Frequency conversion principle. Frequency converter - types, principle of operation, connection diagrams

Frequency conversion is the transfer (transposition) of the spectrum of a signal (usually narrow-band) along the frequency axis “up” or “down” to a certain distance w g, specified by a local oscillator - a low-power harmonic oscillation generator. In this case, the type of modulation and structure of the signal spectrum are preserved, only its position on the frequency axis changes.

The frequency converter consists of a frequency mixer and a local oscillator (Fig. 3.32).

The frequency mixer is implemented on a parametric or nonlinear basis, because at its output it is necessary to obtain an oscillation of the combination frequencies of the second-order input signals (sum or difference). The average frequency of the output signal is called intermediate. As a matter of fact, there is nothing new for us in the frequency conversion operation; we have already encountered it when considering the properties of the Fourier transform (item 9), the properties of the analytical signal (item 5) and the parametric implementation of a single-sideband modulator (Fig. 3.20). The circuit shown in Fig. 3.20 can be used as a parametric frequency converter without any changes. A nonlinear frequency converter can also be made according to the amplitude modulator circuit discussed above (Fig. 3.16) when setting up the load oscillatory L.C. circuit to intermediate frequency.

Frequency converters are part of the vast majority of modern radio receiving devices (superheterodynes). Their use allows the main pre-detector processing of signals in these receivers - filtering and amplification - to be carried out not at the signal frequency (which can be too high and vary over a wide frequency range), but at a fixed intermediate one. This allows you to significantly improve the sensitivity and selectivity of receivers, as well as simplify their tuning in a wide range of received frequencies.

Control questions

1. Which FU is called a frequency converter?

2. Give the algorithm and circuit of the parametric frequency converter.

3. Explain the purpose of each element of the parametric frequency converter circuit.

Signal frequency conversion transfers the signal frequency to another area on the frequency axis. Let's consider the meaning of this signal processing operation.

Classical frequency conversion system consists of an input filter, local oscillator, mixer, and output intermediate frequency (IF) filter.

Purpose input filter- limit the frequency band of the input signal. To simplify, we assume that this signal is sinusoidal with frequency f 1, given by the function X(t)=sin(2πf 1 t + ϕ 1), where f 1 is the frequency of the input signal, ϕ 1 is the initial phase of the input signal, π = 3.141...

Heterodyne is a sinusoidal generator with a constant frequency f 2 and an initial phase ϕ 2. Let us describe the local oscillator signal with the function Y(t)=sin(2πf 2 t + ϕ 2).

Mixer is a signal multiplier. At the output of the mixer, a complex signal with the function X(t) * Y(t) is generated. Taking into account the trigonometric relation sin α * cos β = ½ (sin(α + β) + sin(α - β)), it becomes clear that the signal at the mixer output will consist of the sum of sinusoidal signals with frequencies f 1 + f 2 and f 1 - f 2 and the corresponding initial phases.

An intermediate frequency filter (this is the traditional name from radio engineering) is designed to isolate one of the frequency components: f 1 + f 2 or f 1 - f 2. If a filter is used that passes the frequency f 1 + f 2, then the corresponding frequency conversion operation occurs with increasing frequency output signal relative to the input. If a filter is used that passes the frequency f 1 - f 2, then the conversion occurs with frequency reduction.

Taking into account the fact that the input physical signal- this is not a single frequency f 1, but the sum of frequencies in the decomposition of a real signal with a limited bandwidth, it is clear that frequency conversion operation can shift the frequency band of a signal either left or right on the frequency axis. And by adjusting the local oscillator frequency, you can control either the output frequency shift or the input frequency shift, depending on the purpose of the conversion.

The use of frequency downconversion followed by digitization of the signal allows the use of an ADC with a lower conversion frequency.

The frequency conversion operation can be thought of as special case using the intermodulation effect to your advantage. Here as nonlinear element acts as a multiplier, and based on its theoretical properties shown above, an ideal multiplier and an ideal sinusoidal local oscillator produce exclusively first-order intermodulation.

1. Signal frequency conversion. In this case, the signal at the input of the device with variable amplitude and (or) phase, concentrated along the spectrum near frequency f 1, is converted at the output of the device into a signal having the same shape (K and - constants), but concentrated along the spectrum near frequency.

When converting frequency up, f 2 is greater than f 1. When converting frequency down, f 2 is less than f 1.

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

2. Frequency converter. A frequency converter is a device that allows you to move the spectrum of an input signal up or down the frequency scale.

A nonlinear amplifier with an oscillating circuit at the output tuned to a special (combination) frequency can be used as a frequency converter, Fig. 3.1.

Figure 3.1. Converter circuit when converting frequency up

Upward frequency conversion is carried out by multiplying two oscillations and and isolating an oscillation with a combination frequency (w + Ω) at the output, following the formula:

cos(x)×cos(y) = (1/2)

In this case we have:

Impact:

Helpful reaction:

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

Downward frequency conversion is carried out using the same nonlinear amplifier circuit (Fig. 3.2) by multiplying two input oscillations and isolating an oscillation with a combination frequency at the output, following the formula:

cos(x)×cos(y) = (1/2)

Figure 3.2 - Converter circuit when converting frequency down

In this case we have:

Impact:

Helpful reaction:

In general, a low-frequency signal can be represented as a sum of several harmonic oscillations. A low-pass filter is needed to isolate the beneficial response.

3. Amplitude modulation ( AM) has historically been the first type of modulation mastered in practice. Currently, AM is used mainly only for radio broadcasting on a relatively low frequencies(not higher than short waves) and for transmitting images in television broadcasting. This is due to the low efficiency of using the energy of modulated signals.

AM corresponds to the transfer of information s(t) to the amplitude U(t) at constant values ​​of the parameters of the carrier vibration: frequency w and initial phase j 0. The AM signal is the product of the information envelope U(t) and a harmonic oscillation of its filling with higher frequencies. Recording form of amplitude-modulated signal:

u(t) = U(t)×cos(w o t+j o), (3.1)

U(t) = U m ×, (3.2)

where U m is the constant amplitude of the carrier vibration in the absence of an input (modulating) signal s(t), m is the amplitude modulation coefficient

The value m characterizes depth amplitude modulation. In the simplest case, if the modulating signal is represented by a single frequency harmonic vibration with amplitude S o , then the modulation coefficient is equal to the ratio of the amplitudes of the modulating and carrier oscillations m=S o /U m . The value of m must be between 0 and 1 for all harmonics of the modulating signal. At 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 of m tends to 1 at the extreme points of the function s(t).

One hundred percent modulation (m=1) can lead to signal distortion when the transmitter is overloaded, if the latter has a limited dynamic range in terms of the amplitude of carrier frequencies or limited transmitter power (increasing the amplitude of carrier oscillations in peak signal intervals U(t) requires doubling the transmitter power four times).

When m>1 the so-called overmodulation, an example of which is shown in Fig. 3.6. The shape of the envelope during overmodulation is distorted relative to the shape of the modulating signal, and after demodulation, if its simplest methods are used, the information may be distorted.

4.Monoharmonic amplitude modulation . The simplest form of a modulated signal is created with a monoharmonic amplitude modulation – modulation of a carrier signal by a harmonic oscillation with one frequency Ω:

u(t) = U m × cos(w o t), (3.3)

The values ​​of the initial phase angles of the carrier and modulating oscillations here and in what follows, to simplify the resulting expressions, we will take equal to zero. Taking into account the formula cos(x)×cos(y) = (1/2) from expression (3.3) we obtain:

u(t) = 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 frequency Ω moves to the frequency region w o and splits into two oscillations with frequencies respectively w o + Ω upper side frequency, and w o - j - lower side frequency. These frequencies are located on the axis symmetrically relative to the frequency w o , Fig. 3.7. The amplitudes of oscillations at side frequencies are equal to each other, and at 100% modulation they are equal to half the amplitude of oscillations of the carrier frequency. If we transform equation (3.3) taking into account the initial phases of the carrier and modulating frequencies, we obtain a phase change rule similar to the frequency change rule:

The initial phase of the modulating oscillation for the upper side frequency is added to the initial phase of the carrier,

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

The physical width of the spectrum of the modulated signal is twice the width of the spectrum of the modulating signal.

Under frequency conversion understand the process of transferring the signal spectrum to another frequency region without any distortion.

Frequency conversion is used to place the signal spectrum in a given portion of the frequency range of a communication channel, as well as to increase the sensitivity and selectivity of superheterodyne type receivers.

The principle of conversion is illustrated in Fig. 3.9, 3.10.

The signal at the converter input depends on time and the primary signal:

In the multiplier it is multiplied by the local oscillator signal

and then filtered bandpass filter.

The input signal can be modulated (continuously or discretely) in amplitude, phase, and carrier frequency. Let the spectral density of any modulated signal consist of spectral components concentrated around frequencies +co 0 (Fig. 3.10, A):

Rice. 3.9. Block diagram of the frequency converter:

1 - multiplier;2 - bandpass filter

Rice. 3.10.

Spectral density is characterized by the spectral density of amplitudes and phase characteristics. If these characteristics are necessary for the corresponding calculations, they need to be calculated using formulas and presented in the form of graphs.

In other cases, precise data is not required and spectral densities can be depicted arbitrarily: for example, in the form of bell-shaped spectra or triangles for continuous spectral densities or arrows for discrete ones, as is done in this book.

Let's calculate the spectral density of the local oscillator signal using expression (A.1.3) of the delta function:

Assuming we get

Spectral density of a harmonic cosine oscillation with a zero initial phase (Fig. 3.10, b) is determined by the product of the amplitude of this oscillation, increased by l times, and the sum of two delta functions located at points on the frequency axis co = +co r. Let us also calculate the spectral density of the product of the input signal and the local oscillator using formula (2.51):

Where - intermediate frequency; ? VX (/Ъ), 5 g (/co) - spectral densities of the input signal and local oscillator, respectively.

In the spectral density of the product shown in Fig. 3.10, V, contains a useful conversion product (spectral components near intermediate frequency values

co = +(O pr), as well as interfering components near frequencies -co 0 - co g, COo + Wp

Useful components (see Fig. 3.10, c, d) pass to the output of the bandpass filter, and the interfering ones are significantly attenuated by them. Spectral components at the output of the bandpass filter (Fig. 3.10, d ) are determined by the expression

if the bandpass filter transmission coefficient /C(/co) = 1 in a given frequency band. They are accurate to a constant factor equal to A/ 2, coincide with the spectral components of the signal at its input, and the spectrum of the converted signal is grouped around new frequency values ​​equal to co = +co ex.

Frequency conversion is used in signal modulation and detection.

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

Frequency frequencies are used in radio receivers and can be measured. 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. frequencies, called intermediate. The constancy of the intermediate frequency = const when tuning the signal frequency is ensured at the same time. tuning the local oscillator frequency Thus, signal amplification in devices with frequency frequency is carried out at a relatively low, usually standard frequency.

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

Since there are differences in the mixer output circuit. combination frequency, then to isolate the difference or sum frequency, the output circuit must be selective, i.e., resonant, tuned to the desired frequency.

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

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988 .

See what "FREQUENCY CONVERSION" is in other dictionaries:

frequency conversion- The process of linearly transferring the 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: engl. 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: engl. 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 the radio frequency band occupied by a signal to another part of the frequency spectrum. [GOST 24375 80] Topics radio communications General terms radio reception Synonyms frequency conversion ... Technical Translator's Guide

converting frequency to number code- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN frequency to number conversion ... Technical Translator's Guide

frequency conversion in the direction of decreasing it- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN frequency down conversionFDC ... Technical Translator's Guide

frequency to voltage conversion- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN frequency to voltage conversion ... Technical Translator's Guide

frequency down conversion- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN frequency down conversion ... Technical Translator's Guide

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

frequency conversion based on Raman scattering- Ramano dažnio keitimas statusas T sritis radioelektronika atitikmenys: engl. 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 engineering circuits and signals (set of 2 books), I. S. Gonorovsky. The book is a textbook for the new course “Radio Engineering Circuits and Signals” and corresponds to the program of this course for the specialty “Radio Engineering”. The first part describes the spectral and...