Refinement of the frequency counter FC250. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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A set of parts for the manufacture of an amateur frequency counter FC250 has been produced for many years. Earlier, the author published descriptions of attachments to this frequency meter - a remote amplifier-shaper [1] and a remote probe-frequency divider by 10 [2]. They made it possible to bring the maximum measured frequency up to 300 MHz. In a new article, he describes a number of probes he made for this frequency meter, assembled in a case as a complete design. When carrying out various measurements, they increase the convenience of working with the device.

To assemble the device, a plastic case with dimensions of 116x78x38 mm was chosen. The main board of the frequency meter has been improved. Above the K1554LA3 chip (DD2 according to the scheme given in [3]), a driver amplifier board is installed [1]. Below the DD2 microcircuit, on the board section freed from the input amplifier parts on the VT1 transistor, four holes with a diameter of 4 mm were drilled for mounting the board with the frequency meter input connector. They can be seen in the photograph in Fig. 1. In addition, the integral stabilizer DA1, which prevented the installation of the main board close to the front panel of the case, was moved to the side of this board opposite to the installation of other parts.

Rice. 1. Board with mounting holes

The scheme of the board with the input connector is shown in fig. 2, and its drawing - in Fig. 3. Before installing the XS1 connector on this board, its contact springs (they are supplied separately from the case) are modified according to fig. 4. In each of the springs 1, a segment of a single-core uninsulated wire 3 about 20 mm long is embedded. A piece of copper or rigid plastic tube 2 is put on this wire and pushed close to the shank of the spring. The total length of the spring and the piece of tube is 12 mm. Only after that the contacts are inserted into the housing.

Rice. 2. Board layout with input connector

Rice. 3. Drawing of the board with the input connector

Rice. 4. Refinement of the contact springs of the XS1 connector

The free ends of the wires are passed through the holes in the board. The connector is pressed against it so that the pieces of the tube put on the wires rest against the surface of the board. The wires are cut and soldered to the appropriate pads. The gap between the connector housing and the board is filled with hot-melt adhesive. The assembled board is installed as shown in fig. 5, on insulating posts 15 mm high above the main board and fixed with M3 screws through the drilled holes. Insulating washers are placed under the nuts screwed on them to avoid short circuits.

Rice. 5. Collected fee

The XS1 connector is brought to the front panel of the frequency meter (Fig. 6), for which a cutout is made in the instrument case. Another cutout is for the LED indicator and is covered with organic glass. On the right side is the power connector.

Rice. 6. Connector XS1 on the front panel of the counter

When measuring a frequency from 100 kHz to 100 MHz, a node is connected to the XS1 connector, the circuit of which is shown in fig. 7, and the drawing of the printed circuit board - in fig. 8. When working with signals with a low fundamental frequency, their high-frequency components and interference can distort the measurement results. The low-pass filter, the circuit of which is shown in Fig. 9, helps to eliminate such distortions. 10, and the printed circuit board - in fig. 200. Filter cutoff frequency - about XNUMX kHz.

Rice. 7. Node diagram

Rice. 8. PCB drawing

Rice. 9. Low pass filter circuit

Rice. 10. PCB drawing of low pass filter

If the amplitude of the measured signal is too large, it can be reduced using an attenuator assembled according to the circuit shown in Fig. eleven . Its printed circuit board is shown in Fig. 11. With a low internal resistance of the source, it attenuates the signal by about a factor of two. The input impedance of the attenuator is about 12 kOhm.

Rice. 11. Attenuator circuit

Rice. 12. Attenuator PCB drawing

In all three cases considered, flexible wires 350 ... 500 mm long in insulation of different colors are soldered to the contact pads "Input" of printed circuit boards. The wires are fixed on the ledges of the boards with bandages made of adhesive tape. The boards are protected from external influences by pieces of a heat-shrinkable PBF tube put on them. Probes are soldered to the free ends of the wires, made from pieces of a copper tube with an inner diameter of 0,9 mm (from the refrigerator thermostat) and a length of 10 ... 12 cm. A hard copper wire with a diameter of 0,75 mm is soldered inside each tube so that its ends protrude from the ends of the tube by 3 ... 5 mm. To give the desired thickness, each probe is wrapped with several layers of adhesive tape and placed in a heat-shrinkable tube.

The frequency divider by 10 (more precisely, its part, the diagram of which is circled in Fig. 1 in [2]) is also connected to the XS1 connector. For this, a board was made and connected to the divider by a bundle of four wires, the drawing of which is shown in Fig. 13. Harness length - 600...800 mm. In several places, segments of a PVC tube are put on it. The value of the resistor R1 is 1 kOhm.

Rice. 13. Board drawing

All boards described are designed to accept 1206 surface mount capacitors and resistors.

Literature

1. Panshin A. Pre-amplifier-shaper for frequency counter FC250. - Radio, 2015, No. 2, p. 18-20.
2. Panshin A. Remote probe-frequency divider by 10 for frequency counter FC250. - Radio, 2015, No. 4, p. 26, 27.
3. Set of parts FC250. Frequency meter-designer up to 250 MHz. - URL: 5v. ru/pdf/fc250.pdf.

Author: A. Panshin

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