
To determine the value of C gene, turn off the capacitors C1 and measure the frequency F3 with the inductance used. Now Cgen can be calculated by the formula 
In the author's version of the attachment with KT3107B transistors, the capacitance Cgen is 85 pF, and with KT326B transistors it is 39 pF. Compared to the required value of 25330 pF, this is less than 0.4%, which allows the use of almost any high-frequency transistors without a noticeable effect on measurement accuracy.
- Due to the large self-capacitance of the attachment, when measuring inductance up to 0.1 H, the error introduced by the self-capacitance of the coils is insignificant. So, when measuring the inductance of the primary winding of the output transformer from transistor receivers, the value L = 105.6 mH was obtained. When the oscillatory circuit was supplemented with a reference capacitor of 5970 pF, another value was obtained - L = 102 mH, and the self-capacitance of the winding Str = Cmeas - C1 = 25822 - 25330 = 392 pF.
- The amplitude on the measuring oscillatory circuit of 70 ... 80 mV turns out to be less than the threshold for opening silicon p-n junctions, which in many cases makes it possible to measure the inductance of coils and transformers directly in the circuit (of course, de-energized). Due to the large self-capacitance of the attachment (25330 pF), if the capacitance in the measured circuit is not more than 1200 pF, the measurement error will not exceed 5%.
So, when measuring the inductance of the coil of the IF circuit (the circuit capacitance is not more than 1000 pF), a value of 92.1 μH was obtained directly on the transistor receiver board. When measuring the inductance of a coil soldered from the board, the calculated value turned out to be less - 88.7 μH (error less than 4%).
To connect to the inductors placed on the boards, the author uses probes with connecting wires 30 cm long, twisted in increments of one twist per centimeter. They introduce an additional inductance of 0.5 ... 0.6 μH - this is important to know when measuring small quantities, to evaluate it, it is enough to close the probes together.
In conclusion, a few more useful tips.
You can determine the magnetic permeability of an annular magnetic circuit without marking using the following method. Wind 10 turns of wire, evenly distributing it around the ring, and measure the inductance of the winding, and substitute the resulting inductance value into the formula: 
In practical calculations, it is convenient to use a simplified formula for calculating the number of turns on ring magnetic circuits 
The values of the coefficient k for a number of widespread ring magnetic circuits according to V. T. Polyakov are given in tab. 1.
Table 1
| Size | К18х8х4 | К18х8х4 | К18х8х4 | К18х8х4 | К18х8х4 | К18х8х4 |
| Magnetic permeability | 3000 | 2000 | 1000 | 2000 | 1000 | 400 |
| k | 21 | 26 | 37 | 31 | 44 | 70 |
For widespread armored magnetic circuits made of carbonyl iron, it is more convenient to calculate the inductance in microhenry, so we introduce the coefficient m, and the formula will change accordingly: 
Some values for common armored magnetic circuits are given in tab. 2.
| Core | SB-9a | SB-12a | SB-23-17a | SB23-11a |
| m | 7.1 | 6.7 | 4.5 | 4.0 |
To compile a similar table for your existing ring and armored magnetic circuits, using the proposed prefix, will not be difficult.
LITERATURE
1.Hyduk P. The frequency meter measures the inductance. - Radio amateur, 1996, No. 6, p. thirty.
- L-meter with linear scale. - Radio, 1984, No. 5, p. 58, 61.
- Poliakov V. Coils of inductance. - Radio, 2003, No. 1, p. 53.
- Poliakov V. Radio amateurs about the direct conversion technique. - M.: Patriot, 1990, p. 137, 138.
- Semiconductor receiving-amplifying devices: Handbook of a radio amateur. / Tereshchuk R. M. and others / ― Kyiv: Naukova Dumka, 1987, p. 104.
S. Belenetsky, US5MSQ Lugansk Ukraine Radio, 2005, No. 5, p.26-28
You can discuss the article, express your opinion and suggestions on forum
The main parameter characterizing the loop coils, chokes, transformer windings is the inductance L. In high-frequency circuits, coils with inductance from hundredths of a microhenry to tens of millihenries are used; coils used in low frequency circuits have inductances up to hundreds and thousands of henries. It is desirable to measure the inductance of high-frequency coils that are part of oscillatory systems with an error of no more than 5%; in most other cases, a measurement error of up to 10-20% is acceptable.
Rice. 1. Equivalent circuits of the inductor.
Each coil, in addition to the inductance L, is also characterized by its own (interturn) capacitance C L and active loss resistance R L distributed along its length. Conventionally, it is believed that L, C L and R L are concentrated and form a closed oscillatory circuit (Fig. 1, a) with a natural resonant frequency
f L = 1/(LC L) 0.5
Due to the influence of capacitance C L, when measuring at a high frequency f, it is not the true inductance L that is determined, but the effective, or dynamic, value of the inductance
L d \u003d L / (1-(2 * π * f) 2 * LC L) \u003d L / (1-f 2 / f L 2)
which may differ markedly from the inductance L measured at low frequencies.
With increasing frequency, losses in the inductors increase due to the surface effect, energy radiation, displacement currents in the winding insulation and frame, and eddy currents in the core. Therefore, the active resistance R d of the coil can significantly exceed its resistance R L, measured with an ohmmeter or a DC bridge. The quality factor of the coil also depends on the frequency f:
Q L \u003d 2 * π * f * L d / R d.
On fig. 1, b, shows the equivalent circuit of the inductor, taking into account its operating parameters. Since the values of all parameters depend on the frequency, it is desirable to test coils, especially high-frequency coils, at a power source oscillation frequency corresponding to their operating mode. When determining the test results, the index "d" is usually omitted.
To measure the parameters of inductors, the methods of a voltmeter - ammeter, bridge and resonance are mainly used. Before measurements, the inductor must be checked for the absence of an open circuit and short-circuited turns in it. An open is easily detected with any ohmmeter or probe, while short circuit detection requires a special test.
For the simplest tests of inductors, cathode-beam oscilloscopes are sometimes used.
Indication of shorted coils
A check for the absence of a short circuit is most often carried out by placing the coil under test near another coil, which is part of the oscillatory circuit of the autogenerator, the presence of oscillations in which and their level are controlled using telephones, a pointer, electronic light or other indicator. A coil with short-circuited turns will introduce active losses and reactance into the circuit associated with it, reducing the quality factor and the effective inductance of the circuit; as a result, the oscillations of the self-oscillator will be weakened or even disrupted.
Rice. 2. Scheme of a resonant capacitance meter using the absorption phenomenon.
A sensitive device of this type can be, for example, a generator made according to the circuit in Fig. 2. A coil with short-circuited turns, brought to the loop coil L1, will cause a noticeable increase in the readings of the microammeter μA.
The test circuit may be a series circuit tuned to the frequency of the power supply (see "Radio", 72-5-54); the voltage on the elements of this circuit, controlled by some indicator, under the influence of short-circuited turns of the tested coil will decrease due to detuning and increasing losses. It is also possible to use a balanced AC bridge, one of the arms of which in this case should be the coupling coil (instead of the L x coil); short-circuited turns of the coils under test will cause the bridge to be unbalanced.
The sensitivity of the test device depends on the degree of connection between the coil of the measuring circuit and the coil under test, in order to increase it, it is desirable to put both coils on a common core, which in this case is open.
In the absence of special devices for testing high-frequency coils, you can use a radio receiver. The latter is tuned to some well-audible station, after which the tested coil is placed near one of its active loop coils, for example, a magnetic antenna (preferably on the same axis with it). In the presence of short-circuited turns, the volume will noticeably decrease. A decrease in volume can also occur if the tuning frequency of the receiver is close to the natural frequency of the coil under test. Therefore, in order to avoid error, the test should be repeated when tuning the receiver to another station, sufficiently distant from the first in frequency.
Measurement of inductances using the voltmeter - ammeter method
Voltmeter - ammeter method used to measure relatively large inductances when the measuring circuit is powered by a low frequency source F = 50...1000 Hz.
The measurement scheme is shown in fig. 3, A. The impedance Z of the inductor is calculated by the formula
Z = (R2+X2) 0.5 = U/I
based on AC instrument readings V ~ and mA ~ . The upper (according to the diagram) output of the voltmeter is connected to the point A at Z<< Z в и к точке b at Z >> Z a, where Z in and Z a are the input impedances, respectively, of the voltmeter V ~ and milliammeter mA ~. If the losses are small, i.e. R<< X = 2*π*F*L x , то измеряемая индуктивность определяется формулой
L x ≈ U/(2*π*F*I).
Coils of high inductance in order to reduce their dimensions are usually made with steel cores. The presence of the latter leads to a nonlinear dependence of the magnetic flux on the current flowing through the coil. This dependence becomes especially difficult for coils operating with bias, through the windings of which flow both alternating and direct currents. Therefore, the inductance of coils with steel cores depends on the value and nature of the current flowing through them. For example, with a large constant current component, magnetic saturation of the core occurs and the inductance of the coil decreases sharply. In addition, the permeability of the core and the inductance of the coil depend on the frequency of the alternating current. It follows that the measurement of the inductance of coils with steel cores must be carried out under conditions close to their operating mode. In the diagram in fig. 3, A this is ensured by supplementing it with a DC circuit shown by a dashed line. The required bias current is set by the rheostat R2 according to the readings of the DC milliammeter mA. The separating capacitor C and the inductor Dr separate the DC and AC power circuits, eliminating the mutual influence between them. AC devices used in this circuit should not respond to the constant components of the current or voltage they measure; for a voltmeter V ~ this is easily achieved by connecting a capacitor in series with it with a capacity of several microfarads.
Rice. 3. Schemes for measuring inductance using a voltmeter - ammeter method.
Another variant of the measuring circuit, which makes it possible to dispense with an alternating current milliammeter, is shown in Fig. 3, b. In this circuit, rheostats R1 and R2 (they can be replaced by potentiometers connected in parallel with power sources) set the required test mode for AC and DC. In switch position 1 IN voltmeter V ~ measures the alternating voltage U 1 on the coil L x. When the switch is moved to position 2, the value of the alternating current in the circuit is actually controlled by the voltage drop U 2 across the reference resistor R o. If the losses in the coil are small, i.e. R<< 2*π*F*L x , то измеряемую индуктивность можно рассчитать по формуле
L x ≈ U1*R o /(2*π*F*U 2).
Bridge method for measuring the parameters of inductors. Universal measuring bridges
Bridges designed to measure the parameters of inductors are formed from two arms of active resistance, an arm with a measurement object, the resistance of which is generally complex, and an arm with a reactive element - a capacitor or an inductor.
Rice. 4. Scheme of a magazine bridge for measuring inductances and loss resistances.
In measuring bridges of the magazine type, it is preferable to use capacitors as reactive elements, since in the latter the energy losses can be made negligible, and this contributes to a more accurate determination of the parameters of the coils under study. A diagram of such a bridge is shown in Fig. 4. The regulated element here is a capacitor C2 of variable capacity (or a store of capacities), shunted by a variable resistor R2; the latter serves to balance the phase shift created by the loss resistance R x in the coil with the inductance L x . Applying the amplitude equilibrium condition (Z 4 Z 2 = Z 1 Z 3), we find:
(R x 2 + (2*&pi*F*L x) 2) 0.5: ((1/R 2) 2 + (2*&pi*F*C 2) 2) 0.5 = R 1 R 3 .
Since the phase angles φ1 = φ3 = 0, the phase equilibrium condition (φ4 + φ2 = φ1 + φ3) can be written as an equality φ4 + φ2 = 0, or φ4 = -φ2, or tg φ4 = -tg φ2. Taking into account that the formula (tg φ =X/R) is valid for the arm with L x, and the formula (tg φ =R/X) for the arm with capacity C 2 with a negative value of the angle φ2, we have
2*&pi*F*L x / R x = 2*&pi*F*C 2 R 2
Solving together the above equations, we get:
L x = C 2 R 1 R 3 ; (1)
R x \u003d R 1 R 3 / R 2. (2)
It follows from the last formulas that the capacitor C2 and the resistor R2 can have scales for directly assessing the values of L x and R x, and the amplitude and phase adjustments made by them are mutually independent, which allows you to quickly balance the bridge.
To expand the range of measured values, one of the resistors R1 or R3 is usually made in the form of a resistance box.
If it is necessary to measure the parameters of coils with steel cores, the bridge diagram in fig. 4 is supplemented by a constant voltage source U o, a rheostat R o and a DC milliammeter mA, which serve to adjust and control the bias current, as well as the choke Dr and capacitor C, separating the circuits of the variable and constant components of the current.
Rice. 5. Scheme of a magazine bridge for measuring inductances and quality factors
On fig. 5 shows a diagram of another version of the store bridge, in which the capacitor C2 has a constant capacitance, and the resistors R1 and R2 are taken as variables. The measurement range is extended by including resistors R3 of various ratings in the bridge. From formulas (1) and (2) it follows that the adjustments of the amplitudes and phases in this circuit are interdependent, therefore, the balancing of the bridge is achieved by alternately changing the resistances of the resistors R1 and R2. The inductance L x is evaluated on the scale of the resistor R1, taking into account the multiplier determined by the setting of the switch IN. The reading on the scale of the resistor R2 is usually made in the values of the quality factor of the coils
Q L \u003d 2 * π * F * L x / R x \u003d 2 * π * F * C 2 R 2.
at frequency F of the power supply. The validity of the last formula can be verified if the left and right parts of equality (1) are divided into the corresponding parts of equality (2).
With the data indicated on the diagram, the measuring bridge allows you to measure inductances from about 20 μH to 1, 10, 100 mH; 1 and 10 H (without steel cores) and quality factor up to Q L ≈ 60. The power source is a transistor generator with an oscillation frequency F ≈ 1 kHz. The unbalance voltage is amplified by a transistor amplifier loaded on Tf telephones. Double T-shaped RC filter, tuned to 2F ≈ 2 kHz, suppresses the second harmonic of the source oscillations, which makes it easier to balance the bridge and reduce measurement error.
Bridge meters of inductances, capacitances and active resistances have a number of identical elements. Therefore, they are often combined in one device - a universal measuring bridge. High-precision universal bridges are based on magazine circuits such as those shown in fig. 5. They contain a constant voltage source or rectifier (feeding the R x measurement circuit), a low-frequency generator with an output power of several watts, a multi-stage unbalance voltage amplifier loaded on a magnetoelectric galvanometer; the latter, when measuring active resistances, is included directly in the measuring diagonal of the bridge. The required measurement scheme is formed using a rather complex switching system. In such bridges, indicators of the logarithmic type are sometimes used, the sensitivity of which drops sharply if the bridge is not balanced.
Rice. 6. Scheme of a universal reochord bridge for measuring resistances, capacitances and inductances
Much simpler are universal bridges of the reochord type, which measure the parameters of radio components with an error of the order of 5-15%. A possible scheme of such a bridge is shown in Fig. 6. The bridge is powered for all types of measurements with a voltage with a frequency of approximately 1 kHz, which is excited by a transistor generator, made according to the inductive three-point circuit. The balance indicator is a high-resistance phone Tf. Resistors R2 and R3 have been replaced by a wire rheochord (or, more commonly, a conventional potentiometer), which allows the bridge to be balanced by a smooth change in the ratio of resistances R2 / R3. This ratio is measured on the reochord scale, the range of indications of which is usually limited to the extreme values \u200b\u200bof 0.1 and 10. The measured value is determined with a balanced bridge as the product of the reading on the reochord scale and the multiplier determined by setting switch B. Each type and measurement limit corresponds to the inclusion in the bridge circuit the corresponding supporting element of the required rating - a capacitor C o (C1), a resistor R o (R4) or an inductor L o (L4).
A feature of the scheme under consideration is that the measured elements R x and L x are included in the first arm of the bridge (with the support elements R o and L o located in the fourth arm), and C x, on the contrary, in the fourth arm (with C o - on the first shoulder). Due to this, the assessment of all measured quantities is carried out according to similar formulas of the type
A X \u003d A o (R2 / R3),
where A x and A o are the values of the corresponding measured and reference elements.
The variable resistor R5 serves to compensate for phase shifts and improve the balance of the bridge when measuring inductances. For the same purpose, a variable resistor of small resistance is sometimes included in the circuit of the reference capacitor C on the measurement limit of large capacities, which often have noticeable losses.
In order to exclude the influence of the operator's hand, the slider of the reochord is usually connected to the body of the device.
Resonant inductance meters
Resonance methods make it possible to measure the parameters of high-frequency inductors in the range of their operating frequencies. Schemes and measurement methods are similar to those used in resonant measurements of capacitor capacitances, taking into account, of course, the specifics of measurement objects.
Rice. 7. Resonance scheme for measuring inductances with a reading on the generator scale
The investigated inductor can be included in a high-frequency generator as an element of its oscillatory circuit; In this case, the inductance L x is determined on the basis of the readings of the frequency meter, which measures the oscillation frequency of the generator.
More often, the L x coil is connected to a measuring circuit associated with a source of high-frequency oscillations, for example, a generator (Fig. 2) or the input circuit of a radio receiver tuned to the frequency of a broadcasting station (Fig. 8). Let us assume that the measuring circuit consists of a coupling coil L with a tuning core and a variable capacitor C o.
Rice. 8. Scheme for measuring capacitances by the resonant method using a radio receiver
Then the following measurement procedure is applicable. The measuring circuit at the maximum capacitance C o1 of the capacitor C o by adjusting the inductance L is tuned into resonance with a known frequency f of the oscillation source. Then, a coil L x is included in the circuit in series with its elements, after which the resonance is restored by reducing the capacitance Co to a certain value C o2. The measured inductance is calculated by the formula
L x \u003d * (C o1 -C o2) / (C o1 C o2).
In wide-range resonant meters, the measuring circuit is made up of a reference capacitor C o and the investigated coil L x . The circuit is connected inductively, and more often through a capacitor C 1 of a small capacity (Fig. 7 and 9) with a high-frequency generator. If the oscillation frequency of the generator f 0 is known, corresponding to the resonant tuning of the circuit, then the measured inductance is determined by the formula
L x \u003d 1 / [(2 * π * f o) 2 * C o]. (3)
There are two options for constructing measuring circuits. In the circuits of the first variant (Fig. 7), the capacitor C is taken with a constant capacitance, and the resonance is achieved by changing the tuning of the generator operating in a smooth frequency range. Each value of L x corresponds to a certain resonant frequency
f 0 \u003d 1 / (2 * π * (L x C x) 0.5), (4)
therefore, the generator loop capacitor can be provided with a scale with a reading in L x values. With a wide range of measured inductances, the generator must have several frequency subranges with separate scales for estimating L x on each subrange. If the device uses a generator that has a frequency scale, then tables or graphs can be compiled to determine L x from the values of f 0 and C o.
To exclude the influence of the self-capacitance C L of the coil on the measurement results, the capacitance C o must be large; on the other hand, it is desirable to have a small capacitance C o in order to provide a sufficiently large ratio L x / C o when measuring small inductances, which is necessary to obtain noticeable indicator readings at resonance. In practice, they take C o \u003d 500 ... 1000 pF.
If the high-frequency generator operates in a limited frequency range that is not divided into sub-ranges, then several switched capacitors C o are used to expand the limits of measuring inductances; if their capacitances differ by a factor of 10, then L x can be estimated at all limits using the same generator scale using multipliers to it that are multiples of 10. However, such a scheme has significant drawbacks.
The measurement of relatively large inductances with a significant intrinsic capacitance C L occurs at the limit with a small capacitance C o, and, conversely, the measurement of small inductances is performed at the limit with a large capacitance C o with an unfavorable ratio L x / C o and a small resonant voltage on the circuit.
Rice. 9. Resonance circuit for measuring inductances with reference to the scale of the reference capacitor
In resonant meters, the circuits of which are made according to the second option (Fig. 9), inductances are measured at a fixed generator frequency f 0 . The measuring circuit is tuned to resonance with the frequency of the generator using a variable capacitor C o, the reading on the scale of which, in accordance with formula (3), can be performed directly in the values of L x . If we designate through C m and C n, respectively, the maximum and initial capacitance of the circuit, and through L m and L n - the maximum and smallest values of the measured inductances, then the measurement limits of the device will be limited by the ratio
L m / L n \u003d C m / C n.
Typical variable capacitors have a capacitance overlap of approximately 30. In order to reduce the error when measuring large inductances, the initial capacitance C n of the circuit is increased by including an additional capacitor C d, usually of a tuning type, in the circuit.
If we denote by ΔС o the largest change in the capacitance of the capacitor C o, equal to the difference in its capacitances at the two extreme positions of the rotor, then to obtain the selected ratio L m / L n, the circuit must have an initial capacity
C n \u003d ΔC o: (L m / L n -1). (5)
For example, at ΔC o = 480 pF and the ratio L m / L n = 11, we get C n = 48 pF. If the values of C n and L m / L n in the calculation are the initial data, then it is necessary to use a capacitor C o, which has a capacitance difference
ΔC o ≥ C n (L m / L n -1).
For large values of C n and L m / L n, it may be necessary to use a double or triple block of variable capacitors.
The frequency f 0 at which the generator must operate is determined by formula (4) when the values of L m and C n or L n and C m are substituted into it. To expand the overall measurement range, the generator is operated at several switched fixed frequencies. If the neighboring frequencies of the generator differ by 10 0.5 ≈ 3.16 times, then at all limits it is possible to use the common scale of capacitor inductances C o with multipliers to it, multiples of 10 and determined by setting the frequency switch (Fig. 9). A smooth overlap of the entire range of measured inductances is provided with a ratio of the capacitances of the circuit C m / C n ≥ 10. If the capacitor C about is a logarithmic type, then the inductance scale is close to linear.
Instead of a fixed frequency generator, you can use a measuring generator with a smooth change in frequency, which is set depending on the required limit for measuring inductances.
Resonant circuits for measuring inductances and capacitances are often combined in one device, since they have a number of identical elements and a similar measurement technique.
Example. Calculate a resonant inductance meter operating according to the circuit in fig. 9, for a measurement range of 0.1 μH - 10 mH when using a dual block of variable capacitors, the capacity of the sections of which can be changed from 15 to 415 pF.
Solution
1. The greatest change in the capacitance of the circuit ΔС о \u003d 2 * (415-15) \u003d 800 pF.
2. We choose the ratio L m / L n = 11. Then the device will have five measurement limits: 0.1-1.1; 1-11; 10-110; 100-1100mcg and 1-11mH.
3. According to (5), the circuit must have an initial capacitance C n \u003d 800/10 \u003d 80 pF. Given the initial capacitance of the block of capacitors, equal to 30 pF, we include a tuning capacitor C d in the circuit with a maximum capacitance of 50 ... 80 pF.
4. The maximum capacity of the circuit C m \u003d C n + ΔC o \u003d 880 pF.
5. According to (4), at the first measurement limit, the generator must operate at a frequency
f 01 \u003d 1 / (2 * π * (L n C m) 0.5) ≈ 0.16 * (0.1 * 10 ^ -6 * 880 * 10 ^ -12) ≈ 17 MHz.
For other measurement limits, we find, respectively: f 02 = 5.36 MHz; f 03 = 1.7 MHz; f 04 = 536 kHz; f 05 = 170 kHz.
6. We perform the inductance scale for the measurement limit of 1-11 μH.
Q-meters (kumeters)
Devices designed to measure the quality factor of elements of high-frequency circuits are often called kumeters. The action of the meters is based on the use of resonance phenomena, which allows the measurement of the quality factor to be combined with the measurement of inductance, capacitance, natural resonant frequency and a number of other parameters of the elements under test.
Kumeter, a simplified diagram of which is shown in fig. 10 contains three main components: a high frequency generator, a measuring circuit and a resonance indicator. The generator operates in a wide, smoothly overlapping frequency range, for example from 50 kHz to 50 MHz; this allows many measurements to be carried out at the operating frequency of the elements under test.
The investigated inductor L x , R x through clamps 1 and 2 is included in the measuring circuit in series with the variable reference capacitor C o and the coupling capacitor C 2 ; the capacitance of the latter must satisfy the condition: C 2 >> C o.m, where C o.m is the maximum capacitance of the capacitor C o. Through a capacitive divider C 1, C 2 with a large division factor
N \u003d (C 2 + C 1) / C 1
a reference voltage U about the required high frequency f is introduced into the circuit from the generator. The current arising in the circuit creates a voltage drop U C on the capacitor C o, which is measured by a high-frequency voltmeter V2.
The input resistance of the voltmeter V2 within the operating frequencies of the meter must be very large. With a sufficiently high sensitivity, the voltmeter is connected to the measuring circuit through a capacitive voltage divider, the input capacitance of which is taken into account as a component of the initial capacitance of the capacitor C o. Since all the capacitors that make up the measuring circuit have very low losses, it can be assumed that the active resistance of the circuit is mainly determined by the loss resistance Rx of the coil under study.
Rice. 10. Simplified scheme of the kumeter
By changing the capacitance of the capacitor C o, the measuring circuit is tuned into resonance with the generator frequency f according to the maximum readings of the voltmeter V2. In this case, a current will flow in the circuit I p ≈ U o / R x, creating a voltage drop on the capacitor
U C \u003d I p / (2 * π * f * C o) ≈ U o / (2 * π * f * C o R x).
Considering that at resonance 1/(2*π*f*С o) = 2*&pi*f*L x , we find
UC ≈ U o (2*π*f*L x)/R x = U o Q L ,
where Q L \u003d (2 * π * f * L x) / R x is the quality factor of the coil L x at a frequency f. Therefore, the readings of the voltmeter V2 are proportional to the quality factor Q L. At a fixed voltage U o, the scale of the voltmeter can be linearly graduated in the values Q L ≈ U C / U o. For example, with U o \u003d 0.04 V and the measurement limit of the voltmeter U p \u003d 10 V, the voltages at the input of the voltmeter 2, 4, 6, 8 and 10 V will correspond to a quality factor Q L equal to 50, 100, 150, 200 and 250.
The rated voltage U about is set by adjusting the mode of the output stage of the generator. The control of this voltage is carried out according to the readings of a high-frequency voltmeter V1, which measures the voltage U 1 \u003d U about N at the output of the generator. For example, if the quality scale of the voltmeter V2 is made at a voltage Uо = 0.04 V, and the division factor N = 20, then the voltage U x = 0.04 * 20 = 0.8 V should be maintained at the output of the generator. The measurement limit of the voltmeter V1 should slightly exceed the calculated voltage value U 1 and is equal, for example, to 1 V.
An increase in the upper limit of measuring the quality factors is achieved by reducing the voltage U about to a value several times less than the nominal value. Suppose that at a voltage U o \u003d 0.04 V, a direct reading of the quality factors to the value Q L \u003d 250 is provided. If, however, the voltage U o is reduced by half, to 0.02 V, then the needle of the voltmeter V2 will deviate to the full scale with a quality factor Q L = U p / U o = 10 / 0.02 = 500. Accordingly, to increase the upper limit of measurements by four times, to the value of Q L = 1000, measurements should be carried out at a voltage of U o = 40/4 = 10 mV.
There are two ways to reduce the voltage U about to the required value: by changing the division factor N by switching capacitors C 1 of different ratings or by adjusting the output voltage U 1 of the generator. For the convenience of measuring high quality factors, the V1 voltmeter (or division ratio switch) is equipped with a scale (marking), the reading on which, characterizing the degree of voltage decrease U about compared to its nominal value, is a multiplier to the quality factor scale of the V2 voltmeter.
To check the operation of the kumeter and expand its capabilities, reference coils L o are used with known inductance and quality factor. Usually there is a set of several replaceable coils L o, which, together with a variable capacitor C o, provide resonant tuning of the measuring circuit within the entire operating frequency range of the generator.
When measuring quality factor of inductors Q L 10-15 minutes before the start of work, turn on the power to the device and tune the generator to the required frequency. After warming up, the voltmeters V1 and V2 are set to zero. The coil under test is connected to terminals 1 and 2. By gradually increasing the output voltage of the generator, the needle of the voltmeter V1 is deflected to the nominal mark. Capacitor Co tune the circuit to resonance with the frequency of the generator. If at the same time the arrow of the voltmeter V2 goes beyond the scale, the output voltage of the generator is reduced. The value of the quality factor Q L is determined as the product of readings on the scale of the quality factors of the voltmeter V2 and on the multiplier scale of the voltmeter V1.
Quality factor of the oscillatory circuit Q K is measured in the same order when the circuit coil is connected to terminals 1 and 2, and its capacitor to terminals 3 and 4. In this case, the capacitor C o is set to the minimum capacity position. If the capacitor of the circuit under study has a variable capacitance, then the circuit is tuned to resonance at the required generator frequency f; if this capacitor is constant, then the resonant tuning is carried out by changing the frequency of the generator.
Cometer measurement coil inductance L x is produced in the manner discussed above in connection with the circuit in fig. 9. The generator is tuned to a reference frequency, selected according to the table, depending on the expected value of L x . The coil under test is connected to terminals 1 and 2. The measuring circuit is tuned to resonance with a capacitor C o, on a special scale of which the value of L x is estimated, taking into account the division value indicated in the table. At the same time, by varying the parameters of the contour, it is possible to determine and own capacity of the coil C L . With two arbitrary values of capacitances C 01 and C 02 of the capacitor C, by changing the generator settings, the resonant frequencies of the circuit f 1 and f 3 are found. Desired capacity
C L \u003d (C 02 f 4 2 -C 01 f 1 2) : (f 1 2 -f 2 2)
The measurement of containers with a cumeter is performed by the substitution method. The tested capacitor C x is connected to terminals 3 and 4, and one of the support coils L o is connected to terminals 1 and 2, which provides resonant tuning of the circuit in the selected frequency range. At the same time, you can also determine the loss tangent (quality factor) of the capacitor:
tg δ \u003d 1 / (2 * π * f * C x R p)
(where R p - loss resistance). To do this, with two values of capacitances C 01 and C 02, corresponding to the resonant settings of the circuit without a capacitor C x and when the latter is connected, the quality factors of the circuit Q 1 and Q 2 are found, and then they are calculated by the formula
tan δ \u003d Q 1 Q 2 / (Q 1 -Q 2) * (C 01 -C 02) / C 01
If necessary, the kumeter generator can be used as a measuring generator, and electronic voltmeters can be used to measure voltages in a wide frequency range.
The vast majority of amateur inductance meters on controllers measure the frequency of an oscillator operating at frequencies around 100 kHz, and although they supposedly have a resolution of 0.01 μH, in reality, at inductances of 0.5 and below, they are a good random number generator, not a device. The designer of RF devices has three ways:
- break off
- buy an industrial impedance meter and starve for a while
- to do something more high-frequency and broadband.
The presence of many online calculators drastically simplifies the task, you can get by with just one generator connected to the frequency meter, not losing much in convenience, but gaining in functionality.
The prefix can measure inductances from 0.05 μH. The output voltage is about 0.5V. Own inductance of the conclusions is 0.04 μH. Output frequency range: xs...77MHz.
The broadband generator is made according to the well-known two-point scheme and is not very sensitive to the quality factor of the frequency-setting circuit. 
To measure the smallest inductances, the capacitance was chosen to be 82pf, together with the input capacitance, the calculated one (for the calculator) turns out to be about 100pf (round numbers are more convenient), and max. generation frequency is about 80 MHz. From the circuit, voltage is applied to the follower vt2 and from it to the emitter vt1, thus the POS is implemented. The sometimes used direct connection of the gate with the circuit leads to unstable operation of the generator at frequencies of 20-30 MHz, therefore, an isolation capacitor c1 is used. The field-effect transistor must have an initial drain current of at least 5mA, otherwise the transistor must be slightly opened with a resistance of several hundred kOhm from the plus to the gate. It is better to use the transistor in a high transconductance, this will increase the output voltage taken from the source. Although the generator itself is practically insensitive to the types of transistors.
Online calculators are used to calculate
The most convenient
most uncomfortable
glamorous but with character
The setting capacity in the device can be any, even Chinese clay. It is better to have reference coils, and already substitute the measured capacitance into the calculator, although in reality this is not necessary.
The foil on the back is used as a screen.
Conclusions to the coil are made in the form of flexible flat leashes from a braid 2 cm long. with crocodiles.
http://edisk.ukr.net/get/377203737/%D0%B8%D0%BD%D0%B4.lay6
Features of use.
For power supply, it is better to provide an appropriate terminal on the frequency meter.
Coil leads should be as straight as possible if ultra-low inductances are to be measured. From the result, you need to subtract your own inductance of the leads 0.04 μH. The minimum measurable inductance is about the same.
To measure inductances up to 100 μH, a standard capacitance is suitable, higher it is better to use additional capacitances from 1n, otherwise there will be an error from the interturn capacitance of the coil.
To measure the interturn capacitance, you need to measure the true value of the inductance with C 10-100n, then the frequency is measured with a standard capacitance (100pF), entered into the calculator, then the total capacitance is calculated, from which 100pF must be subtracted.
Example. axial choke 3.8 mH, with a standard capacitance, frequency 228 kHz, total capacitance 128pF, interturn 28.
The capacitances in the circuits are calculated in the same way.
To measure chokes on low-frequency LV magnetic cores, they must have a sufficiently large number of turns, for example, on 2000NN rings at least 20, otherwise the frequency may be higher than the operating one for them (up to 400 kHz), and the generation will be disrupted at best, and at worst pulsed, as in a blocking generator, with a frequency of kilohertz. For small turns, additional capacity is needed.