How to improve the technical characteristics of a frequency meter according to A. Denisov’s scheme. Scheme. Fees. Input frequency divider Technical data and application

Digital microcircuits and their applications

The presented divider is an attachment to a digital frequency meter. Thanks to its use, it is possible to measure frequencies up to 1.2 GHz with a frequency meter with a maximum measurement range of 10 MHz. The input stage of the divider uses a monolithic digital divider ECL, which divides the input frequency in a ratio of 1: 64.

The input of the microcircuit is protected from damage by signals with too large an amplitude by Schottky diodes. The device works well at frequencies from 30 MHz to 1.2 GHz. The sensitivity of the divider is ~20 mV with an input resistance of 50 Ohms.

Frequency divider circuit diagram

The input frequency divided by 64 is fed to the following 5/4 dividers. Transistor T1 brings the ECL level to the TTL standard. The role of 5/4 dividers is performed by the 74LS390 chip, which has 2 binary decimal counters in its structure.

Each of them has a divider by 2 and by 5. The 5/4 split consists of the fact that out of every five input pulses only 4 are passed through. If 1600 pulses arrive at the input of the divider, then after dividing by 64 there will be 25 of them. After passing through the first their divisor will be 20, and after passing through the second one - 16. Thus, division by 100 is realized.

You should remember to multiply the readings of a frequency meter operating with a divider by 100.

The device can be built into an existing frequency meter or used as a probe. The leads of the elements used for the divider should be as short as possible. Integrated circuits should be soldered directly onto the board. After installation, the entire console should be shielded.

The divider requires a supply voltage of 5 V. The prototype circuit consumed ~70 mA.

List of elements

Attention! After installing the circuit, you should connect the pins of the US3 integrated circuit (74LS132) together.

The first digital IC design made by radio amateurs in the 80s and 90s was usually an electronic clock or frequency meter.
Such a frequency meter can still be used today when calibrating instruments, or used as a reading device in generators and amateur transmitters, when setting up various radio-electronic devices. The device may be of interest to those who have K155 series microcircuits lying around idle, or who are starting to get acquainted with automation and computer devices.

The described device allows you to measure the frequency of electrical oscillations, the period and duration of pulses, and can also work as a pulse counter. Operating frequency from a few Hertz to several tens of MHz with an input voltage of up to 50 mV. The maximum operating frequency of counters based on K155IE2 integrated circuits is about 15 MHz. However, it should be borne in mind that the actual speed of flip-flops and counters exceeds the specified value by 1.5... 2 times, so individual instances of TTL microcircuits allow operation at higher frequencies.

The minimum LSB price is 0.1 Hz when measuring frequency and 0.1 μs when measuring period and duration.
The operating principle of the frequency meter is based on measuring the number of pulses arriving at the counter input within a strictly defined time.

The circuit diagram is shown in Fig. 1

The signal under study is fed through connector X1 and capacitor C1 to the input of the rectangular pulse shaper.

The wideband amplifier-limiter is assembled using transistors V1, V2 and V3. Field-effect transistor V1 provides the device with high input resistance. Diodes V1 and V2 protect transistor V1 from damage if it accidentally comes into contact with the input of a high voltage device. Chain C2-R2 carries out frequency correction of the amplifier input.

Transistor V4, connected as an emitter follower, matches the output of the amplifier-limiter with the input of logic element D6,1 of microcircuit D6, which ensures the further formation of rectangular pulses, which, through an electronic switch, are sent to the control device on chip D9, and pulses of reference frequency that open key for a certain time. A burst of pulses appears at the output of this key. The number of pulses in a packet is counted by a binary decimal counter; its state after closing the key is displayed by a digital display unit.

In the pulse counting mode, the control device blocks the reference frequency source, the binary decimal counter continuously counts the pulses arriving at its input, and the digital display unit displays the counting results. The counter readings are reset by pressing the “Reset” button.

The master clock generator is assembled on a D1 (LA3) chip and a Z1 quartz resonator at a frequency of 1024 kHz. The frequency divider is assembled on K155IE8 microcircuits; K155IE5 and four K155IE1. In measurement mode, the accuracy of the “MHz”, “kHz” and “Hz” settings is set by push-button switches SA4 and SA5.

The power supply of the frequency meter (Fig. 3) consists of transformer T1, from winding II of which, after the rectifier VDS1, a voltage stabilizer on the DA1 microcircuit and a filter on capacitors C4 - C11, a voltage of +5V is supplied to power the microcircuits.

A voltage of 170V from winding III of transformer Tr1 through diode VD5 is used to power gas-discharge digital indicators H1..H6.

In the pulse shaper, the field-effect transistor KP303D (V3) can be replaced with KP303 or KP307 with any letter index, transistor KT347 (V5) with KT326, and KT368 (V6, V7) with KT306.

Choke L1 type D-0.1 or homemade - 45 turns of PEV-2 0.17 wire, wound on a frame with a diameter of 8 mm. All switches are P2K type.

Setting up the device comes down to checking the correct installation and measuring the supply voltages. A correctly assembled frequency meter confidently performs its functions; the only “capricious” unit is the input driver, the configuration of which must be given maximum effort. Having replaced R3 and R4 with variable resistors 2.2 kOhm and 100 Ohm, you need to set the voltage on resistor R5 to approximately 0.1...0.2V. Having supplied a sinusoidal voltage with an amplitude of about 0.5V from the signal generator to the input of the shaper, and replacing resistor R6 with a variable resistor with a nominal value of 2.2 kOhm, it is necessary to adjust it so that rectangular pulses appear at the output of element D6.1. Gradually lowering the input level and increasing the frequency, it is necessary to select elements R6 and SZ to achieve stable operation of the shaper over the entire operating range. You may have to select the resistance of resistor R9. During the installation process, all variable resistors should have leads no longer than 1...2 cm.

When the installation is completed, they should be unsoldered one at a time and replaced with constant resistors of a suitable value, each time checking the operation of the driver.

In the design, instead of IN-17 indicators, gas-discharge indicators IN-8-2, IN-12, etc. can be used.

In the pulse shaper, KT368 transistors can be replaced with KT316 or GT311; instead of KT347, you can use KT363, GT313 or GT328. Diodes V1, V2 and V4 can be replaced with KD521, KD522.

Schematic and board in sPlan7 and Sprint Layout format - schema.zip *

* This circuit was assembled by me back in 1988 in the same housing with a sound generator and was used as a digital scale.

As an independent device, it was designed recently, so it is possible that an error could have crept somewhere in the circuit diagram and design of the printed circuit board.

Bibliography:

To help a radio amateur No. 084, 1983

Digital Devices on Integrated Circuits - © Radio and Communications Publishing House, 1984.

Radio magazine: 1977, No. 5, No. 9, No. 10; 1978, no. 5; 1980, no. 1; 1981, no. 10; 1982, no. 1, no. 11; No. 12.

Amateur radio digital devices. - M.: Radio and communication, 1982.

When using a software frequency meter and oscilloscope, measuring signal frequencies is usually limited to the boundaries of the frequency range of the computer's sound card. To be able to measure frequencies above 20 kHz, as well as view their shape and spectrum on an oscilloscope, you can use a simple frequency divider. The easiest way to implement it is with the help of digital microcircuits - decimal counters. Each such counter produces a signal at the corresponding output that is 10 times lower than the input frequency. Figure 1 shows a diagram of such a frequency divider. When using two counter chips, the input signal can be divided by 10 twice, that is, you can get an output signal whose frequency will be 10 and 100 times less than the input one. The frequency division ratio is switched using a simple two-position switch S1.

Any MC (decimal counters) can be used as counters, preferably CMOS technologies, since such microcircuits are not critical to the supply voltage and work well with different signal levels, both digital and analog. In the above circuit, K164IE2 microcircuits are used, you can use others that are functionally similar, for example K561IE4, K 176IE4……… Unused input and control pins of the microcircuits should be connected to a common wire, as shown in the diagram (pins 1,4,5,6,7 ,9), to exclude the possibility of induced interference voltage appearing on them.

The design of the divider is shown in the photo below (sorry for the low resolution of the picture, there is no better photo at the moment!). The circuit is assembled on a printed circuit board, on which only the contact pads for the pins of the microcircuits are etched. All connections are made with a single-core wire in insulation, since the circuit is simple and there are a minimum of connections.

The divider probe is made from a piece of wire in the screen. As the tip of the probe, you can use, for example, a thin nail 4–5 cm long. The wire is soldered to the nail with any ordinary solder on an aspirin tablet (simple “Soviet”). Aspirin is a good substitute for soldering acid when soldering iron. Then the wire with the nail tip can be inserted, for example, into the body of an empty ballpoint pen.

The screen of the input wire must be connected to the common conductor of the divider (minus power). Power to the divider can be supplied from the device whose frequency we are measuring. To do this, the ends of the power wires can be equipped with small alligator clips. The output cord with a connector for the computer sound card input is also shielded. The connector wiring diagram is shown in the figure.

If you take power from the circuit being measured, then the connection to the common wire will be provided through the negative supply wire. If the divider is powered separately, for example from a Krona battery, then the common wire of the divider should be connected to the common wire of the circuit being measured with a separate conductor.

To better match the input of the divider with the circuit being measured and to reduce mutual influence, you can add some simple matching stage with the highest possible input impedance at the input of this divider. For example this:

The trimmer resistor VR1 is used to set the operating mode of the transistor so that there is no limitation (“cutting”) of the input signal in amplitude from below and above (you can control the signal shape using a software oscilloscope at the output of the frequency divider). Transistor – any low-power one, for example KT315, KT342, KT3102….

To clearly demonstrate the operation of the divider, below is a screenshot where an RF signal with a frequency of about 900 kHz is measured (switch S1 in the “1/100” position). The frequency meter readings in this case, naturally, need to be multiplied by 100.

The use of the U664BS high-frequency digital frequency divider chip helped to simplify the design of the device, which the author used to expand the measurement range of a relatively low-frequency frequency meter. The divider can be designed as an adapter between the input jack and the cable to the signal source, or it can be built into an existing amateur frequency meter with an additional high-frequency input.

Frequency divider circuits (for example) intended for use with a low-frequency frequency meter have already been published in the amateur radio literature. The divider described in, with its relative simplicity, allows you to increase the upper frequency limit of the device by only 10 times. The frequency divider has a division coefficient of 100, but, in my opinion, its design is unjustifiably complicated both in terms of the range of parts used and in technical implementation.
Meanwhile, using a modern element base, it is possible to significantly simplify the frequency divider circuit without the need for programming in the case of using a microcontroller. The described divider has a division coefficient of 100 and a stable operation range of 25 MHz... 1 GHz (the upper limit corresponds to the nameplate frequency of the input divider). The sensitivity of the divider is 20 mV with an input resistance of 50 Ohms.

The divider circuit is shown in rice. 1. The U664BS (TELEFUNKEN) chip is a monolithic digital frequency divider with a ratio of 1:64. This microcircuit is made using ECL technology (smitter-coupled logic), its transistors have a cutoff frequency frp = 4.5 GHz. Schottky barrier diodes (VD1, VD2) are used to protect the input of the DD1 microcircuit from high-amplitude signals.
As is known, the logical levels of ESL in a standard connection are in the voltage region of negative polarity and therefore they are not directly compatible with the logical levels of TTL and CMOS microcircuits. To convert ESL levels to TTL levels when powering the ESL microcircuit from a voltage of positive polarity, a matching stage on transistor VT1 is used.

The input signal with a frequency divided by 64 is fed to the next two dividers, made on microcircuits DD2 (K555IE20) and DD3 (K155TLZ). The K555IE20 microcircuit contains two four-bit binary-decimal counters, each of them has a trigger with input C1, output 1 and a frequency divider by 5 with input C2 and outputs 2, 4, 8. In this device, counters DD2 operate in the mode of a frequency divider by five with input C2 and output 8. By the way, based on my practice, the upper operating frequency of the entire device is determined by the maximum frequency for the DD2.1 counter (K555IE20), which for the C2 inputs is usually at least 20 MHz, i.e., in fact, at least 1.28 GHz . Each of the dividers into DD2.1, DD3.1, DD3.2 and DD2 2, DD3.3. DD3.4 has a fractional division coefficient KR = 1.25 (or 5/4).
The essence of the used method of fractional frequency division is as follows. Let there be a sequence of pulses following with frequency F (Fig. 2). If n pulses are excluded from each packet formed by m pulses, then the average frequency of pulses in the new sequence
F o =(m-n)/m *F
The division coefficient has the form of the ratio of two numbers TO D = m/(m-n) ,: i.e., in general, it represents an improper fraction.
In general, the block diagram of a fractional frequency divider is shown in rice. 3. It is based on the divisor A1 by the integer t. Shaper A2 creates a pulse of duration equal to n periods of the input pulse sequence.: Matcher A3 selects m-n pulses from each sequence in n pulses.
In our case, m = 5 and n = 1. The Schmitt triggers of the logic elements of the D03 chip allow the use of a coincidence circuit to clearly select only four pulses out of every five input pulses arriving at the counters of the DD2 chip. In Fig. Figure 4 shows timing diagrams explaining the operation of each of the two cascaded fractional dividers.
Thus, if a signal with a frequency, for example, F = 1000 MHz, is received at the input of the described frequency divider, then after the first divider DD1 the frequency F 2 = F1 /64 = 15.625 MHz . After the second divider (with DD2.1) the frequency will become equal F 2 = F1 /1.25 = 12.5 MHz and after the third - F 2 /1.25 = 10 MHz
All elements of the divider are placed on a board made of foil fiberglass. The printed circuit board drawing is shown in Fig. 5. The board should be placed in a metal shield. The input and output of the divider are connected to the frequency meter with an RF cable
If the frequency meter is made in the form of a complete small-sized structure, the divider can be designed as an adapter between the input jack and the cable to the signal source. To do this, the board must be placed in a rectangular screen, at the ends of which the CP-50-75 connectors must be mounted: on one side - the male part of the connector, on the other - the female part.
The frequency divider was tested in conjunction with the frequency meter described in and showed excellent results.

LITERATURE

1. Biryukov S. A. Preliminary divider. - Radio, 1980, No. 10, p. 61.
2. Zhuk V. Preliminary frequency divider for the range 50 - 1500 MHz. - Radio, 1992, No. 10. p. 46, 47
3. Nechaev I. Frequency divider range 0.1. .3.5 GHz. - Radio. 2005. No. 9, p. 24-26.
4. Biryukov S. Digital frequency meter - Radio, 1981, No. 10, p. 44 47.