Combined frequency counter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The peculiarity of the proposed frequency meter is that, in addition to its main function, it allows you to determine the inductance of various coils, the resonant frequency of the circuits, and the capacitance of capacitors. That is why the frequency meter is called combined.

A combined device, the circuit diagram of which is shown in Fig. 1, will be a good assistant to the amateur radio designer. Its manufacture does not require scarce parts; it is easy to set up and operate. The device can measure the frequency of signals with an amplitude of 0,1...5 V of a sinusoidal or rectangular shape in the range from 50 Hz to 500 kHz, as well as inductance from 4 μH to 1 H.

For ease of reading, the operating range of values ​​is divided into four subranges. The first of them is installed when measuring frequencies up to 500 Hz. The second is when measuring frequency from 500 Hz to 5 kHz or inductance from 40 mH to 1 H. The third - at a signal frequency from 5 to 50 kHz, inductance values ​​of 0,4...40 mH. And the fourth sub-range - with a signal frequency from 50 to 500 kHz and inductance values ​​of 4...400 μH. The required subrange is set with switch SA2, and the measurement mode (frequency or inductance) with SA1. The frequency measurement error does not exceed 5%.

The operating principle of the frequency meter is based on converting the input signal into a sequence of rectangular pulses with stable duration and amplitude and subsequent measurement of the average current value of this sequence with a microammeter. The operation of the frequency meter is explained in a simplified way by the diagrams shown in Fig. 3. The signal under study (Fig. 3, a) is supplied to the input of the buffer node, which is made on transistor VT1. The purpose of the node is to provide high input impedance and minimal input capacitance of the frequency meter. From the output of the node, the signal goes through section SA1.1 of switch SA1 to a converter made on elements DD4.1, DD4.2. It serves to form a sequence of rectangular pulses from an arbitrary-shaped input signal, which from the output of element DD4.2 (Fig. 3, b) goes either directly to the input of the inverter on transistor VT2 (if you set the first sub-band), or to the input of the frequency divider (when work on other subbands) performed on counters DD1 - DD3. Each of the counters divides the frequency of the input signal by 10, therefore, regardless of what subrange is set, the frequency of the pulse sequence at the input of the transistor inverter will be no more than 500 Hz.

The DD4.3 inverter and the DD4.4 element are equipped with a pulse generator that is stable in amplitude and duration. The high-level voltage from the collector of transistor VT2 (Fig. 3, c) is supplied to the input of the inverter DD4.3 and to the integrating circuit R8 R9 C6.

At the top input of the DD4.4 element in the circuit, the voltage is set to a low level (Fig. 3, d), and at its lower input - to a high level (Fig. 3, e), but with a time delay, which depends on the value of the time constant of the integrating circuit. The duration of the delay is adjusted by trimming resistor R8, and its value determines the duration of the pulses t at the output of element DD4.4 (Fig. 3, e).

The average current value of the sequence of these pulses is measured using a microammeter PA1. The current value is proportional to the frequency of the input signal.

Rice. 1 (click to enlarge)

Fig. 2

How does an inductance meter work? For this mode, switch SA1 is moved to position "L". The converter based on elements DD4.1, DD4.2 turns into a generator, the frequency of which is determined by the value of the capacitance of capacitor C2 and the inductance of the coil Lx - it is connected to sockets X2, X3.

The frequency value is measured with a frequency meter (its operation is described above), and the inductance is calculated using the formula: Lx = 1/f^2, where Lx is in μH, af is in MHz.

For ease of reading, the instrument scale can be additionally calibrated in inductance values ​​or a recalculation scale can be made separately for each subrange and the scales can be glued to the instrument body.

Ris.3

The measurement accuracy depends on the stability of the pulse amplitude at the output of the comparison element DD4.4. The amplitude, in turn, depends on the stability of the supply voltage. That is why the device is powered through a parametric voltage stabilizer made on transistors VT3, VT4. The emitter junction of transistor VT4 was used as a zener diode, and a Krona battery was used as the main power source (Korund or a 7D-0,115 battery is suitable).

The capabilities of the device can be expanded by taking into account the ability of 561IE14 microcircuits to operate at frequencies up to 2 MHz and installing another frequency divider (it is not shown in Fig. 1). Then the upper measurement limit of the frequency meter will increase to 1,5...2 MHz and, accordingly, the range of inductance measurements will expand - up to 1 μH. The number of subbands will increase to five.

It is also easy to provide the possibility of measuring the resonant frequency of an unknown circuit or the capacitance value of a capacitor. To do this, you need to replace the switch SA1 with a three-position one and install additional input jacks (in Fig. 1 these additions are shown with a dashed line). By connecting the circuit to sockets X4, X5, its resonant frequency is found - according to the readings of the frequency meter. Based on the known (or previously measured) inductance of the coil, the capacitance value is calculated using the formula: Cx = 25,33/f^2*L, where f is in kHz, L is in mH and Cx is in µF.

The following parts can be used in the device. Transistors: VT1-KP303A-KP303V; VT2-VT4-KT315A-KT315I or KT312A-KT312V. Capacitor S2-K73MBM (if it is not possible to select a capacitor of such capacity from those available, it is made up of several parallel-connected capacitors of different capacities). Adjusted resistor R8 - SP3-3. Switch SA2 - PG-2 or P2K.

In the absence of a microammeter, you can use a magnetoelectric measuring head of an avometer, for example, Ts20 or TL-4 (avometer measurement mode - direct current). In this case, the device itself can be made in the form of an attachment. It is only necessary that the connecting wires be of the shortest possible length. Alligator clips can be used to connect inductors.

Mount the device parts (with the exception of switches SA1, SA2, capacitor C2 and the input unit) on a printed circuit board (Fig. 2) made of foil fiberglass. The input assembly parts are placed in the remote probe housing. This is done in order to reduce the capacitance that the device introduces into the measured circuit. The probe is connected to the device with a shielded wire. The sockets for connecting the probe are from a microtelephone. Capacitor C2 is attached directly between the terminals of sockets X2, X3.

Setting up the device begins by setting the resistor R8 slider to the lowest position according to the diagram, and then turning on the power. A voltmeter is used to monitor the voltage on capacitor C5. It should be within 5,5...7 V and not change when the device supply voltage increases from 9 to 12 V. Then the remote probe is turned off, and switch SA1 is moved to position “F” - frequency measurement. If the instrument needle deviates noticeably from the zero mark, this indicates that the converter is excited (elements DD4.1, DD4.2). It may be caused by parasitic interference through closely spaced connecting wires of switches SA1, SA2. To eliminate excitation, the conductors should be separated or a capacitor with a capacity of up to 4.2 pF should be installed between the output of the DD100 element and the common wire.

Next, connect the measuring probe and, by connecting its terminals to each other, again control the operation of the converter. Only after making sure that there is no excitation do they begin to calibrate the frequency meter.

Switch SA2 is switched to the first sub-range, and a sinusoidal signal with an amplitude of 1...2 V and a frequency of 500 Hz is supplied to the input of the measuring probe. Using trimmer resistor R8, set the microammeter needle to the end mark of the scale. By changing the amplitude of the input signal from 0,2 to 5 V, one is convinced of the stability of the frequency meter readings. Otherwise, the sensitivity of the input node is equalized by selecting resistor R2.

To set up the inductance meter, switch SA1 is moved to the “L” position, and SA2 is moved to the fourth subrange. A coil whose inductance is known (2... 3 µH) is connected to sockets X4, X10. Using the first of the above formulas, the frequency value is calculated, and then by selecting capacitor C2, the readings of the frequency meter correspond to this value.

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