DE2658113A1 - METHOD OF REDUCING INTERMODULATION NOISE IN AN ELECTRONIC AMPLIFIER CHAIN - Google Patents

Description

22, D sz. 1978

DIpL-In ₀ - JOrcen WE ₁ NMILLER FATENTASSESSOR

SOSP! GmbH I

80OO Munich 8O
ZeppeJinstr. 63

COMPAOIIE INDUSTRIELLE DES TELECOMMUNICATIONS

CIT-ALCATEL S.A. 12, rue de la Baume, 75008 PARIS, France

METHOD OF REDUCING INTERMODULATION NOISE IN AN ELECTRONIC AMPLIFIER CHAIN

The invention relates to a method for reducing intermodulation noise in an amplifier chain and is particularly applicable to analog telephone links with repeaters.

Intermodulation noise occurs when an electrical signal passes through a non-linear conduction organ. This creates an interference signal from intermodulation products, which are combinations of the fundamental frequencies of the input of the non-linear conduction organ given signal.

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The intermodulation noise of an entire chain of amplifiers is made up of shares of all amplifiers in the chain · The total amplitude of the noise increases with the Number of amplifiers in the chain and limits the number of amplifiers that can be used.

To reduce the geearata phenomenon of the intermodulation noise In an amplifier chain, one tries to find the intermodulation noise that can be traced back to each individual amplifier in the chain to reduce as much as possible by using the components in such a way that they are in the linear characteristic range work. However, this endeavor takes place in the characteristic value scatter the switching elements its limit.

The invention aims to measure the overall amplitude of the intermodulation noise of an amplifier chain via this Limit beyond decrease.

This goal is defined by what is defined in the main claim Procedure achieved.

Preferred embodiments of the invention are in characterized the subclaims.

. The invention is described below on the basis of an exemplary embodiment explained in more detail with the help of six figures.

Fig. 1 shows a vector diagram to illustrate the Composition of the intermodulation products in an amplifier, chain.

FIG. 2 shows a vector diagram of the intermodulation products using the method according to the invention.

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- * - 2658Ί13

FIGS. 3 and 4 show vector diagrams for the composition of the intermodulation products in a chain of amplifiers with a gain factor of 1, which by and large have the same characteristics.

Fig. 5 is a block diagram showing an amplifier for a through a non-linear switching element of the amplifier generated intermodulation product corresponds.

Fig. 6 shows schematically an analog transmission line with a plurality of amplifiers, in which the inventive Method for reducing the noise is used.

1 relates to the composition of the noise in the general case of the intermodulation products at a given frequency of a chain of η non-linear amplifiers 1, 2, ... or n, each of which has a gain or attenuation factor G ₁ , G, ... or G and for a given input signal an intermodulation product

with the value I ₁ , I_, ... or I. χ Λ η

The total intermodulation product at the given frequency that is generated by the chain of the η amplifiers is equal to the vector sum of the intermodulation products I, ... I, as they appear at the end of the chain.

The intermodulation product I. generated by the amplifier j is amplified in the chain by the amplifiers j + 1, j + 2 ... η. Its final amplitude is t

After they have been generated, the intermodulation products 1: te X ₁ ... I no longer experience any phase shift in relation to one another,

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because they have the same frequency. The phase shift angle at the output of the chain between two intermodulation products I. and I -.- i »those on two successive amplifiers j and j + 1 are due to be equal to the phase shift angle ΔΤ-ii-iii between these two intermodulation products at the output of the amplifier j + 1.

Is the amplitude of the intermodulation products I, I at the output of the chain and the phase shift angle

. ■ ·, between two on two successive amplifiers j and j + 1 to be traced back intermodulation products I. and Ι ·, -ι known, then the sum of the intermodulation products I, ... I are represented vectorially, as they appear at the output of the chain, and then the entire intermodulation product at the given frequency of the amplifier chain.

In Fig. 1, the total intermodulation product at a given frequency is given by the vector 01, 12 existing vector train between 0 and η reproduced; the individual vectors represent the through each amplifier generated intermodulation products, as they occur at the output of the chain.

Looking at Figure 1, the important role played by the phase shift angle between the on each amplifier traceable intermodulation products for the determination of the amplitude of the total intermodulation product the chain clearly to learn more about this role, will be described below with reference to Figures 2, 3 and

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the case is described in which the amplifiers in the chain have a gain factor of 1 and, by and large, the same characteristic values exhibit. This case is of particular importance in practice, as it involves analog transmission using repeaters lines provided, where the problem of intermodulation noise is the most acute. In this case, the amplitudes of the intermodulation products I, ... I at the output of the chain are practically the same, and the same applies to the phase shift CJi. · between them.

In Fig. 2, the value of the angle Δ ι · · ₍₁ was chosen so that it lies between 7C / 2 and 3/2 * 3 "C.

Those to be traced back to every amplifier 1. .... η Intermodulation products Il ... In form a broken line in the vector diacnramm, the shape of which is similar to that of a star-shaped polygon is.

The amplitude of the total intermodulation product in this case is smaller than the intermodulation product generated by an amplifier. This case is therefore very much advantageous.

In Fig. 3, the value of the angles A1- ^ _{1 became} ^so

chosen so that it lies between 0 and «7 ^ / 2 or 3/2 'S ^ and 2 TC . The vector diagram of the intermodulation products I ₁ ... I generated by the amplifiers has a form which is appropriate

J- η

approaches that of a regular polygon. The amplitude of the total intermodulation product is greater than that Amplitude of the intermodulation generated by the amplification

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-λ-

Product with the exception of the cases where the ends of the Line train unite.

4 shows the extreme case in which the angles / λΡ · · _{ιΊ are} equal to 0. The amplitude of the total intermodulation product of the chain is equal to the sum of the amplitudes of the intermodulation products generated by each of the amplifiers. This is the worst case scenario.

In general, the three cases shown in the figures just described are encountered at the same time, since the value of the angle Ζ \ τ ■ ·, -t functionally of the frequency

3 D + J

of the entire considered intermodulation product depends.

The characteristic values of the intermodulation products to be traced back to an amplifier depend on those of the input the signal supplied by the amplifier. The amplitudes of these intermodulation products are for certain frequencies bigger than for others. It is advantageous if one can act on the angle / ΐτ · ■, -, on the one hand, so that one can obtain a

3 ι 3 + J-

The best possible compensation for the intermodulation products whose amplitudes are the greatest, even if this action results in poorer compensation for the other intermodulation products and, on the other hand, to avoid the worst case scenario where the intermodulation products add up in terms of tension.

The following shows how to do it in most

In cases it is possible to change the displacement angle / ly. . . between

33+ ¹

two by two amplifiers j and j + 1 of an amplifier chain, receiving an input signal χ generated intermodulation

products I. and Ι ·, _Ί of frequency P to change without the 3 3 + J-

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to change relative phase shifts existing between the fundamental frequencies of this input signal χ by between a phase shifter is placed on the amplifiers j and j + 1.

A non-linear amplifier to which an input signal χ is applied, which has frequencies A, B, ... N as the basic component and generates an intermodulation product with a frequency P, which is a function g of the basic frequencies, can be a linear for the intermodulation product Amplifier which has a certain gain factor and follows a certain phase law and a signal would be received at the input whose frequency is equal to the frequency P and whose phase is equal to the function g of the phases W _o CA) _t "^ (greater fundamental frequencies A, B ... N is at the input of the non-linear amplifier. Subsequently, the between two intermodulation products I. and I -.- i of the frequency P, which, based on a linear function g of the fundamental frequencies of the input signal, is generated by two amplifiers j and j + 1 are generated, new phase shift angles cX τ · · .-, between the two

3 t J ⁺ - ¹ -

Intermodulation products I. and Ι · ₊₁ # if a phase shifter with a phase shift law H ^ is interposed between the amplifiers j and j + 1, and the difference between these two phase shift angles is calculated.

The intermodulation products found at the output of an amplifier receiving an input signal χ are determined by generates the various switching elements of the amplifier that are non-linear in relation to the input signal χ. the

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non-linear switching elements form a signal χ in Signal y of the following form:

(1) M * I ^ * ^; Often = constant

It is assumed that the amplitudes of the non-linear switching elements of the amplifier from an input signal χ generated intermodulation products are sufficiently small that the secondary intermodulation products, which in turn could cause them can be neglected. Under these conditions an intermodulation product, which was generated by the amplifier on the basis of a signal χ applied to its input, as a sum the one available at the output of the amplifier and through each The intermodulation products generated by the non-linear switching elements of this amplifier can be considered.

One of the non-linear switching elements considered here The intermodulation product originating from the amplifier is generated on the basis of the fundamental frequencies of the signal χ, see above as they arrive in the non-linear switching element. Then the intermodulation product is sent to the output of the amplifier transfer. To calculate the phase of this intermodulation product, one can use the amplifier as a sequence of three in Include switching elements connected in series, which are shown in Fig. 5:

a first phase shift element 1 which obeys a phase shift law ψ _β and represents the part of the amplifier through which the input signal χ passes before it arrives at the non-linear switching element 2 under consideration here;

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- The non-linear switching element 2, which hypothetically is a signal x. into a signal y, of the following form:

<*: * * f ((/ _£ = constant)

and a second phase shift element 3 which obeys a phase law ife and which represents the part of the amplifier through which the intermodulation product due to the non-linear component 2 passes before it reaches the output of the amplifier.

It is assumed that the input signal χ given to the amplifier contains a certain number of given fundamental frequencies A, B ... N, the amplitudes of which are a, a, a. _T for periods U>, U », W.« And for original phases iff (A) (B), -. % (N) are:

_{The signal X 1} available at the output of the phase shift element 1 and at the origin of the intermodulation product generated in the non-linear switching element 2 has the form

to simplify ^ the notation is used:

In response to the signal x, the non-linear one sends Switching element 2 a signal y ,, which by inserting x ^ in

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the above equation for switching element 2 is obtained:

If one develops this expression by converting the cosine products into a sum, one obtains:

ι? * egg

where ι? * is a constant, where Onß is a relative integer and G represents the entirety of the linear forms g. If one replaces the terms Λ * · with their values, one obtains the Formula:

3 € 6 ^?

This signal y is fed to the phase shifting component 3, which _{converts it into a signal y o} which is present at the output of the amplifier and has the following form:

with ) r constant.

To simplify the expression, g is used to denote the linear function of a set of N elements which are represented by the Coefficient Ce £ is defined, with h varying from A to N. is :

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4t

It is then used for

This results in

Μα * Σ

This equation shows, firstly, that by a
non-linear switching element of an amplifier to which an input signal χ consisting of the fundamental frequencies A, B ... N is fed, consists of components whose frequencies are the linear combinations of the fundamental frequencies, and secondly, that from the point of view of each of these components the amplifier behaves like a linear amplifier, which has a certain gain factor and follows a certain phase law and which receives a signal at the input which has the frequency of the component under consideration, defined by a linear function g of the fundamental frequencies A, B ... N and has the linear function g of the original phases Te (A / Vers} ·· of the fundamental frequencies as the original phase.

This property also remains for the sum X of

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fs

Components of the intermodulation product obtained from the All of the non-linear components of the amplifier based on one and the same linear function g of the fundamental frequencies of the input signal χ because they all have the same frequency:

(I) X ₃ * <f ₅ α «  [<J (U >> t ♦ ^>

Consequently, the on an amplifier to which an input signal χ is fed, which as fundamental frequencies the Frequencies Ä, B ... N, to be traced back intermodulation products formed from different components, each having a frequency equal to a linear function g of fundamental frequencies A, B ... N, as well as having a phase equal to the sum of one from the amplifier and one from the linear Function dependent phase shift coefficients y £ and of the linear function g of the phase input of the amplifier of the fundamental frequencies A, B ··· N.

KFun, two amplifiers j and j + 1, arranged as a tandem in an amplifier chain to which a signal χ is fed with the frequencies A, B ... N as the fundamental frequency, are considered. The phase difference PjQ present at the output of amplifier j + 1 between the intermodulation product X generated by amplifier j on the basis of a linear function g of the fundamental frequencies A, B and the intermodulation product X generated by amplifier j + 1 on the basis of the same linear function g is used ".I ₊₁ calculated.

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Let: γζ £ (or γίψ-f) be the phase shift law of amplifier j (or j + 1) for signals belonging to the frequency band of the input signal χ? "\ U * J Ö-> zw · y ^ fe ^) is the phase shift coefficient of the component X. (Or X.,) Of the linear form g of the fundamental frequencies A, B ... K obtained intermodulation product; ] οΐί A) _t % dß) · · · · ► ¥ L ~ LN) (or [M «.ΓΑ)» · · Ψ · (if) öi- ^e phases of the fundamental frequencies Ä , B ... N at the input of the amplifier j (or j + 1)? J44 ^ \ | ^ the phases at the output of the amplifier j + 1 of the intermodulation product X. (Or X.,) man:

The phase of the intermodulation product X. , at the The output of the amplifier j + 1 is equal to s according to equation (2)

The phase Hd ff ^ f OO of a fundamental frequency k at the input of the amplifier j + 1 is equal to the sum of the phase shift Yf {^ J} # that the frequency k experiences through the action of the amplifier j, and from the phase shift φ , / Jr \ at the input of the amplifier j this same frequency ks

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It follows :

From this it can be deduced:

The phase ^ f * \ Λ4ϊ / at the output of the amplifier j + 1

of the intermodulation product X. is equal to the phase * γΈ (Xq \ of this intermodulation product at the output of the amplifier j, increased by the phase shift ^ t £ 4J * which is given by the amplifier j + 1 at the frequency g (A, ... N):

The phase of the intermodulation product X. at the output of the amplifier j is according to equation (2):

This results in :

If you combine equations (3), (4) and (5), you get:

According to the invention, a phase shifter which provides a phase shift is connected between the amplifiers j and j + 1

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for those belonging to the frequency band of the input signal χ Introduces signals and recalculates the phase difference 4JTQ at the output of the amplifier j + 1 between the intermodulation products X. and X

gj g

If you put:

this is how you get:

If you put:

this is how you get:

* IX

It follows :

Subtracting equation (6) from equation (7) results in t

It can also be stated that if one chooses for Ä% K ** where K is a constant and f is the frequency, one holds i * ^ i * ^ 'i ^ Q * This leads to the fact that in the following one only the phase spacing is considered in relation to a linear law as a function of the frequency.

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Equation (8) shows that turning on a

Phase shifter with a phase shift t ^ between two series-connected amplifiers j and j + 1 it allows the phase shift at the output of the second amplifier j + 1 between the one hand from an amplifier j and on the other hand from an amplifier j + 1 starting from a linear puncture g of the fundamental frequencies of the input signal generated intermodulation products X. and X. _{to change Ί} , on condition that the algebraic sum of the coefficients of the function g considered here differs by +1.

In practice, the amplitudes of the intermodulation products generated by an amplifier rapidly increase in direction higher order, and one only considers the second and third order intermodulation products. the Second order intermodulation products are of the form A + B or A-B. If equation (8) is applied to them, so you get :

do) α-B g (n) = »,. (Α) - η (%) j

The third order intermodulation products have the form A + B + C, A + B-C or A-B-C. Will use the equation on it (8) applied, one obtains:

If the phase shift is constant, we get:

* Λ it;

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by

It can be seen that switching on a

Phase shifter between the two amplifiers j and j + 1 die

for the intermodulation products generated by each of the amplifiers of the second and third order with the exception of the third order intermodulation products of the shape A + B-C existing phase shifts can be changed.

The method according to the invention for reducing the intermodulation noise in an amplifier chain consists from an application of the above results. It consists in switching phase shifters between certain amplifiers, which introduce practically constant phase shifts in the frequency band of the signal arriving at the input of the amplifier chain can be given. These phase shifters do not change the shape of the input signal as they have all the fundamental frequencies of the Give signals the same relative phase shifts. On the other hand, they change the phase shift angle between the intermodulation product components to be traced back to the amplifiers preceding these phase shifters in the chain of the second and third order (with the exception of those of the form A + B-C) and that of the subsequent ones in the chain Intermodulation product components to be traced back to the amplifier.

The points at which the phase shifters are switched on in the chain, as well as the value of their phase shift can be carried out experimentally on the basis of the characteristics of the amplifiers in the chain and on the input of this chain Signal are determined in such a way that for those intermodulation products, their components to be traced back to each of the amplifiers

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have the strongest amplitudes, phase shift angles which lead to the best possible compensation * They can also be determined from the outset in such a way that that the worst! compensation cases for the components of the intermodulation products can be avoided.

For example, in the case of an amplifier chain with a gain factor of 1 and with practically the same characteristic values for all amplifiers, as applies to an analog transmission line with intermediate amplifiers, it is advantageous to avoid the components having intermodulation products of the second order those whose angles of the relative phase shift are greater than or equal to the voltage add. For this purpose, Uiri-reversing transformers are used as phase shifters, which are regularly arranged after every k-th amplifier, if it is an analog transmission line with intermediate amplifiers, the number Ik being greater than 2. According to equations (9) and | 10), each inverting transformer changes the angle of the relative phase shift between the components of the

return

formator respectively preceding amplifier ^ supplying intermodulation products of the third order and the components of the intermodulation products to be traced back to the respectively following amplifiers by a value + JC · It follows that the components of the intermodulation products that were added up to now are now subtracted. The reverse process also occurs, but is insignificant since the number k is chosen to be greater than 2. On the other hand, the additional switching on of inverse transformers leads to all k amplifiers,

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that at the level of the groups of k amplifiers, partial sums of the components of the intermodulation products occur which add up while they previously compensated each other. To limit this interference, it is advantageous to use the monotonous arrangement change by the ^ ahl k different Values for the same amplifier chain are given, e.g. two different values k, and k, of which one is roughly a multiple of the other.

Fig. 6 shows schematically a telephone connection between two terminals A and B, thirteen of the same through Has circles shown amplifier. The circles are shown filled when a reversing transformer is switched on is. This example uses a reversing transformer

alternately every two and four amplifiers switched on. Thanks to the reversing transformers, this could be due to intermodulation noise in the connection can be reduced by half.

In addition to amplifier chains, the invention can also be used with several stages of an amplifier or several stages connected in cascade Amplifiers, such as applied to a telephone line terminal.

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Claims (1)

PATENT CLAIMS Method for reducing the intermodulation noise in an amplifier chain, thereby characterized in that phase shifters are switched between certain amplifiers, each introduce a practically constant phase shift in the frequency band of the input signal of the chain. 2 - Method according to claim 1, characterized marked that the phase shifters evenly all k amplifiers are arranged along the chain. 3 - Method according to claim 1, characterized in that the phase shifters are arranged at uneven intervals in the chain. 4 - Method according to one of the preceding claims, characterized in that it is the phase shifters are reversing transformers. OTOGiNAL INSPECTED 709827/0698