FR2476846A1 - Nonlinear amplifier characteristic measuring device - utilises calculator controlled frequency generators and phase shifters forming intermodulation products - Google Patents

Abstract

The characteristics and saturation power of an amplifier are measured utilising several frequency generators. This permits measurement at all frequencies and not just at several frequencies. Intermodulation products are generated from two frequency generators (61,62) whose outputs are mixed by coupling circuits and applies, via attenuators (A1,2), to phase shift circuits (D1,2). The shift circuits comprise motor driven elements and are controlled from a calculator (91) with the values stored in its memory. The calculator balances each generator arm to eliminate the fundamental generator frequency. The amplifier under test (BA) is connected into a measuring arm and a measuring receiver (R) is connected to it and to the calculator data bus.

Description

The subject of the present invention is a device for measuring

non-linear characteristics of an electric organ and, as an application,

  measuring the saturation power of amplifiers

  cators. = An electrical transmission member is said to be linear when the electrical signal it delivers is directly proportional to the signal it receives. For a number of organs,

the output signal is actually expressed by a function

  polynomial tion of the input signal, which reflects their non-linearity. Such organs, receiving a signal at a pure frequency f, generate an output signal whose spectrum contains the frequency

  f but also the harmonic frequencies 2f, 3f ...

  The same organs, receiving two frequency signals

  these pure f1 and f2 generate inter-

  frequency modulation f1 f2 (order 2>, 2f1 f2

or 2f2 fl (order 3) etc ...

  In transmission systems, for example, the imperfectly linear members are generally the amplifiers. But there are other non-linear organs, including coils with a ferromagnetic core, transformers and

filters. In the following description, we

  will hold to amplifiers without this constituting

  naturally a limitation for the invention.

  Non-linearity measurements require

  one or more generators as well as a receiver.

  We will assume, in what follows, that there is no problem of frequency discrimination and that we can distinguish two frequencies, as close as they are. This is generally the case with devices

  which, for example, for the function ranges

from 1 to 100 MHz have measurement windows of the order of 10 to 20 Hz. The receiver

generally includes, at its input, an amplifier-

  and / or a mixer. But these organs are themselves nonlinear elements. When attacked by high level signals, they generate harmonics and intermodulation products which combine with those desired

measure, and that disrupt the measurement.

  It is a minor annoyance if the organ to be measured is strongly non-linear, the level of non-linearities to be measured then masking those of the receiver. But in the case of analog link amplifiers, the measurement is disturbed to the point of being made impossible. So he is

  necessary to remove or at least mitigate strong-

the fundamental frequency (s) using

  a filter. This filter must have characteristics

  as it attenuates the fundamental frequency (s) while letting the frequencies pass

harmonics and frequencies of the inter-

modulation to be measured. These requirements raise two problems:

  1) - a passive filter has a characteristic impedance

  which is only determined in its bandwidth. Apart from this, this

  impedance is poorly determined. Now, it's just-

In this zone, the fundamental frequency (s) fall (s). The member to be measured therefore does not see the characteristic impedance before charging it during normal operation. However, the rate of non-linearity depends

precisely of this load impedance.

  The measurement cannot therefore be made under normal operating conditions. This problem is usually overcome by inserting between the organ to be measured and the 2476e46

  filter, an attenuator which improves the coef-

  think of the latter, twice its attenuation. Experience shows that a 7 to 8 dB attenuator is generally necessary. However, this attenuator reduces the sensitivity of the receiver and therefore the minimum level of the non-linearity signal as much as possible.

wish to measure.

2) - For harmonic measurements, we know how to perform

without difficulty filters stopping a

  quence f1 and letting frequencies 2fl, 3f1 pass ... It is quite different with intermodulation products. This is how two signals at frequencies f1 and f2 sent

  simultaneously in the same device non

  linear not only generate harmoni-

ques, but also products of frequency f + f2 'fl - f2, products of frequencies 2f1 + f2, 2f2 + f1 and products of frequencies

  2f1 -f2, 2f2 - fl to limit itself to order 3.

  If you want to measure these

  nier products, we observe that the difference between the frequency of one of them and the frequency of a fundamental is equal to the difference between the two fundamental frequencies. Consequently, if the two fundamental frequencies are

  very close, the frequency of the corresponding products

  laying will also be very close. The filter placed at the input of the receiver must have a significant stiffness in order to discriminate these frequencies. For example, a discrete component filter allows a deviation of 5% of the fundamental frequencies and a quartz filter allows a deviation of 1%. But these

are very expensive.

  A measuring device, according to art

  an intermodulation product of

  quence equal for example to 2f1 - f2 is then con-

form in the diagram of figure 1.

This device comprises two generators G1 and G2 delivering signals at frequencies f1 and f 2, two source filters 11 and 12 of low-pass type (which are intended to improve the spectral purity of the generators), a coupler 14, a member 16 which is the organ to be measured, an attenuator 18, a bandpass filter 20 centered on the frequency 2f1 - f2 and finally a receiver R. The use of a bandpass filter in reception allows, with a single filter, to carry out the measurement with several pairs of frequencies

f1 and f2 provided of course that the product of

  modulation falls at the center frequency of the filter.

  To remedy the drawbacks mentioned above (and which are linked to the use of a filter), the present invention proposes to achieve the attenuation of the fundamental frequencies by another means than by filtering, namely by a

  interferometric type means. To this end, the

vention proposes to use a coupler to take the signals at fundamental frequencies at the output of the generators (or at the input of the organ to be measured) and to reinject them at the input of the receiver (or at the output of said organ) with a suitable amplitude and phase to suppress (or all

  at least strongly attenuate) each fundamental signal

  mental. More specifically, the invention has

  The object is a device for measuring the characteristics

non-linear ticks of an electrical component, this device comprising a plurality of signal generators at different fundamental frequencies supplying a measurement branch comprising the component in question, means for attenuation, in the signal delivered by said branch, signals at different fundamental frequencies, and a measurement receiver, characterized in that these attenuation means consist of as many compensation branches as there are generators,

  each branch comprising a phase shifter and a

  adjustable nuator and being connected in parallel on the measuring branch by means of couplers, part of the signals emitted by the generators being thus superimposed on the input of the receiver to the signal delivered by the measuring branch, the phase shifter and the attenuator of each compensation branch being adjusted to minimize the component of the signal applied to the receiver at the fundamental frequency corresponding to each

generators.

  The advantages of the device of the invention are numerous: - it allows the measurement of non-linearity of any organ,

- it does not require a filter at the reception entrance-

tor,

- non-linearity measurements can be performed

  killed at any frequency and no longer at a few frequencies, - the proximity of the fundamental frequencies is no longer limited by the stiffness of a filter and the fundamental can be as close as it

  is desirable (at least within the limit of

  receiver interference which can be in the range of 10-7) - the minimum measurable level is lowered because there is only a 3 dB coupler between the organ to be measured and the receiver (and possibly

2476S46

an attenuator). The gain on the minimum level of the fundamental is a few decibels; the natural noise of the source is compensated near the fundamental: using the device of the invention with two frequencies sufficiently

  close, we can measure inter-

  modulation which are normally drowned in the

source noise.

In any case, the characteristics and advantages of the invention will appear better after

  the following description, of exemplary embodiments

given for explanatory purposes and in no way limitative.

  This description refers to attached drawings

on which: - Figure 2 represents the variations

  attenuation of a signal at a fundamental frequency

mental when superimposed on it a signal of the same phase, but of different amplitude, - Figure 3 represents the variations

  attenuation of a signal at a fundamental frequency

  mental when superimposed on it a signal of the same amplitude, but of different phase, - Figures 4, 5 and 6 represent

  three particular embodiments of a provision

  sitive according to the invention, - Figure 7 shows the result

  of a spectral analysis of a signal seen by a re-

  receiver preceded by a device devoid of compensation branches, - Figure 8 represents the result of the spectral analysis of the signal seen by a receiver preceded by a compensation device according to the invention, - Figure 9 represents an example of

  variations of an intermodulation product of

  quence 2f1 -f2 depending on the level of the fundamental, - Figure 10 illustrates a device according to the invention provided with an automatic analog adjustment, - Figure 11 illustrates a device according to the invention provided with a suitable computer

to make the adjustment.

  The invention is essentially based on an interferometric process between two alternating signals. If the phase of the signal fed back to the input of the receiver is exactly opposite to the phase of the output signal of the organ to be measured

  and if the amplitudes of these two signals are rigorous

strongly equal, then the attenuation is perfect, that is to say infinite. However, this ideal situation is never obtained in reality and there are always slight differences in phase AE Effect of amplitude Ak between the two signals. Attenuation is not

then more infinite.

The curves of FIGS. 2 and 3 give the variations of this attenuation A (expressed in decibels and plotted on the ordinate) respectively as a function of Ak (expressed in dB and plotted on the abscissa) respectively when the phase shift AP is zero and as a function of Av (expressed in degrees and plotted on the abscissa) when the amplitude difference

Ak sucks.

  FIG. 4 represents a first embodiment of a device according to the invention, which makes it possible to obtain the desired attenuation of the fundamental frequencies. The illustrated case corresponds to the measurement of intermodulation products making

intervene three fundamental frequencies fly f2 and f3.

  The device then comprises: three generators G1, G2 and G3 delivering sinusoidal signals respectively at frequencies fl, f2 and f3; three source filters F1, F2 and F3; three couplers C1, C2 and C3; three compensation branches BC1, BC2 and BC3 each consisting of an adjustable phase shifter (D1, D2, D3) and an adjustable attenuator (A1, A2 and A3); two couplers C4, C5; a measurement branch which, in the illustrated case, is an amplification branch BA comprising an amplifier framed by two attenuators 31 and 32; three couplers C6, C7 and C8 and finally a receiver R. FIG. 5 illustrates a second embodiment which differs from the first by the use of coupling resistors R1, R2 and R3, instead of the couplers C1, C2 and C31 and by the use of C10 and Cil couplers connected differently

couplers C4 and C5.

  FIG. 6 illustrates yet another embodiment which differs from the first by the connection of the couplers C12 and C13 and by the use, in each compensation branch, of an amplifier (a1, a2 and a3) followed by a filter

  low pass (FC11 FC2, FC3) letting pass through

  quences fit f2 and f3 specific to each branch.

The adjustment of the device can be done very simply by proceeding successively

at the extinction of each of the fundamental frequencies

  tales. For this, a wodulation between the input and the output of the device shows a curve with minima regularly spaced and whose depth is more or less marked (we can use a network analyzer instead of a wodulator, the curve then appearing on a screen). The adjustment then consists in acting on the phase shifters to move the minima and bring one of them to the desired frequency. By then acting on the attenuators, this minimum is made as low as possible. Finally, by reconnecting the generator and adjusting the receiver to the desired frequency, we can perfect the compensation

by acting on fine adjustments.

The attenuators of the compensating branches

  tion can be attenuators with studs, or amplifiers whose gain varies by action on a feedback. The variable phase shifters can be lengths of switched lines having a large bandwidth. But it is also possible to use phase shifters with discrete components, circuits using wave devices

  surface, operational amplifiers, etc ...

  Figures 7 and 8 illustrate the attenuation of the fundamental frequencies that can be obtained

using a device according to the invention.

  These figures represent the result of a spectral analysis of the signal present at the input of the receiver in the case of the use of two frequencies

  fundamental, in the absence of branches of compensation

sation (figure 7) and in the presence of these branches

according to the invention (Figure 8).

  The central frequency Fo is equal to 56,000, 150 kHz. The frequency f1 is at Fo - 150 Hz and the frequency f2 at Fo + 150 Hz. On the abscissa

one division represents 100 Hz.

We see in Figure 7 the presence of lines of order 3 at 55,999, 700 kHz and at 56,000, 600 kHz, but also lines of order 6 at 55,999, 400 kHz and 56,000, 900 kHz. All these lines are due to saturation of the input mixer as

indicated above.

  The curve of FIG. 8 presents the same spectral analysis with 60 dB of compensation of the signals at fundamental frequencies obtained with the device of the invention. It can be seen in this figure that the background noise of the generator has also been compensated, at least in the very narrow window (2 kHz) where the analysis is carried out. The order 5 lines have disappeared and the order lines

3 have decreased considerably. Because of the dyna-

  mique of the receiver these lines are due only to the amplifier in measure. We see that the fundamentals are only 30 dB above

intermodulation products.

  We can notice that it is arbitrarily that we chose a difference of 300 Hz between the

  fundamental frequencies. As the receiver discri-

mine without difficulty even closer frequencies, it would have been possible to take a smaller difference, for example 50 Hz. A filter which would allow the same result according to the technique of the prior art should therefore be a stop-band of 60 dB of attenuation and

8 stiffness. Such a filter is impractical.

The curve in FIG. 9 represents the level Ni of the frequency intermodulation product

2f1 - f2 depending on the NF level of the fundamentals.

  The two fundamentals are issued at the same level and it is their common value that is represented

on the x-axis.

  It will be observed that this measurement corresponds to a very important dynamics of level of the fundamentals (-1 to +22 dBm) and with frequencies

very close to each other.

The measures described above correspond to

are based on manual adjustment of the compensation branches

  station. They require a certain training on the part of the operator to be carried out quickly. This is why the invention provides

  variants in which this setting is self-

matic. Two types of automation can be used, one analog, the other logic. These two automatisms, the principle of which is the same, are distinguished by the fact that the first operates from DC voltages and the second from

  digital signals supplied to a computer.

  They are illustrated in Figures 10 and 11 in

  the particular case of a measurement of inter-

  modulation in 2f1 - f2 'which requires two generators. The device represented in the figure is of the kind of that of FIG. 4. It comprises two compensation branches each composed of a generator (G1, G2), a coupler (C1I C2), an adjustable phase shifter (D1 , D2) and an attenuator

  (A1, A2). Couplers C5, C6 combine these branches

  to the amplifying branch. BA. The assembly is connected to a receiver R. The receiver R has an input 40 and an output 50 delivering a continuous signal proportional to the measured level of the intermodulation product. The phase shifters D1 and D2 and the attenuators A1 and A2 are controlled by an automatic adjustment circuit 60 which comprises: an adapter

level 62; a delay device 64 connected to the adapter

  guardian; a comparator 66 with two inputs 67 and 68, the first being connected to the adapter 62 and the second to the delay device 64; a circuit for counting the changes in sign of the signal applied to it and which comes from comparator 66; a switch 72 having a

  control input 73 connected to circuit 70 of

  number, one input 74 connected to comparator 66, and two outputs 75, 76; a control circuit

- 2476846

attenuation 80A connected to the output 75 of the switch 72; and finally, a phase shift control circuit

  D connected to output 76 of this same switch.

The device shown also includes switches or relays: a switch 82

  to an input connected to the standby control circuit

  nuation 80A and two outputs connected respectively

to the adjustable attenuators A1 and A2 of the two branches

ches; a switch 84, to an input connected to the phase-shift control circuit 80D and to two

  outputs connected respectively to the adjustable phase shifters

bles D1 and Dz of the two branches; two relays 86, 88 arranged between the generators G1, G2 and the couplers C1, C2. The four organs 82, 84, 86

  and 88 are controlled by a circuit 90.

  The operation of this circuit is as follows. Each of the fundamentals is first compensated before the measurement is undertaken. For this purpose, the switch 90 controls the relays 82, 84, 86 and 88. To compensate for the signal at the frequency f1 coming from G1, 86 is closed, 82 and 84 are connected on the side of the variable attenuator and phase shifter

  from the first lane and 88 is open; and vice

vice versa for signal compensation at frequency f2. In the measurement position, the relays 86 and 88 are closed while the switches 82 and 84 are decoupled from the attenuators and the phase shifters. Under these conditions, the voltage taken from the output of the receiver R passes through the level adapter 62 which has the role of making the levels compatible

  Operating. The voltage delivered by the re-

  counter R is compared at all times by 66 with that of the previous instant. The comparator 66 delivers a positive or negative voltage depending on the state of the voltages applied to its inputs 67 and 68. This voltage acts in one direction or in t4? I444 the other on the attenuation command 80A or of

  phase shift 80D, depending on the position of switch 72.

  The latter successively implements the attenuation and phase shift commands. For this, the circuit 70 counts the changes in sign of the output of the comparator 66. After a

  fixed number of sign changes, it switches 72.

The role of the BOA and 80D circuits is then to act on the attenuators and phase shifters so that the voltage at input 67 of the comparator is

  lower than the voltage at input 68.

  Variable attenuators A1 and A2 and

  the variable phase shifters D1 and D2 are with memory.

These are, for example, motors acting on

variable resistances.

  The fundamental from G1 being compensated, the same process is used for the compensation

of the fundamental from G2.

  All these functions can be fulfilled

  by digital means of information processing

mation. The synoptic diagram of the device corresponds to

laying is shown in Figure 11 which includes

a computer 91 connected by a bus of 92 to the genera

  G1, G2, the R receptor, and the organs of

adjustment-Al A2 and D1, D2-

The computer performs balancing

of each of the branches as follows.

  The generator G2 is first switched off and G1 is brought to its nominal level. The receiver being set on fl, an action on A1 and Dl causes the extinction

  of the fundamental fl. When the level of this fundamen-

  tal is below a predetermined level, the

  generator G1 is disconnected and G2 is put into service

  vice. Compensation is then carried out on the ground.

  damental f 2 it remains to perform the measurement and the computer controls the setting of the receiver

on 2f1 - f2 or 2f2 - f1.

  The device which has just been described finds a preferred application in the measurement

  of the saturation power of the amplifiers.

We know that an amplifier's role is to deliver a signal as similar as possible to the input signal, but amplified. If the input signal increases, there may be a distortion of the output signal due to the saturation of the amplifier. In addition, an amplifier is a non-linear member, which means that for an input signal at frequency f., The amplifier

  outputs a signal at this frequency f., but also

  signals at harmonic frequencies 2f, 3f ... which we want to keep as low as possible. The theory shows that in linear operating mode, the harmonic 3 varies, in level, three times faster than the fundamental. As we approach saturation this

slope varies and increases rapidly.

  The C.C.I.T.T. therefore gave, in opinion G223, volume III-1 of the orange book, a definition of saturation which is as follows: "The level of saturation of an amplifier (or usable power level) is the absolute level of power at the exit for

  which the absolute level of power of the third

  5th harmonic increases by 20 dB when the signal level applied to the input of this

amplifier increases by 1 dB.

This definition is no longer applicable when the measurement frequency is so high that the third harmonic falls outside the band transmitted by the amplifier. We can then apply the following definition: Second definition. Saturation level

  (or usable power level) of an am-

  amplifier is 6 dB higher than the common value, at the amplifier output, of the absolute power levels of two waves

  sinusoids of the same amplitude and frequency

  these respective A and B when we adjust them so that if we increase by 1 dB

  the level of each at the input of this amplifier

  the output level of the product

  frequency intermodulation (2 A-B) increased

would lie by 20 dB ".

  It turns out that with the use of transistors the maximum slope of harmonic 3 does not exceed 12 to 15, which makes the definition

previous unusable.

  It is therefore necessary to find out what is the change of state of an amplifier which characterizes its saturation. A certain number of criteria can be considered 1) - gain variation: The gain of an amplifier is theoretically independent of the level of the signal which is - injected into it. We note, however, from a certain level, that this gain varies. Saturation could be defined from the level for

  which gain has varied by a certain amount.

2) - phase rotation:

As with gain, the phase of an amplifier

level is independent of the level except when approaching saturation. Saturation could be the level for which the phase

has varied by a certain amount.

  3) - change of slope of harmonics 2 and 3 t It is this definition (on harmonic 3)

  which is temporarily retained by the C.C.I.T.T.

  Depending on the level of the fundamental and in unsaturation regime, the harmonic 2 varies with a slope of 2 and the harmonic 3 with a slope of 3. When approaching saturation, this slope increases.

The previous definition of C.C.I.T.T.

  was based on the appearance of a slope of 20 on harmonic 3. We could naturally propose

a definition of saturation based on the pairing

  tion of a lower slope (6 dB for 1 dB for example) on one of the two harmonics or on the level where the harmonic deviates by a quantity

  determined from its theoretical value.

All of these definitions, although

However, geables have a drawback: they are based on the appearance of an elbow on a curve and experience shows that this elbow is generally not very accentuated. These definitions therefore lead, by their nature, to an imprecision in the measurement and none of them can be retained for a precise definition of the

  saturation power of an amplifier.

  The device of the invention allows just-

  ment to base the measurement of such a parameter on - another criterion much more satisfactory than the previous ones. According to this criterion, we no longer take into account one or the other of the magnitudes envisaged

  higher (gain, phase shift, etc.), but a large

  which combines all of the effects of saturation. This quantity is the amplitude of the signal precisely delivered by the device

  of the invention. Indeed, the signal from the dispo-

  sitive of the invention can be considered as an error signal, which is generated by the 2476i46 'saturation which affects the amplifier. This signal is the result of all the effects indicated above. Its variation is therefore particularly brutal when approaching saturation. The device of the invention then allows a precise measurement of

  the power at which this phenomenon appears.

Claims (6)

  1. Device for measuring the characteristics   ques nonlinear of an electric organ, this dis- positive comprising a plurality of generators   (G1, G2, G3 ...) of signals at fundamental frequencies   different mental (f, f2, f3 ...) supplying a measurement branch (BA) comprising the organ in question, means of attenuation, in the signal delivered by said branch, signals at different fundamental frequencies, and a measurement receiver (R), characterized in that these attenuation means consist of as many compensation branches (BC1, BC2, BC3 ...) as there are generators, each branch comprising a phase shifter (D1, D2, D3 ...) and an attenuator (A1, A2, A3 ...) adjustable and being connected in parallel on the measuring branch by means of couplers (C1, C2, C3 ...), part of the signals emitted by the generators being thus superimposed on the input of the receiver to the signal delivered by the measurement branch, the phase shifter and the attenuator of each compensation branch being adjusted to minimize the component of the applied signal to the receiver at the corresponding fundamental frequency   laying on each of the generators.   2. Device according to claim   1, characterized in that the phase shifters and attenuation the various compensation branches   are provided with manual adjustment means.   3. Device according to claim   1, characterized in that the phase shifters and attenuation   the various compensation branches are controlled by an automatic adjustment circuit comprising: a level adapter (62) connected 2476846-   at the output of the receiver (R), a delay device (64) connected to the adapter, a comparator (66) with two inputs, one (67) connected to the adapter and the other (68) to the delay device, a circuit (70) for counting changes in sign of the signal delivered by the comparator, a switch (72) having an input (74) connected to the comparator, a control input (73) connected to said counting circuit and a first and a second outputs (75, 76), a dimming control circuit   tion (80A) connected to the first output (75) of the switch (72) and a phase shift control circuit (80D) connected to the second output (76) of the switch (72).   4. Device according to claim 3,   characterized in that it further comprises a commu-   guard (82) connected to the attenuation control circuit   tion (80A) and having as many outputs as the device comprises compensation branches, each of these outputs being connected to the attenuator   adjustable from one of the branches, and another commu- guard (84) connected to the phase shift control circuit (80D) and having as many outputs as available   sitive includes compensation branches, each of these outputs being connected to the phase shifter of one of the branches; as well as relays (86, 88) inserted between the generators (G1, G2, G3 ..) and the compensation branches (BC1, BC2, BC3 ...) o 5. Device according to claim 1, characterized in that the phase shifters and attenuators   different compensation branches are com- ordered by a computer (91) connected to the output of the receiver.   6. Application of the device according to one any of claims 1 to 5, to the extent   of the saturation power of the amplifiers.