In radio engineering, it is often necessary to shift the signal spectrum along the frequency axis to a certain constant value when the signal structure is saved. Such a shift is called an hour transformation

To determine the essence of the frequency conversion process, we will return to the question of the effect on the nonlinear element of two stresses, briefly discussed in § 8.4. However, in this case, only one of the oscillations is exactly the one that is created by the auxiliary generator (heterodyne), we will consider harmonic. Under the second oscillation, we will mean the signal to be transformed, which can be any complex, but narrow-band.

Thus, two voltages affect the nonlinear element: from heteroodine

from signal source

The amplitude frequency and the initial phase of heterodyne oscillation are constant values. The amplitude and instantaneous frequency of the signal can be modulated, i.e., may be slow time functions (narrowband process). The initial phase of the signal is a constant value.

The frequency conversion task is to obtain a total or difference frequency. As the expression (8.30) follows, for this it is necessary to use quadratic nonlinearity,

As a nonlinear element, we take, as in § 8.9, a diode, however, it is characteristic of it for a more complete detection of signal interaction products and heterodyne oscillations by approximating the four-year polynomial (and not the second, as in § 8.4):

The components containing various degrees only or only interest do not represent. From the point of view of the transformation (shift) of the frequency, members of the form of the right part of the expression (8.72) are arranged by the framework.

Substituting into these works (8.70) and (8.71) and discarding all components whose frequencies are not sum in or by the difference after simple trigonometric calculations, we arrive at the next final result:

From this result, it is seen that the frequency you are interested in arise only thanks to the even degrees of the polynomial, the approximating characteristic of the nonlinear element. However, only a quadratic member of the polynomial (with the coefficient) forms the components whose aplites are proportional to only the first degree higher even degrees (the fourth, sixth, etc.) violate this proportionality, since the amplitudes of the oscillations introduce them also contain the degrees above the first.

It can be seen that amplitudes should be chosen with such a calculation so that in decomposition (8.72) the predominant significance had the components not higher than the second degree. This requires the execution of inequalities

Then the expression (8.73) goes into the following:

In radio and many other devices in which the frequency conversion task is closely related to the signal strengthening task, usually?,

The first term in curious brackets with a frequency (derived from the cosine argument) corresponds to the shift of the signal spectrum to the high frequency area, and the second with the frequency to the low frequency region. To highlight one of these frequencies - a difference or total - you need to apply the appropriate load at the output of the converter. Let, for example, frequencies are very close and you need to highlight the low frequency located near zero. This task is often found in the measuring technique (the method of "zero beats"). In this case, the load should be the same as with amplitude detection, i.e. consisting of a parallel connection R and C, which provides filtering (suppression) of high frequencies and highlighting the difference frequency If the difference frequency lies in the high frequency range, then to highlight it A resonant oscillatory chain should be applied (Fig. 8.42). If the useful to be selected is the total frequency of the outline, respectively, must be configured to the frequency

Typically, the bandwidth band of the oscillatory chain, which is the load of the converter, is designed to width of the modulated oscillation spectrum. In this case, all current components with frequencies close to K pass through the contour evenly and the signal structure at the outlet coincides with the structure of the input signal.

Fig. 8.42. Frequency converter substitution scheme

Fig. 8.43. Signal spectrum at the input and output of the converter:

The only difference is that the frequency at the outlet is equal to or depending on how the resonant frequency of the load chain.

So, when converting the frequency, the laws of change of the amplitude of the frequency and the phase of the input oscillation are transferred to the output oscillation. In this sense, the transformation of the signal conversion is linear, and the device is a linear converter or "mixer".

The rotor of any electric motor is driven under the action of forces caused by a rotating electromagnetic field inside the stator winding. The speed of its revolutions is usually determined by the industrial frequency of the electrical network.

Its standard value of 50 Hertz implies the commission of fifty periods of oscillations within one second. In one minute, their number increases 60 times and is 50x60 \u003d 3000 revolutions. The rotor under the influence of the applied electromagnetic field is shocking the same number.

If you change the magnitude of the network frequency applied to the Stator, you can adjust the rotation speed of the rotor and the drive connected to it. This principle is based on the control of electric motors.

Types of frequency converters

By design, frequency converters are:

1. induction type;

2. Electronic.

Asynchronous electric motors, performed and running into the generator mode, are representatives of the first species. When working, they have low efficiency and are noted small efficiency. Therefore, they did not find wide use in production and are used extremely rarely.

The method of electronic frequency conversion allows you to smoothly adjust the revolutions of both asynchronous and synchronous machines. In this case, one of two management principles can be implemented:

1. By a predetermined characteristic of the dependence of the speed of rotation from the frequency (V / F);

2. Method of vector control.

The first method is the easiest and less perfect, and the second is used to accurately regulate the speed of rotation of the responsible industrial equipment.

Features of vector frequency conversion control

The difference of this method is the interaction, the effect of the converter control device to the "spatial vector" of the magnetic flux rotating with the frequency of the rotor field.

Algorithms for the operation of converters on this principle are created in two ways:

1. inexpensive management;

2. Furnigulation.

The first method is based on the appointment of a certain dependence of the alternation of the inverter sequences for pre-prepared algorithms. In this case, the amplitude and frequency of the voltage at the output of the converter are adjusted by slipping and loading current, but without the use of reverse links in the rotor speed.

This method uses when controlling several electric motors connected in parallel to the frequency converter. Flow regulation involves control of operating currents inside the engine with decomposition of them on active and reactive components and making adjustment to the operation of the converter to exhibit amplitude, frequency and angle for output voltage vectors.

This allows you to improve the accuracy of the engine and increase the boundaries of its regulation. The use of footpooling expands the capabilities of the drives operating on small revolutions with large dynamic loads, such as lifting crane devices or winding industrial machines.

Using vector technology allows you to apply a dynamic adjustment of rotating moments to.

Scheme of substitution

The principled simplified electrical circuit of an asynchronous engine can be represented as follows.


On the windings of the stator with an active R1 and inductive x1 resistances, the voltage U1 is applied. It, overcoming the resistance of the air gap, is transformed into the rotor winding, causing a current in it that overcomes its resistance.

Vector diagram of replacement scheme

Its construction helps to understand the occurring processes inside the asynchronous engine.


Stator current energy is divided into two parts:

    iμ - a streaming share;

    iW - the moment-forming component.

At the same time, the rotor has an active R2 / S resistance, depending on slip.

For non-sensory controls are measured:

    voltage U1;

    current I1.

According to their values, count:

    iμ is a streaming current component;

    iW - a momentum-forming value.

The calculation algorithm has already laid an electronic equivalent scheme of an asynchronous engine with current regulators, in which the conditions for saturation of the electromagnetic field and the loss of magnetic energy in steel are taken into account.

Both of these components of current vectors, differing in the corner and amplitude, are rotated in conjunction with the rotor coordinate system and are recalculated in the stationary orientation system to the Stator.

According to this principle, the parameters of the frequency converter under the load of the asynchronous motor are adjusted.

Principle of operation of the frequency converter

The basis of this device, which is also called the inverter, is laid down double change in the form of the signal of the supply electrical network.


Initially, industrial voltage is fed on a power straightening unit with powerful diodes that remove the sinusoidal harmonics, but leave the ripples of the signal. For their liquidation, the inductance capacitor battery is provided (LC-filter), which provides a stable, smoothed shape to the straightened voltage.

Then the signal enters the input of the frequency converter, which is a bridge three-phase diagram from six IGBT or MOSFET series with protection diodes from the breakdown of reverse polarity stresses. Thyristors used earlier for these purposes do not have sufficient speed and work with great interference.

To enable engine braking mode, a controlled transistor with a powerful resistor scattering is mounted in the diagram. This technique allows you to remove the voltage generated by the engine to protect the filter capacitors from recharging and fail.

The method of vector control of the frequency of the converter allows you to create schemes that automatically adjust the signal to SAR systems. This uses the control system:

1. amplitude;

2. PWM (latitudinal pulse modeling).

The amplitude control method is based on the change in the input voltage, and the PWM is an algorithm of switching power transistors with a constant input voltage.


At the PWM adjustment, the signal modulation is created when the stator winding is connected by strict priority to the positive and negative conclusions of the rectifier.

Since the frequency of the generator's clock is quite high, then in the winding of the electric motor with inductive resistance, they are smoothing to the sinusoids of the normal form.


Methods of PWM control allow us to exclude energy loss as much as possible and provide high conversion efficiency due to the simultaneous frequency control and amplitude. They have become available due to the development of technologies for managing power lockable thyristors of the GTO series or bipolar brands of IGBT transistors with an isolated shutter.

The principles of their inclusion for controlling the three-phase motor are shown in the picture.


Each of the six IGBT transistors is connected along a counter-parallel diagram to its diode of the county. At the same time, the active current of the asynchronous motor passes through the power circuit of each transistor, and its reactive component is directed through diodes.

To eliminate the effect of external electrical interference to the operation of the inverter and the engine in the design of the frequency converter scheme, eliminating:

    radiopomers;

    electric discharges inspected equipment.

Their occurrence signals the controller, and to reduce the impact, the shielded wiring between the engine and the output terminals of the inverter are used.

In order to improve the accuracy of the operation of asynchronous motors, the frequency converter control circuit includes:

    input links with advanced interface capabilities;

    built-in controller;

    memory card;

    software;

    information LED display displaying the main output parameters;

    brake trigger and built-in EMC filter;

    the cooling system of the scheme based on blowing the fans of an increased resource;

    the engine warming function by direct current and some other features.

Connection operational schemes

Frequency converters are created to work with single-phase or three-phase networks. However, if there are industrial sources of direct current with a voltage of 220 volts, then inverters can also be powered from them.


Three-phase models are calculated on the voltage of the network of 380 volts and give it to the electric motor. Single-phase inverters feed from 220 volts and the output is given three phase time.

Connection diagram of the frequency converter to the engine can be performed according to the schemes:

    stars;

    triangle.

Engine windings are collected in the "star" for a converter, driven from a 380 volt three-phase network.


According to the "Triangle" scheme, the engine winding is collected when its converter is connected to a 220 volt single-phase network.


When choosing a way to connect an electric motor to the frequency converter, you need to pay attention to the ratio of power, which can create a working engine in all modes, including a slow, loaded launch, with the inverter capabilities.

It is impossible to constantly overload the frequency converter, and a small supply of its output power will provide him with long and trouble-free operation.

Federal Education Agency

Krasnoyarsk State Technical University

Laboratory work on RTCIS №4

Frequency conversion.

performed:

student c. P53-4: Titov D. S.

checked:

Cashkin V. B.

Krasnoyarsk 2005.

purpose of work

Studying the basic patterns of frequency conversion. The work removed the dependence of the transformation coefficient from the offset voltage, the spectra of signals at the output of the converter with a large and low amplitudes of the heteroodine are investigated.

Homework .

Frequency converter scheme

Dependence of differential steepness from the input voltage.

Known: The frequency of the heterodine Fg, the frequency of the filter of the intermediate frequency FF. Determine the frequencies of the signal in which the voltage at the output of the converter reaches the maximum.

A) if the amplitude of the heteroodine is small, the converter works in quadratic mode, therefore

B) If the amplitude of the heteroodine is large, the mode will not be quadratic.

where M and N are some whole positive numbers.

In the current case there will be a strong distortion of the signal at the output of the converter.

Using dependence (UB0) in frequency conversion mode ie With the simultaneous submission to the input UC and UG and FC \u003d | FG ± FF |.

This dependence is as non-linear as the input characteristic of the transistor.

experimental part

We remove the dependence of the voltage at the output of the converter from the offset voltage in the direct pass mode at UC \u003d 10 mV and Fc \u003d Fn and the heterodine turned off.

The calculated intermediate frequency of the filter F \u003d 121 kHz (C \u003d 2200pf L \u003d 780 μH).

Experimentally found frequency of heterodyne F \u003d 261 kHz, intermediate frequency of the filter F \u003d 104 kHz.

The frequency of the signal is set to the maximum voltage at the output of the converter.

The resulting characteristic is clearly nonlinear because The input characteristic of the transistor is nonlinear.

Select the operating point in the middle of the linear section of the dependences of the UB0 (UB0). Ub0 \u003d 0.5 V.

We will remove and construct the voltage dependence on the output of the signal frequency converter at UC \u003d 10 mV, bring the voltage value at the output value at the maximum output and the maximum frequencies.

With a small amplitude of heteroodine AG \u003d 10 mV.

With a large amplitude of heterodyne AG \u003d 250 mV.

Oscillogram of AM voltage at the input of the converter.

Am-voltage oscillograms at the output of the converter with a large amplitude of the heteroodine and the offset Ub0 \u003d 0.5 V, at the frequency of the signal

1) fc \u003d Fg + ff fc \u003d 365 kHz

2) FC \u003d FG-FP FS \u003d 158 kHz

3) fc \u003d 3fg + fp ff \u003d 840 kHz

4) fc \u003d 3fg-fp ff \u003d 630 kHz

We will remove the dependence of the UB (UB0) with a large amplitude of the heteroodine.

From the obtained data we calculate and construct a graph of the dependence of the conversion coefficient from the offset voltage.

Output: In the course of laboratory work, the processes occurring when converting the frequency of the AM signal were investigated.

The dependence of the voltage at the output of the converter from the offset voltage in the direct pass mode was removed, this dependence is nonlinear.

The frequencies and amplitudes of maxima at a small and large amplitude of the heteroodine were measured. It was found out that at the output of the frequency converter, the signal has a complex spectrum with highs on several frequencies.

Oscillograms of signals at the output of the converter at different frequencies of the input AM signal were obtained. It turned out that the outlet signals are smallly distorted.


1. Signal frequency conversion. In this case, the signal at the input of the device with a variable amplitude and (or) phase focused on the spectrum near the frequency F 1 turns to the output of the device into a signal having the same form (K and - constant), but focused on the spectrum near the frequency.

When converting the frequency up F 2 larger than F 1. When converting the frequency down F 2 is less than F 1.

Frequency conversion is often used in modern devices when receiving signals with both amplitude and angular modulation;

2. Frequency converter. The frequency converter is called a device that allows you to transfer the spectrum of the input signal up or down the frequency scale.

A nonlinear amplifier with an oscillatory output loop, configured to a special (combinational) frequency, rice, can be used as a frequency converter. 3.1.

Figure 3.1. Converter Scheme when frequency conversion up

The frequency conversion is carried out by multiplying two oscillations and and oscillation isolation with a combination frequency (W + Ω) at the output, following the formula:

cOS (X) × COS (Y) \u003d (1/2)

At the same time we have:

Impact:

Useful reaction:

In the general case, the low-frequency signal can be represented as a sum of several harmonic oscillations. To highlight a useful reaction, a filter is required.

The frequency conversion is carried out by the same nonlinear amplifier (Fig. 3.2) scheme by multiplying the two input oscillations and the release of oscillations with a combination frequency at the output, following the formula:

cOS (X) × COS (Y) \u003d (1/2)

Figure 3.2 - Converter diagram when converting frequency down

At the same time we have:

Impact:

Useful reaction:

In the general case, the low-frequency signal can be represented as a sum of several harmonic oscillations. Low frequency filter is required to highlight a useful reaction.

3. Amplitude modulation ( AM historically was the first type of modulation, mastered in practice. Currently, AM is used mainly only for broadcasting at relatively low frequencies (not higher than short waves) and to transfer the image in television broadcasting. This is due to low efficiency of using the energy of modulated signals.

AM complies with the transfer of information S (T) into the amplitude U (T) with constant values \u200b\u200bof the parameters of the carrier oscillation: frequencies W and the initial phase J 0. AM is a product of the information envelope U (T) and the harmonic oscillation of its filling with higher frequencies. The form of an amplitude-modulated signal recording:

u (T) \u003d U (T) × COS (W O T + J O), (3.1)

U (T) \u003d U M ×, (3.2)

where u m is the constant amplitude of the carrier oscillation in the absence of an input (modulating) signal S (T), M - the coefficient of amplitude modulation

M value characterizes depthamplitude modulation. In the simplest case, if the modulating signal is represented by one-frequency harmonic oscillation with an amplitude S o, the modulation coefficient is equal to the ratio of the amplitudes of the modulating and carrier oscillation m \u003d s o / u m. M must be between 0 to 1 for all harmonics of the modulating signal. With the meaning of M.<1 форма огибающей несущего колебания полностью повторяет форму модулирующего сигнала s(t), что можно видеть на рис.3.4 (сигнал s(t) = sin(w s t)). Малую глубину модуляции для основных гармоник модулирующего сигнала (m<<1) применять нецелесообразно, т.к. при этом мощность передаваемого информационного сигнала будет много меньше мощности несущего колебания, и мощность передатчика используется неэкономично.

Fig..3.4 - modulated signal Fig. 3.5 - Deep modulation

Figure 3.5 shows an example of the so-called Deep modulation,at which the value m strives for 1 in the extreme points of the function S (T).

One hundred percent modulation (m \u003d 1) can lead to signal distortions during transmitter overloads, if the latter has a limited dynamic range by amplitude of the carrier frequencies or limited transmitter power (increasing the amplitude of carrier carriers in the peak intervals of the signal U (T) twice requires an increase in transmitter power four times).

When M\u003e 1, the so-called reproduction, example of which is shown in Fig.3.6. The shape of the envelope when modified is distorted relative to the form of the modulating signal and after demodulation, if its simplest methods are used, the information may be distorted.

4. Monogarmonic amplitude modulation . The simplest form of the modulated signal is created during monogarmonic amplitude modulation - modulation of the carrier signal with a harmonic oscillation with one frequency ω:

u (T) \u003d U M × COS (W O T), (3.3)

The values \u200b\u200bof the initial phase angles of the carrier and modulating oscillation here and in the future, to simplify the obtained expressions we will take equal to zero. Taking into account the COS (X) × COS (Y) \u003d (1/2) formula (y) \u003d (1/2) from the expression (3.3) we get:

u (T) \u003d U M COS (W O T) + (U M m / 2) Cos [(W O + Ω) T] + (U M m / 2) Cos [(W O - Ω) T] (3.4)

It follows that the modulating oscillation with the frequency ω is moved to the frequency area W O and is split into two oscillations with frequencies, respectively, W O + Ω, the upper side frequency, and W o - J - lower lateral frequency. These frequencies are located on the axis symmetrically relative to the frequency W O, Fig. 3.7. The amplitudes of oscillations on the side frequencies are equal to each other, and at 100% modulation is equal to half the amplitude of the carrier frequency oscillations. If you convert equation (3.3), taking into account the initial phases of the carrier and modulating frequency, then we obtain a phase change rule, similar to the rule of frequency change:

The initial phase of the modulating oscillation for the upper side frequency is folded with the initial carrier phase,

The initial phase of the modulating oscillation for the lower - subtracted from the carrier phase.

The physical width of the modulated signal spectrum is two times the width of the modulating signal spectrum.

With the simultaneous action of the signal and the heterodyne on the nonlinear element, current frequencies of the species appear in the output circuit where M and N are integer numbers of natural rows and determine the nonlinearity of the converting element relative to the signal and heterodyne. If the converter with respect to the signal is linear, then m \u003d 1, if the heterodyne generates a harmonic signal, then n \u003d 1.

In all three inputs of the frequency converter, selective systems, configured, respectively, are connected to the resonance at the input with the frequency of the signal. At the same time, a heterodyne system is connected to the clamp 3-3 (specify n \u003d 1), the selective system is connected to the clamping 2-2, for example, a simple oscillating circuit.

The main equations that describe the operation of the 6-poles are the equations of the form:

(1)

(2)

In the expression (1) and (2) the time does not include, since the 6-polenik, we consider it uninterrupted. When withdrawing equations describing the frequency conversion process, we assume that the voltage of the signal U C has the order of a tent - hundreds of MKV, which allows the frequency converter linear. At the same time, the voltage with the frequency of the heterodyne U g has the order of the tenths and units. Therefore, neither U C nor u. Taylor in the degrees of small variables U C and U PR, that is, limiting the consideration of the decomposition members with U C and U PR in the first degree.

(3)

Derivatives that are coefficients of the series are determined by and, that is, under action only the voltage of the heterodyne;

for

Physical meaning:

This is an input current under the action of u.

- Input conductivity.

- Conducting reverse transformation.

Output current under the action of heteroodine, in the absence of a signal.

- steepness.

- output conductivity.

Since the heterodyne voltage is considered harmonic, for example, cosineidal: , then the steepness S (T), as a periodic function of time, can be represented as a row of Fourier:

After substitution in (3) and (4), we obtain the direct and reverse conversion equation:

a) direct conversion ,

where I pr - current intermediate frequency;



b) reverse transformation .

Converter parameters.

1. CRF transducer:

(k. z. at the output)