resonant frequency meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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It is known that even the simplest measuring instruments make it possible to quickly and better adjust and test this or that radio structure. Today we introduce you to the description of a resonant frequency meter - a device that will be very useful in amateur radio practice.

It will help you determine the presence and frequency of unknown electrical oscillations, the relative voltage level of the fundamental frequency and its harmonics, check the laying of the band boundaries, the stability of the local oscillator of the receiver, high-frequency generator or transmitter on amateur bands.

The appearance of the device is shown in the intro of the article. It is a small-sized structure assembled on semiconductor parts. The principle of its operation reveals the name itself - the resonant method of measurement is the basis.

Five switched operating bands allow covering the full range of frequencies allocated for broadcasting with amplitude modulation of the signal and placed within the boundaries of 150 kHz-26 MHz, which covers long, intermediate, medium and short waves.

The frequencies are divided into bands in the following order: I - 150-430, II - 430-1200, III - 1200-3700, IV - 3700-11000 and V - 11000-26000 kHz. Tuning within each range is smooth. The measured frequency is measured on a scale calibrated directly in units of MHz.

The accuracy of tuning to resonance is determined by the maximum readings of a pointer indicator - a DC microammeter connected to the output of the device.

The device has an autonomous power source - a galvanic cell of the "316" type. Current consumption does not exceed 0,5 mA. The weight of the structure is about 0,6 kg. Overall dimensions - 110x155x55 mm.

The circuit contains five oscillatory circuits L1C2C3, L2C2C4, L3C2C5; L4C2C6 L5C2C7 operating in the five frequency bands mentioned above. The necessary switching is carried out by switch P1, smooth adjustment - by a variable capacitor C2.

With the help of tuning cores of coils L1-L3 and semi-variable capacitors C3-C7, the initial laying of the boundary frequencies of each range is performed.

From the input jacks G1 and G2, the signal under study is fed to the working circuit through a separating capacitor C1 of small capacity and switch P1. The high-frequency voltage selected last in the process of tuning into resonance from a part of the coil through switch P2, combined with P1, is fed to the detector - diode D.

After converting the high-frequency signal into a constant component, the voltage of the latter is fed to the input of a single-stage amplifier assembled on the transistor T1. To eliminate the possible ingress of alternating voltage, the input of the amplifier - base T1 - is blocked by a large capacitor C9. The input circuit does not have a special input level control, since other means can be dispensed with without complicating the circuit.

The constant component of the signal, entering the base of the transistor in negative polarity, controls the collector current Ik. At the moment of tuning to resonance, the collector current reaches its maximum value, which is recorded by a microammeter connected to the output to sockets G3 and G4.

In addition to the current Ik caused by the input voltage, the pointer device also records the initial collector current Ikn. Its value for some transistors is relatively large, this causes the indicator arrow to shift in the absence of voltage at the amplifier input. To get rid of this shortcoming, the frame of the device is shunted by a resistor R1 and blocked by a large capacitor C8 from the ingress of an alternating voltage.

DETAILS

To assemble the frequency meter, you need: capacitors of constant capacitance: C1 and C9 - ceramic, such as KT, KT-1a, KD and K10-7V (two 0,047 each, in parallel), KLS, MBM, respectively; C8 - electrolytic, type K50-3, K50-6. Capacitor of variable capacitance C2 (one section of a double block with a solid dielectric) type KP4-5, from a transistor receiver "Quartz", portable radio "Mriya". It can be replaced by a capacitor with an air dielectric of the KPE-4 type from the Alpinist receiver. Semi-variable capacitors C3-C7 - ceramic, type KPK-M.

Range switch P1-P2 - biscuit, any type for five positions and two directions. G1-G4 sockets - telephone. Transistor T1 - type P13, P14, P15, P16, P40, P41 or any other analogue. Diode D1 - type D1, D2, D9. Switch Vk- - single-pole toggle switch.

For loop coils L1-L5, plastic frames are needed (see fig.). These parts are not widely sold, so you need to purchase ready-made contour coils, full-fledged or substandard. For winding L1-L3, four-section frames of long-wave or medium-wave heterodyne coils of the Selga receiver are suitable, and for L4-L5 - smooth frames of input or heterodyne short-wave coils of Sokol-4, Russia, etc. magnetodielectric rod cores pressed into plastic screw plugs with a slot for a screwdriver bit. For coils L1-L2, ferrite cores of the brand F \u600d 3 are needed, and for L5-L100 - F \uXNUMXd XNUMX. You can distinguish the brand of ferrite by the color of plastic plugs. The first one is white, the second one is black.

The winding of all coils on the frames begins from the side where the tuning core is placed. This end is the beginning and is connected to the common wire of the frequency counter circuit.

Coils L1-L3 are wound in bulk, evenly placing turns in all sections of the frame, L4 - in one layer, turn to turn, and L3 - in one row, with a step of 0,35-0,4 mm. The beginning and end of the last two coils are fixed on the frame with threads. Finished coils are lightly coated with BF-4 glue. The winding data of the coils are given in the table.

The wiring of the coil leads to the pins of the bases of the frames is carried out in accordance with the designations given in the figure. The letter H indicates the beginning, O - branch and K - the end of the winding.

The circuit board is cut out of foil-coated getinaks or fiberglass with a thickness of 1,5-2 mm. When using a variable capacitor KP4-5, its dimensions are 93x80 mm.
The drawing of field connections is made on the foil with some quick-drying paint, for example, nitro-lacquer. After the paint has dried, the unnecessary foil is etched in an aqueous solution of ferric chloride.

Placement of parts and installation of the board is carried out according to the figure. The numbers indicate the connection points of the board elements with other parts of the circuit.

The front panel of the device is cut out of aluminum 2-3 mm thick. Holes are drilled on the workpiece, the front side is treated with fine-grained sandpaper in the longitudinal direction until a smooth matte surface with slight risks is formed.

On the washed and dried panel, inscriptions are applied with paint and covered with a thin layer of colorless varnish.

The scale of the device is made of thick paper. Five semicircles are applied in ink, according to the number of working ranges, and other inscriptions.

The paper scale is covered with a 1-2 mm thick organic glass casing.

The sighting pointer is also made of plexiglass, but 2,5-3 mm thick. In the middle of the strip, a deep thin line is made, which should be clearly visible against the background of the scale. In places corresponding to the placement of semicircles on the scale, holes of 1 mm are drilled, which are necessary for applying reference points during graduation. The pointer is fixed on the handle.

The axis of the variable capacitor is lengthened. For a galvanic cell, contact springs are made to ensure its electrical connection with the circuit.

The case of the device is plastic or wooden.

After assembling and checking the installation, proceed to the adjustment and graduation of the frequency meter scale. To perform them, you need an industrial generator of standard signals such as GSS-6, G4-1a, G4-I8 or a well-calibrated amateur analogue.

Establishment begins with checking the performance of the frequency meter on all ranges. To do this, through sockets G1 and G2, the input of the device is connected to the output of the generator. To sockets G3 and G4, observing the polarity, connect a DC microammeter at 100-200 μA. By setting the switch P to position 1, and the sighting pointer to the middle of the scale, check the first range of the frequency meter.

To do this, by supplying a high-frequency voltage of 100-200 μV from the generator and tuning the frequency within 15-430 kHz, they find the moment of coincidence of the instrument settings, the moment of resonance will be fixed by the microammeter.

If the indicator arrow deviates by a slight angle, then you need to change the transistor. A normal position is when, at the moment of resonance, the arrow deviates by at least two-thirds of the scale.

After checking the performance of the frequency meter on other ranges, they begin laying the cutoff frequencies.

Start it again from the first range. The sighting pointer is placed in the position of the maximum capacitance of the variable capacitor. The lowest frequency of the range, equal to 150 kHz, is supplied from the generator, and by rotating the tuning core of the coil L1, the circuit is tuned to resonance. After that, the capacitance of the capacitor C2 is changed to the maximum and, by applying a signal with a frequency of 430 kHz, the rotation of the rotor of the capacitor C3 again achieves resonance. The borders are laid in the same way on other ranges. It is quite acceptable that the boundaries of the range are 10-20% wider than the norm.

Having finished laying, proceed to the graduation of the scale. The first range can be calibrated every 10 kHz, the second up to 0,6 MHz - also every 10 kHz, and the rest and the third range - every 50 kHz. The fourth up to 6 MHz - through 100 kHz, and the rest and the fifth - also through 0,5 MHz.

For the convenience of working with the frequency meter, it is necessary to highlight the marks of the standard intermediate frequency of 465 kHz and the boundary - stretched short-wave ranges. They have the following values: 25 m - 11,5-12,1 MHz, 31 m - 9,4-9,8 MHz, 4 m - 7,0-7,5 MHz, 49 m - 5,9-6,3, XNUMX MHz.

Author: M. Rumyantsev

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