Attachment to the frequency meter for resonant measurements. Scheme, description

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Attachment to the frequency meter for resonant measurements. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The prefix is designed to determine the resonant properties of inductors, capacitors, as well as to determine the capacitance of pn junctions of varicaps, diodes, transistors.

The attachment diagram is shown in fig. 1. It contains a generator on transistors VT1, VT2, a voltage amplifier on transistors VT5, VT6 and an AGC unit (transistors VT3, VT4). The generator circuit differs from the known one by the presence of elements R1, R4, C2, C4, which improve the conditions for self-excitation at low values of the characteristic resistance of the circuit.

Rice. 1 (click to enlarge)

The investigated inductor and capacitor are connected to sockets XS1-XS4. The generation frequency is measured by a frequency meter connected to XP1, XP2 plugs. Using formulas

all parameters of interest can be calculated. In these formulas, L is in μH, C is in pf, f₀ - in kHz.

To expand the capabilities of the set-top box, the generator can be implemented on the "internal" elements L1, C3 using switches SA1, SA2.

A voltage amplifier is used to match the frequency meter. The transfer coefficient is . By selecting a resistor R12, the output signal can be increased.

In the AGC node on the transistor VT4, an RF signal detector is assembled. By selecting the resistor R7, you can change the sensitivity of the detector.

The tuned resistor R6 provides a closing voltage at the p-n junction of a semiconductor device, which is connected to sockets XS5 and XS2 (or XS4) when measuring its capacitance:

The prefix is assembled on a board made of fiberglass laminated on one side. Its drawing is shown in Fig. 2. Instead of switches SA1, SA2, you can use pins with jumpers. Sockets XS1-XS5 - from connectors. The prefix was developed for use in conjunction with the VC86A multimeter, which allows you to measure the frequency up to 30 MHz. For this reason, the output pins XP1, XP2 are from a mains plug or similar. They are soldered to the board from the side of the printed conductors. They allow you to connect the set-top box directly to the input jacks of the multimeter. The set-top box is powered from a network unit with an output voltage of 5 ... 6 V, the current consumption (excluding the LED) is approximately 10 mA. I use a cell phone charger.

Prefix to the frequency meter for resonant measurements
Fig. 2

Field effect transistor - any of the KP303 series. Instead of the KT315B transistor, you can use KT315A, KT315V-KT315Zh.

Coil L1 - any suitable in size. After mounting all the elements on the board, from the side of the printed conductors, the right (on the board) output of the resistor R11 should be connected to the left (on the board) output of the resistor R3 with a jumper from an insulated wire. Correctly assembled from serviceable parts, the prefix does not need to be adjusted.

To work with the attachment, it is necessary to determine its own capacity, which is calculated by formulas (1) - (3) with the SA2 switch closed. Record the measured frequency f₁. Then, a capacitor with a minimum tolerance is connected to the sockets XS1, XS2 (or XS3, XS4). The author used a 4700 pF capacitor with a tolerance of 1% (Set), which can be found in the radio channel module of 3rd (or 4th) generation TVs. Write down the value of the frequency f₂ with a reference capacitor connected. The own capacity of the set-top box is calculated by the formula

In this formula, the capacitance value is substituted in picofarads, and the frequencies are substituted in megahertz. The same measurements and calculations must be carried out for socket XS5. The obtained values of the self-capacitance of the set-top box should be taken into account in calculations and measurements.

Author: V. Kalendo

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