Remote probe - frequency divider up to 500 MHz for FC250 frequency counter. Encyclopedia of radio electronics and electrical engineering

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Before making a remote probe for the FC250 frequency counter, which allows it to measure frequencies up to 500 MHz, the author assembled several frequency dividers on the K193IE2 chip described on the Internet. In them, the measured signal was applied directly to the input of the counter or to the input of a pre-amplifier based on a transistor. All samples showed a strong dependence of the output frequency on the amplitude of the input signal and low noise immunity.

According to the technical description of the K193IE2 frequency divider [1], its normal operation is possible only when pulses with sufficient amplitude and steepness of the drops are applied to the input. Therefore, an ADCMP500 comparator [606] was installed at the input of the probe, which was called VSC-2, and sensitivity adjustment was introduced. This probe divides the frequency of the input signal by 10. Together with the FC250 frequency counter, it allows you to measure the frequency from 2 to 500 MHz with a resolution of 100 Hz. Its sensitivity in the entire range of measured frequencies is not lower than 0,65 V. From a supply voltage of 5 V, the probe consumes a current of 80 ... 85 mA. Input differential impedance - about 70 kOhm.

The probe diagram is shown in fig. 1. It uses DA1 - ADCMP606BKSZ-R2 chips (CML standard comparator with a maximum operating frequency of 750 MHz) and DD1 - K193IE2 (frequency divider by 10 to 500 MHz). According to the standard, the CML comparator inputs are biased close to +3,3 V supply voltage. But the ADCMP606 chip belongs to the Rail-to-Rail category and therefore can operate at input voltages from 0 to positive supply voltage. In the VSC-500 probe, a bias equal to half the supply voltage was applied to the DA1 inputs. From the input contacts of the probe, through the chains R1C1 and R2C2, the measured signal is fed to the inputs (pins 3 and 4) of the comparator DA1. A circuit of resistors R3-R7 allows the variable resistor R3 to adjust the sensitivity of the probe by changing the constant voltage between the inputs DA1 from 0 to 0,5 V. The anti-phase outputs DA1 are connected to the positive power supply through resistors R8 and R9. Due to their low resistance (56 ohms), the relatively large input resistance of the frequency divider DD1 connected to one of the outputs of the comparator does not violate the symmetry of the latter.

Rice. 1. Diagram of the probe

The input of the frequency divider chip DD1 receives rectangular pulses with an amplitude of 0,4 V with a duration of 160 ps. Resistor R10, lowering the constant voltage at the input of the divider, prevents its self-excitation in the absence of a signal.

When connected to the FC250 frequency meter, according to the description in [3], the +5 V probe supply voltage comes from the voltage regulator present in the frequency meter. From the outputs of the divider DD1, an antiphase signal with a frequency of up to 50 MHz is fed to the inputs of the pre-amplifier-shaper of the frequency meter FC250 [4]. The XP1 connector is located on a separate board connected to the main bundle of four wires 600...800 mm long. Resistor R12 is located on the same board.

In contrast to the remote probe described in [5], which operates up to a frequency of 300 MHz, the VSC-500 operates at a frequency of up to 500 MHz. Both probes, without using an external divider, can register sinusoidal and rectangular oscillations with an amplitude of up to 5 V. Adjusting the sensitivity with a variable resistor R3 allows you to suppress interference if its amplitude is less than the amplitude of the useful signal.

At frequencies of 100 ... 200 MHz, the VSC-500 can respond to harmonics of irregularly shaped signals with an amplitude of more than 0,5 V, which leads to a doubling of the output frequency. If by adjusting the sensitivity it is not possible to tune out harmonics, you can lower the input impedance of the VSC by temporarily soldering a resistor with a resistance of 100 Ohm to 1 kOhm parallel to its input contacts. The capacitance of the capacitor C6 is 4,7 microns.

A drawing of the printed circuit board of the probe is shown in fig. 2, and the location of parts on it - in Fig. 3. The board manufacturing technology is described in detail in [5]. All fixed resistors and capacitors installed on the board are 1206 size for surface mounting. Variable resistor R3 - R-0904n-A1K (RP1-74). Before installing on the board, bend its side mounting tabs at a right angle in different directions. Cut the ends of the petals in place and solder to the foil of the common wire. Remove the unused pins of the K193IE2 chip.

Rice. 2. PCB drawing of the probe

Rice. 3. Location of parts on the board

Connector XP1 (WF-4R) is mounted on a small board similar to the one shown in fig. 13 in [3]. Resistor R12 is marked as R1 there. The appearance of the assembled probe VShch-500 is shown in fig. 4.

Rice. 4. Appearance of the assembled probe VShch-500

After assembling the VShch-500 probe board without comparator DA1 and resistors R3 and R10, solder the wiring harness with Xp1 connector to the corresponding pads of the board and connect it to the frequency meter, modified according to [3]. Usually the frequency divider DD1 is self-excited. Eliminate self-excitation by selecting a resistor R10, then install a comparator and the missing resistors on the board. If the self-excitation of the DA1 comparator cannot be stopped by the variable resistor R3, the likely reason for this is an open or poor soldering of one of the DA1 outputs.

Literature

1. Nefedov A. V. Integrated circuits and their foreign analogues. Handbook, vol. 3 - M.: IP RadioSoft, 2000.
2. Rail-to-Rail, Very Fast, 2.5 V to 5.5 V, Single-Supply CML Comparators ADCMP606/ ADCMP607. - URL: analog.com/media/en/technical-documentation/data-sheets/ADCMP606_607. pdf.
3. Panshin A. Refinement of the frequency meter FC250. - Radio, 2016, No. 3, p. 23, 24.
4. Panshin A. Pre-amplifier-shaper for frequency counter FC250. - Radio, 2015, No. 2, p. 18.
5. Panshin A. Remote probe - frequency divider by 10 for frequency counter FC250. - Radio, 2015, No. 4, p. 26, 27.

Author: A. Panshin

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