Comparator probe for frequency meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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One of the measuring instruments that a radio amateur needs is a frequency counter, which can often also measure the repetition period or pulse duration. Today, the most popular frequency meters are assembled on microcontrollers. They are distinguished by the relative ease of manufacture. Depending on the microcontroller used, the maximum measurable frequency ranges from several hundred kilohertz to several tens of megahertz.
It is necessary to apply a signal with logic levels to the input of the microcontroller, therefore, the frequency meter, as a rule, includes an input signal amplifier on an op-amp or on transistors, much less often - a comparator. To increase the sensitivity of the frequency meter, the amplifier or comparator is often made in the form of an external probe.
Fig. 1
It is this design that is offered to the attention of readers. The scheme of the device is shown in fig. 1. The maximum input frequency is about 100 MHz, the input resistance of the probe is 50 ... 60 kOhm, the input capacitance is no more than 10 pF. The basis of the device is a high-speed comparator DA1. The applied MAX999EUK-T chip is operable in the supply voltage range of 2,7...5,5 V, the maximum propagation delay time is no more than 10 ns, the hysteresis is 3,5 mV, the current consumption is about 5 mA.
The inverting input of the comparator DA1 receives a constant voltage (half of the supply voltage) from the resistive divider R2R3, and to the non-inverting input - from the resistive divider R4R5 connected to the divider R2R3. Therefore, the voltage at the inverting input is approximately 25.30 mV higher than the voltage at the non-inverting one. In this case, the comparator output is logic low, and the sensitivity of the probe is determined by the specified difference. When an input signal with an amplitude of more than 30 mV is applied, the comparator switches and rectangular pulses are formed at its output, which are fed to the measuring input of the frequency meter. Diodes VD1, VD2, together with resistor R1, protect the non-inverting input of the comparator, capacitors C1 and C2 are blocking.
Fig. 2
The printed circuit board drawing is shown in fig. 2. It is made of fiberglass laminated on both sides with a thickness of 1 mm. The side free from parts is left completely foiled and is used as a common wire. Through the holes, it is connected by pieces of tinned wire to the contact pads on the other side. The location of the elements on the board is shown in fig. 3.
Fig. 3
The device uses elements for surface mounting. Resistors (RN1-12) and capacitors (K10-17v) - size 1206. Diodes 1N4148UR-1 can be replaced by any low-power fast (preferably Schottky) for surface mounting and a capacity of not more than a few picofarads. Connector XP3 - plug with a diameter of 3,5 mm from stereo phones. It is connected to the board with three twisted wires placed in an insulating tube. The central contact (1) of this plug is supplied with supply voltage, the external contact (3) is a common wire, the middle contact is the output signal. An appropriate socket is installed on the frequency meter case.
Fig. 4
The XP1 pin is made of a steel paper clip, it is soldered to a metallized pad on the board and glued with epoxy glue for strength, XP2 is a crocodile clip. The board is placed in a plastic case of a suitable size, for example, from a ballpoint pen (Fig. 4), and fixed there with hot glue.
Author: I. Nechaev
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