With the simultaneous action of a signal and a local oscillator on a nonlinear element, combination frequency currents of the form appear in the output circuit , where m and n are natural numbers and determine the nonlinearity of the conversion element with respect to the signal and the local oscillator. If the converter is linear with respect to the signal, then m = 1, if the local oscillator generates a harmonic signal, then n = 1.

Selective systems are connected to all three inputs of the frequency converter, tuned accordingly to resonance at the input with the signal frequency. In this case, a heterodyne system is connected to terminals 3-3 (set n = 1), a selective system is connected to terminals 2-2 in the form of, for example, a simple oscillatory circuit.

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

(1)

(2)

Expressions (1) and (2) do not include time, since we consider the 6-pole pole to be inertial. When deriving the equations describing the frequency conversion process, we will assume that the signal voltage U c is of the order of tens - hundreds of μV, which allows the frequency converter to be considered linear. At the same time, the voltage with the local oscillator frequency U g is of the order of tenths and units V. Therefore, neither U c nor U pr cause a change in the parameters of the nonlinear element, this does U g. This allows the functions f 1 and f 2 to be expanded in a series Taylor in powers of small variables U c and U pr, that is, limiting ourselves to taking into account the expansion terms with U c and U pr in the first degree.

(3)

The derivatives, which are the coefficients of the series, are determined at and, that is, when only the local oscillator voltage is applied;

at

Physical meaning:

This is the input current under the action of U g.

- input conductivity.

- the conductivity of the reverse transformation.

Output current with LO action, no signal.

- steepness.

- output conductivity.

Since the heterodyne voltage is considered harmonic, for example, cosine: , then the slope S (t), as a periodic function of time, can be represented as a Fourier series:

After substitution in (3) and (4), we obtain the equation of direct and inverse transformation:

a) direct conversion ,

where I pr - intermediate frequency current;

b) reverse transformation .

Converter parameters.

1. Transducer slope:

(k.z. at the exit)

Introduction

In radio engineering, it is often required to shift the spectrum along the frequency axis by a certain constant value while maintaining the signal structure. This shift is called frequency conversion. This is necessary in radio receivers in order to carry out better bandpass filtering. at low frequencies it is more efficient to do this. In radio transmitters, this is needed for modulation.

This problem is solved by the frequency converter. A frequency converter is a device that consists of a mixer and an oscillator called a local oscillator. The purpose of the transducer is to shift the spectrum of the received signal to a lower intermediate frequency.

The main parameters of the frequency converter are: local oscillator frequency, maximum signal frequency, supply voltage, current consumption.

Frequency conversion principle

Modulated (or unmodulated) high-frequency oscillations can be converted to oscillations of a different frequency in such a way that the amplitude and phase relationships between the components of the spectrum are preserved.

Frequency conversion requires an auxiliary voltage, which requires a high frequency oscillator called a local oscillator.

Frequency conversion can be done in one of two ways:

Create beats of two voltages and apply them to a nonlinear element - a diode, transistor, or any other device with a nonlinear characteristic, in order to isolate the components of the total and difference frequencies from them. This method is called additive mixing.

Apply the converted high-frequency oscillation to the element, the transmission coefficient of which changes under the influence of the heterodyne voltage, and extract from the output oscillation the components of the sum or difference frequency. This method is called multiplicative mixing.

The devices performing this task are called frequency converters.

The frequency converter consists of a mixer and an oscillator called a local oscillator. Usually, frequency synthesizers are used as local oscillators in professional radio receivers. This ensures quartz frequency stability, low phase noise and readjustment.

A mixer is a device with two inputs. One of them receives the signal voltage, the other - the local oscillator. At the output of the mixer there is a spectrum of frequencies, among which there is a difference frequency. There are two types of blending: additive and multiplicative.

Multiplicative mixing

With multiplicative mixing, the signal voltage is multiplied with the LO voltage. The functional diagram of this principle is shown in Fig. 1

To obtain oscillations of the difference frequency, it is enough to multiply the signal and local oscillator voltages.

The original of this image is rather cumbersome, so we will only show a graph of the output voltage function.

Thus, the task is to make a voltage multiplier such that its output spectrum contains the minimum number of side components.

Frequency conversion is a process of linear transfer of the radio signal spectrum from one frequency range region to another while maintaining the modulation law and parameters. To simplify the process of amplifying the useful signal in radio receivers, the transfer is carried out to the region of relatively low frequencies.

The principle of operation of the frequency converter is based on the interaction of two high-frequency voltages supplied to a circuit with a nonlinear element. However, of these voltages, it carries useful information of the received signal, and the second is auxiliary, formed by a special generator (local oscillator). If we represent the current-voltage characteristic of a nonlinear element in the form of the simplest limiting series

and apply two voltages to this element

the current of the nonlinear element will contain many combination components of these frequencies. The medium of a number of current components will also be the difference between the frequencies of the local oscillator signal and the useful signal
, which is selected using a filter tuned to this frequency. The output signal of the converter is the voltage drop across the selective load resistance from the current varying with this frequency.

.

The amplitude of the output voltage is determined by the properties of the nonlinear element and the magnitude of the applied voltages, and the frequency and phase are determined by the initial values ​​of these voltage parameters.

In the general case, when the real volt-ampere characteristic of a nonlinear element is determined by a rather complex dependence, a set of combination frequencies is formed in the conversion process, one of which can be selected as an intermediate

,

wherepandqwhole numbers. Ifp= q= 1 transformation is called simple. For other values, it is complex.

As a rule, in receivers the amplitude of the signal voltage is much less than that of the local oscillator. When such voltages are added in a nonlinear circuit, the result of the action can be represented in the form of a small increment, for which the I – V characteristic of a nonlinear element can be considered linear with a certain accuracy, and the slope of the I – V characteristic changes under the influence of a sufficiently large local oscillator voltage. In this case, the conversion process can be represented as the action of the signal voltage on a linear system with variable parameters.

The slope of the characteristic in this case is a periodic function of time, which can be represented as a Fourier series

When a signal is applied to the input of the voltage converter
current is represented as

,

In the case when the law of slope change is complex, in addition to the fundamental frequency of the local oscillator, its higher harmonics appear. The frequencies of the combinational components are determined by the expression
.

Due to the fact, in the receivers, the conversion of weak signals occurs, regardless of how it is carried out (by a nonlinear element, or by a linear system with variables

parameters), the frequency converter belongs to the linear part.

Classification of converters and their main characteristics.

In accordance with the stated principles of frequency conversion, the converter circuit must include a non-linear element (an element with a variable parameter) - a mixer, a local oscillator and a selective load.

As a mixer can be used: vacuum tubes, transistors, semiconductor diodes, as well as nonlinear capacitance or inductance with nonlinear conductivity.

Heterodyne is usually a low-power oscillator with self-excitation, less often special devices that create a set of voltages of different frequencies.

The mixer is loaded with various bandpass filters.

A simple transformation has become more widespread in practice,

which is the result of the interaction of the first harmonics of the signal and local oscillator frequencies.

By the nature of the conductivity of the mixing element, frequency converters are divided into two groups;

- converters on nonlinear elements with active conductivity.

- converters with reactive conductivity.

The first group includes converters based on lamps, transistors and semiconductor diodes.

The second group includes converters based on a nonlinear capacitance of a parametric diode.

Tube and transistor converters can be with a separate local oscillator. In the second variant, the functions of the mixer and the local oscillator are combined in one stage and the effect of the heterodyne voltage on the nonlinear properties of the mixer is carried out through the common current of the nonlinear element. Converters with different local oscillators have more stable characteristics than combined ones.

Tube converters are often divided into pentode, triode, diode. Pentode converters are built according to the scheme with a common cathode and can be one or two grid. In the first case, the voltage of the signal and the local oscillator are applied to the same grid. In the case of supplying signals to different grids, the mutual influence of the input circuit of the local oscillator decreases when they are rebuilt. Triode and transistor converters are built according to schemes with both a grounded cathode (emitter) and

grounded grid (base).

Triode mixers are widely used in the DM wavelength range, because have a lower level of intrinsic noise and constructively are more convenient for matching with oscillatory circuits based on sections of coaxial lines. Recently, diode mixers have been widely used, especially in the DM and CM ranges.

To assess the quality of the converters and for their comparative assessment, the following main indicators are used.

1. Conversion factor. It is the ratio of the voltage amplitude or power of the intermediate frequency signal at the output of the converter to the signal voltage at its input.

;

The value of this coefficient is determined by the type and mode of operation of the mixer and the properties of the load

2. The working frequency range is determined the range of the receiver and is provided by tuning the local oscillator. With a fixed local oscillator setting, the receiver operates at the same frequency.

3. The level of intrinsic noise of the converter. As one of the first stages of the receiver, the frequency converter significantly affects the overall noise floor. The sources of noise are the same elements as in other stages, and the methodology for their assessment is the same.

4. Selectivity ... By analogy with amplification stages, the selectivity of a frequency converter determines its ability to attenuate the output voltage when detuned. Selectivity is determined

resonant properties of its load. However, the specificity of the operation of the frequency converter makes it possible for a number of other frequencies to appear, the voltages of which at the same frequency of the local oscillator during the conversion process will give an intermediate frequency.

Shown here is the frequency dependence of the drive gain. According to the conversion principle, current components of the mixer with different combination frequencies flow through the load. In the case of a simple conversion with a constant local oscillator frequency the same value of the intermediate frequency can be when receiving signals at two frequencies and

;

The additional receive channel differs from the main one in frequency by an amount and is located mirror-like relative to the local oscillator frequency.

In addition to the mirror channel, there are additional channels.

If the frequency of the input channel is equal to the intermediate frequency, the converter works as an amplifier.

The presence of mirror and additional channels is a significant disadvantage of a superheterodyne receiver, which reduces its stability. The main way to weaken the reception on the mirror channel is to improve the selectivity of the stages upstream of the converter. This becomes easier as the IF frequency increases. However, this in turn complicates the formation of the required resonant characteristics of the IF amplifier, especially when a narrow bandwidth is required.

This contradiction is resolved in the double conversion process. At a higher frequency, the influence of the image channel is weakened, and at a lower frequency, the formation of the required band.

Like other elements of a receiver, a frequency converter can be a source of frequency, phase and harmonic distortion. The latter are due to the very principle of transformation. So the appearance of additional components in the signal spectrum due to combination frequencies is equivalent to nonlinear distortions. A decrease in these distortions is achieved by improving the selectivity and choosing an operating mode in which the nature of the change in the displacement conductivity will approach harmonic.

Let's imagine the frequency converter in the form of a circuit with active nonlinear conductivity, with a control voltage of the local oscillator ... A signal voltage is applied to the input of such a system ... The output is loaded voltage drop of which
.

Converter output current with frequency
... Generally dependent on nonlinear conductance characteristics, signal and intermediate frequency

The signal level at the input of the converter is much less than the local oscillator voltage, and the magnitude of the conversion coefficient of the converter is relatively large, and thus the inequality

;

Thus, the output current of the converter is a function of two small variables. Based on this, we expand the stream function in a Taylor series in powers of small variables, limiting the first three

members.

The first term is the component of the converter current, which is due to the action of the local oscillator voltage at
... We denote
... The second term is the increment of the converter current caused by the action of the signal voltage, i.e.
is the conductivity of a nonlinear circuit for ... Under the influence of the local oscillator voltage, the conductivity value periodically changes with the frequency ... Let us denote this conductivity .

The third term characterizes the current increment due to the action of the intermediate frequency voltage on it. At each moment of time, this increment is determined by the conductivity of the nonlinear circuit for
and the instantaneous value of this voltage.

Let's denote it and define it as the conductivity of a nonlinear circuit for
... Thus

Introducing the function ,and in the form of Fourier series and accepting the condition that the intermediate frequency is formed according to the law

we represent the current of the intermediate frequency in the sl. the form

passing to complex amplitudes, the last expression is represented in the form

and it can be called the direct transformation equation. Here - as a harmonic of a functionS. - constant component .

Similarly, you can imagine the circuit of the frequency converter from the output side. Assuming that an intermediate frequency source is connected to the output of the converter, an expression for the output current at the signal frequency can be obtained. In any real frequency converter circuit, the influence of
on due to the presence of reverse conductivity of the nonlinear circuit. This process is usually called inverse frequency conversion. The physical meaning of this influence is as follows. The intermediate frequency voltage applied to the nonlinear conduction is converted by the local oscillator voltage into the current of the signal frequency. As if the frequency is changing places.

Representing the input current as a function and two small variables and
its value can be expressed as a series by analogy with direct transformation, provided that they change places. Separating the component of the input current with frequency, we can obtain the following expression for its complex amplitude

,

where
and - periodic functions of the local oscillator voltage representing, respectively, the amplitude of the k-th harmonic of the reverse conductance of the nonlinear circuit for
... And the constant component of the conductivity of the same circuit for ... These quantities are determined by the type of nonlinear circuit and the amplitude ... The presented expression can be considered the inverse transformation equation. The reverse transformation leads to a change in the input and output conductivity of the converter. To a greater extent, this applies to diode converters and converters with a common grid (base). In this regard, it is necessary to take into account (internal parameters).

- The steepness of the transformation. The ratio of the amplitude of the intermediate frequency current to the amplitude of the signal voltage when the output is shorted.

- Internal conductivity ... The ratio of the amplitude of the intermediate frequency current to the voltage amplitude of the same frequency at a legal input.

- Internal gain of the converter. The ratio of the intermediate frequency voltage amplitude to the signal voltage amplitude.

Based on the equations of direct and inverse transformation, it is possible to draw up an equivalent circuit of the converter, with which it is possible to determine its external parameters.

- Conversion factor

- Input conductivity

is equal to the sum of the input conductance of the nonlinear element at and conductivity due to the inverse transformation.

total input current of the converter at the signal frequency due to the presence of input conductivity of the nonlinear element and inverse transformation.

- Output conductivity

is the sum of the internal conductivity of the converter and the conductivity of the load on
.

Frequency converter noise.

The sources of noise of frequency converters and the method of their assessment are similar to those of UHF, however, take into account the conversion features. In the microwave range, when using diode semiconductor mixers to quantify noise, the concept of relative noise temperature is used

, where
- noise power generated only by the output impedance of the converter at ambient temperature. When matching the converter with the subsequent cascade

,

from here

assuming that matching at the input of the converter is ensured and taking its noise figure equal to

noise temperature can be represented as

,

where
- power conversion factor.

An additional source of internal noise is the image-to-mirror conversion. there is a summation of the noise components that fall into the frequency band of the IF amplifier. This phenomenon is estimated to be an equivalent increase in the effective noise bandwidth of the receiver. Only in the mirror channel, such an equivalent expansion is approximately equal to

,

where
- effective noise bandwidth of the receiver, - weakening Z.K.

The spectrum of the signal by frequency without changing the shape of the spectrum. The P. of h. Arises when the oscillations of the signal n of the heterodyne on the nonlinear device, called. mixer; As a result, in the spectrum of the output signal, along with other frequencies, the difference and sum frequencies are formed: the selection of one of them is the result of the mixer operation. The amount of shift is determined by the frequency of the auxiliary. generator (local oscillator).

P. h. Are used in radio receivers, will measure. technology, reference oscillators, etc., since in this case the signal amplification in a wide range of tunable frequencies is replaced by the amplification of a non-tunable combination. frequency, called. intermediate. The constancy of the intermediate frequency = const during the restructuring of the signal frequency provides at the same time. tuning the frequency of the local oscillator So, the amplification of the signal in devices with P. ch. is carried out at a relatively low frequency, usually a standard frequency.

When transmitting information, the radio frequency oscillation can be modulated by dec. parameters: amplitude frequency p phase (see. Modulated fluctuations). In order for P. h. To be transferred to an intermediate frequency without distortion, it is necessary to perform. conditions: 1) a non-linear device (for example,) must have a current-voltage characteristic close to quadratic or approximated by an even-degree polynomial; 2) the signal amplitude should be much less than the oscillation amplitude of the local oscillator 3) the frequency should be higher

Since in the output circuit of the mixer there are decomp. combinations. frequency, then to select the difference or sum frequency, the output circuit must be selective, that is, resonant, tuned to the desired frequency.

Under P. frequency divider or frequency multiplier. WITH... F. Litvak.

Physical encyclopedia. In 5 volumes. - M .: Soviet encyclopedia. Chief Editor A.M. Prokhorov. 1988 .

See what "FREQUENCY CONVERSION" is in other dictionaries:

frequency conversion- The process of linear transfer of a frequency band occupied by a signal to another region of the frequency spectrum with or without inversion. [L.M. Nevdyaev. Telecommunication technologies. English Russian explanatory dictionary reference book. Edited by Yu.M. Gornostaeva ...

frequency conversion- dažnio keitimas statusas T sritis automatika atitikmenys: angl. frequency conversion; frequency transformation vok. Frequenztransformation, f; Frequenzumsetzung, f; Frequenzumwandlung, f; Frequenzwandlung, f rus. frequency conversion, n pranc.…… Automatikos terminų žodynas

frequency conversion- dažnio keitimas statusas T sritis fizika atitikmenys: angl. frequency conversion vok. Frequenzumsetzung, f; Frequenzumwandlung, f; Frequenzwandlung, f rus. frequency conversion, n pranc. conversion de la fréquence, f… Fizikos terminų žodynas

radio frequency conversion- frequency conversion The process of transferring a radio frequency band occupied by a signal to another part of the frequency spectrum. [GOST 24375 80] Topics radio communication General terms radio reception Synonyms frequency conversion ... Technical translator's guide

convert frequency to number code- - [Ya.N. Luginsky, M.S.Fezi Zhilinskaya, Y.S.Kabirov. English Russian Dictionary of Electrical Engineering and Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts EN frequency to number conversion ... Technical translator's guide

frequency conversion in the direction of its decrease- - [Ya.N. Luginsky, M.S.Fezi Zhilinskaya, Y.S.Kabirov. English Russian Dictionary of Electrical Engineering and Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts EN frequency down conversion FDC ... Technical translator's guide

frequency to voltage conversion- - [Ya.N. Luginsky, M.S.Fezi Zhilinskaya, Y.S.Kabirov. English Russian Dictionary of Electrical Engineering and Electric Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts EN frequency to voltage conversion… Technical translator's guide

down conversion- - [Ya.N. Luginsky, M.S.Fezi Zhilinskaya, Y.S.Kabirov. English Russian Dictionary of Electrical Engineering and Power Engineering, Moscow, 1999] Subjects of electrical engineering, basic concepts EN frequency down conversion ... Technical translator's guide

Radio frequency conversion- 163. Frequency conversion of a radio signal Frequency conversion Source: GOST 24375 80: Radio communication. Terms and definitions original document ... Dictionary-reference book of terms of normative and technical documentation

Raman frequency conversion- Ramano dažnio keitimas statusas T sritis radioelektronika atitikmenys: angl. Raman frequency conversion vok. Raman Frequenzumwandlung, f rus. frequency conversion based on Raman scattering, n pranc. conversion Raman de fréquence, f ... Radioelektronikos terminų žodynas

Books

• Radio circuits and signals (set of 2 books), I. S. Gonorovsky. The book is a textbook for the new course "Radio circuits and signals" and corresponds to the program of this course for the specialty "Radio engineering". The first part sets out the spectral and ...

Frequency conversion is the shift of the signal spectrum along the frequency scale in one direction or another, i.e., in the region of both lower and higher frequencies. With such a shift or transfer, the shape of the spectrum should not change.

An example of frequency conversion (amplitude modulation, detection). When the AM signal is generated, the spectrum of the modulating signal containing the transmitted message is transferred to a higher frequency region to enable the receiving radio signal to be emitted in the form of electromagnetic waves into the transmission line. When detecting a radio signal, its spectrum is also transferred, but already in the opposite direction - to the low-frequency region, which makes it possible to re-isolate the modulating signal, and, consequently, the transmitted message. In this case, of course, it is required that with such transformations the waveform of the signal extracted during detection coincides with the shape of the modulating signal during modulation. The fulfillment of this requirement means that there is no distortion in the submission. A necessary condition for undistorted message transmission is the preservation of the shape of the spectrum of the control signal during its transfer both to the high-frequency region (during modulation) and during its reverse transfer to the low-frequency region (during detection).

The general principle that provides frequency conversion is that the signal to be converted is multiplied by a harmonic oscillation with a frequency of r. This oscillation must be obtained using a special oscillator called a heterodyne oscillator. If the signal spectrum contains a harmonic with a frequency of 0, then by multiplying these harmonic oscillations we get:

i.e., harmonic oscillations with sum and difference frequencies appear at the output of the multiplier, therefore, each harmonic of the signal causes the appearance at the output of the multiplier of two harmonic oscillations with sum and difference frequencies.

The figure shows the AM signal spectrum conversion diagram:

a) AM signal

b) AM signal spectrum

c) local oscillator signal

d) the spectrum of the local oscillator signal

e) spectrum of the signal at the output of the multiplier

f) the frequency response of the difference frequency filter (or the IF filter of the intermediate frequency)

g) signal at the output of the difference frequency filter.

Transistor frequency converter circuit.

In practical circuits of frequency converters, nonlinear elements (semiconductor diodes, transistors, vacuum tubes) are used. In this circuit, the multiplier is performed by a transistor, or rather its input nonlinear circuit: the base-emitter transition. The best conditions for frequency conversion are obtained if the dependence i b = (U b.e) is quadratic, i.e.

i b = i b.e + a 1 U b.e + a 2 U b.e

In the converter, the voltage U b.e is proportional to the sum of the voltages of the signal S (t) and the local oscillator U g (t), i.e., the variable component of this voltage:

U b.e (t) = S (t) + U g (t)

Substituting this expression in (1) we get.

i b = i b. e + a 1 S (t) + a 2 U g (t) + a 2 S 2 (t) + 2a 2 U g (t) S (t) + a 2 U g (t)

Of all the terms in this formula, only one is of interest - the underlined one, which contains the products of the local oscillator voltages and the signal.

For example, S (t) is described by the function

S AM (t) = U m sin (t +)

(Amplitude modulated signal)

and U g (t) = U m g sin (t +), then this term

2a 2 U g (t) S (t) = 2a 2 U m g sin (t +) *) = U m sin (t +) =

A 2 U m g U m (cos [- g) t + -] - cos [(- g) t ++])

If the circuit in the collector circuit of the transistor is tuned to the intermediate frequency pr = - r, then all other oscillations with frequencies, r, - r, 2, 2 r will be filtered out. The component of the collector current of the difference frequency - r causes the appearance of a voltage at the resonant impedance of the circuit u, therefore at the output of the converter