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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Preamplifier-shaper for FC250 frequency counter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The frequency meter made from the FC250 set [1] performed well in operation. But the desire of the author of the proposed article to obtain the maximum measured frequency of 250 MHz promised in the description of the device made him look for a circuit for the pre-amplifier-shaper (PUF) needed for this. But the puf circuits found on the internet either didn't work for the FC250 or were too complicated. The article provides descriptions of two variants of the PUF developed by the author, as well as a remote probe for the FC250 frequency counter. The CMOS comparators MAX999EiKi or ADCMP600BRJZ-R2 in the SOT-23-5 package with one TTL level signal output and ADCMP604BKSZ-R2 in the SOT-323-6 package with two anti-phase outputs of the LVDS standard [2] are used in the described PFUs. With such PIF, the frequency meter based on the FC250 set is capable of measuring the frequency of signals from 50 Hz to 110 ... 250 MHz with their minimum amplitude of 0,25 ... 0,65 V. Additional amplifiers at the input of the comparators had to be abandoned. They led to self-excitation, measures to combat which further reduced sensitivity. When working with the FC250 frequency meter, it was noticed that it creates strong impulse noise propagating along the common wire and the power circuit. To eliminate the influence of these interferences on the object of measurement, the inputs of the PUF and the remote probe are made according to a differential circuit. On fig. Figure 1 shows a diagram of the simplest version of the FPU, which allows you to measure the frequency from 50 Hz to 140 MHz using the comparator ADCMP600BRJZ-R2 [3] or up to 170 MHz with the comparator MAX999EUK [4]. The amplitude of the measured signal at a frequency below 70 MHz must be at least 0,3 V and at least 0,65 V at the limiting frequency.
From the input probes, the measured signal is fed through the circuits R2C1 and R3C2 to the inputs of the comparator DA1. Diodes VD1 and VD2 not only protect these inputs from overvoltage (both types of comparators mentioned above have internal protective diodes), but reduce the likelihood of self-excitation of the comparator, which has a large gain. The supply voltage +5 V to the comparator comes from the frequency meter. The inverting input of the comparator (pin 4) is connected through resistor R4 to a voltage source of +5 V, while in the absence of a measured signal at the output of the comparator (pin 1), which must be connected to pin 2 of the DD2 chip of the frequency meter, the voltage has a low logic level. When enabled in this way, the operating point of the MAX999 and ADCMP600 comparators is set automatically, and the switching characteristic has a hysteresis loop. Diodes VD1, VD2 and resistor R1 make it possible to reduce the width of this loop to a value at which self-excitation does not occur, and the sensitivity is high enough. This version of the PUF works well at low frequencies, up to 50 Hz. Two versions of the printed circuit board have been developed for the considered PPF. Both of them are made of 1...1,5 mm thick fiberglass laminated on both sides by cutting through the foil and mechanically removing its excess sections. One of the boards (Fig. 2, a) is designed for the installation of output diodes and resistors with a power of 0,0–2 W. Capacitors can be surface mount or disc type. The location of the elements on this board is shown in fig. 3. The smaller board shown in fig. 2b is designed for surface mount elements, including 1N4148W diodes. The location of the elements - in fig. 4.
The vias connecting the printed conductors on opposite sides of the boards are shown filled in both cases. Resistors R1 and R2 - output power 0,125 watts. They are inserted with one output into the corresponding holes of the boards and soldered to the foil. Segments of flexible insulated wires 15 cm long with probes are soldered to the free terminals of the resistors. Segments of rigid wire soldered into the holes of the boards, intended for connecting the PPF with the frequency meter, simultaneously serve as racks for fastening the PPF board on the frequency meter board. On fig. Figure 5 shows a diagram of the PUF with an external probe, assembled on three comparators connected in series. Comparators ADCMP604BKSZ-R2 [5] are used in the probe and at the input of the FPU proper. With the outputs of the DA2 comparator connected directly to the inputs of the DA3 comparator, the latter is in a static mode in a state of limitation, which prevents its self-excitation. Increasing the "buildup" voltage of the inputs of the DA3 comparator increased its switching speed, which determines the maximum frequency of the PUF. The bias voltage at the inverting input of the comparator DA2 and the width of the hysteresis loop in its switching characteristic are set in the same way as in the previous PUF.
After connecting a remote probe to the second version of the PPF (using a 50 cm long unshielded bundle of flexible insulated wires), the limit frequency measured by the FC250 exceeded 250 MHz. This is illustrated by the photograph in Fig. 6. The ADCMP604BKSZ-R2 chip is not prone to self-excitation, so there are no back-to-back diodes at the probe input to reduce the input capacitance. The high input impedance and low input capacitance of the probe made it possible to measure the local oscillator frequency of such microcircuits as the TDA7021T and its analogues.
This PUF and its probe are assembled on printed circuit boards made of the same material and by the same method as the previous one. A drawing of the printed conductors of the main board of the PUF is shown in fig. 7, and the arrangement of elements on it - in Fig. 8. The printed circuit board of the remote probe is shown in fig. 9. Details on it are located in accordance with fig. 10. Capacitors C1 and C2 - ceramic disk. They are located on different sides of the board.
The probe board features two rows of vias along its long edges. They are "stitched" with a thin tinned wire, which is then soldered to the foil along the entire length of the board on both sides. This allows you to take the probe by hand without affecting its performance. The length of the measuring probes of the probe is 4 ... 1 cm. Wires 4-XNUMX of the connecting harness are soldered to the corresponding contact pads on different sides of the board. When checking the frequency meter with the described PIFs, a generator assembled according to the circuit shown in Fig. 11 was used as a signal source. 1. Coil LXNUMX in it is replaceable. It is frameless with the number of turns selected depending on the required range of generator tuning.
Despite the results obtained, the normal operation of the frequency meter assembled from the FC250 set is still impossible at frequencies higher than 180...190 MHz. The maximum operating frequency of the K1554 series microcircuits used in it (similar to 74AC) does not exceed 130 MHz. At a higher frequency, they quickly overheat, and after a couple of minutes the frequency meter readings decrease by 2 ... 5 MHz. The inaccuracy and instability of the frequency meter readings at these frequencies is explained by the fact that not all pulses following with a frequency above the limit, which came to the inputs of the K1554LA3 (74AC00) microcircuit and the K1554TM2 (74AC74) D-flip-flop, forced to switch at an unacceptable frequency, correctly reach their outputs . For this reason, I do not recommend using a frequency meter based on the FC250 set to measure frequencies exceeding 110 MHz (with the PPF according to the scheme of Fig. 1 on the ADCMP600 comparator), 120 MHz (with the same PPF on the MAX999 comparator) and 180 MHz (with the PPF according to the scheme Fig. 5 with remote probe). To work with the described PUF, this frequency meter needs to be modified. The transistor VT1 with all its related parts, capacitors C3 and C5 are not installed on its board (or the already installed ones are removed). In both holes for the outputs of the capacitor C5 and in the hole for the output of the capacitor C3, connected to the resistor R4, or R2 (see Fig. 5), a variable resistor with a nominal value of 100.150 kOhm is mounted. With the frequency meter turned on, without touching the inputs of the PUF with your hands, the resistance of this variable resistor is gradually reduced until the PUF stops self-exciting. Then a variable resistor is soldered, its resistance is measured and a constant resistor of the nearest higher value is soldered instead. Similarly, the resistor R5 is selected in a remote probe, already connected to the established main board of the PUF. Literature
Author: A. Panshin
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