Iec 60835 2 4 1998

Microsoft Word 835 2 4F doc NORME INTERNATIONALE CEI IEC INTERNATIONAL STANDARD 60835 2 4 Edition 1 1 1998 02 Méthodes de mesure applicables au matériel utilisé pour les systèmes de transmission numér[.]

Généralités

La CEI 60835-1-1 donne des exemples d'émetteurs de signaux numériques, à modulation en r.f ou en i.f.

For multi-channel radio frequency beams, it is essential to incorporate the emission filter into the transmitter measurement, even though it may be part of the connection device In this context, the RF access of the transmitter is considered to be located at the output of this filter.

It is essential to connect the measuring instruments to this access point, or, if feasible, to an equivalent measurement access During all the measurements described later, only the transmitter under test should be operational, while all other transmitters in the multi-channel radio system must be turned off.

Les mesures à effectuer, éventuellement, à l'accès de sortie d'un modulateur en f.i ne sont pas décrites De telles mesures peuvent néanmoins être exigées pour les modulateurs en f.i. des faisceaux hertziens.

Fréquence du signal de sortie

Le fréquencemètre est branché à la sortie de l'émetteur, par l'intermédiaire d'un atténuateur r.f. appropriộ Il convient de couper la modulation, de faỗon à obtenir une porteuse pure.

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IEC 60835-2-5:1993, Methods of measurement for equipment used in digital microwave radio transmission systems – Part 2: Measurements on terrestrial radio-relay systems – Section 5:

IEC 60835-2-8:1993, Methods of measurement for equipment used in digital microwave radio transmission systems – Part 2: Measurements on terrestrial radio-relay systems – Section 8:

CCIR Recommendation 556: Hypothetical reference digital path for radio-relay systems which may form part of an integrated services digital network; systems with a capacity above the second hierarchical level

CCIR Recommendation 557: Availability objective for a hypothetical reference circuit and a hypothetical reference digital path

CCIR Recommendation 594: Allowable bit error ratios at the output of the hypothetical reference digital path for radio-relay systems which may form part of an integrated services digital network

CCITT Recommendation O.151: Specification for instrumentation to measure error performance on digital systems

CCITT Recommendation G.703: Physical/electrical characteristics of hierarchical digital interfaces

Examples of digital transmitters with r.f and i.f modulators are given in IEC 60835-1-1.

In multi radio frequency (r.f.) channel systems, the transmitting filter, while potentially part of the branching system, must be considered in the transmitter measurements Therefore, the r.f output of the transmitter is treated as the output from this filter.

When conducting measurements, it is essential to connect the measuring instruments to the designated point or an equivalent test point if available During the testing process, ensure that only the transmitter being evaluated is operational, while all other transmitters in the multi-channel system are turned off.

Measurements related to the output port of the i.f modulator, if applicable, are not included.

Nevertheless, such measurements may be required for i.f modulators.

The frequency meter is connected via a suitable r.f attenuator to the transmitter output.

Modulation should be disabled to obtain a c.w carrier.

Spectre r.f à la sortie

L'analyseur de spectre est branché à la sortie de l'émetteur, par l'intermédiaire d'un atté- nuateur r.f approprié, et l'on effectue trois mesures.

First, modulation is removed to achieve a pure carrier signal, and the spectral purity of the unmodulated carrier is measured It is essential that the levels of spurious and harmonic spectral components are below specified threshold values within a defined frequency range This is verified on the calibrated display of a spectrum analyzer.

NOTE – D'autres méthodes sont à l'étude pour le cas ó la modulation ne peut pas être coupée.

The modulation is typically restored, with the digital signal input being driven by a pseudo-random signal generator at the nominal digital rate to create a spectrum that aligns with operational conditions When transmitting multiple main digital streams (for example, 2 x 34 Mbit/s), it is essential for each input access to be driven separately by sufficiently uncorrelated pseudo-random signals This measurement is conducted to ensure that the output signal spectrum complies with a template defining the specified limits.

Enfin, on observe également le spectre du signal de sortie en présence du signal d'indication d'alarme (S.I.A.), pour s'assurer de la conformité avec d'éventuelles spécifications relatives aux interférences.

Puissance de sortie en r.f

The RF wattmeter is connected to the transmitter's output through a suitable attenuator The RF output power is typically defined as the time-averaged power of the vectors representing the modulated signal.

When measuring output power, the input of the digital modulator must be driven through the signal processing unit, as required by a pseudo-random signal generator producing a bit sequence at the nominal data rate It is essential to ensure that the signal states are sufficiently random, particularly for MAQ-n modulated transmitters To achieve this, a sufficiently long pseudo-random sequence should be utilized, ideally following the sequence lengths specified in CCITT Recommendation O.151 for error rate measurements.

NOTE – Dans le cas de modulation à sauts de phase, la puissance de sortie est parfois spécifiée en l'absence de modulation.

Erreurs d'amplitude et de phase

Méthode de mesure

Les amplitudes et les phases des différents états de modulation de la porteuse à la sortie de l'émetteur sont celles du vecteur représentatif de cette modulation dans l'espace des signaux.

Les erreurs d'amplitude et de phase, qui caractérisent l'écart de ces vecteurs de leurs positions nominales, sont des paramètres de base de la qualité d'un modulateur numérique.

The spectrum analyser is connected via a suitable r.f attenuator to the transmitter output and three measurements are carried out.

To ensure spectral purity, the modulation process is initially disabled to measure the continuous wave (c.w.) carrier It is essential that the levels of unwanted spurious and harmonic components within a defined frequency range remain within specified limits, as indicated by the calibrated spectrum analyzer display.

NOTE – In the case where the modulation process is not capable of being disabled, alternative methods of measurement are under consideration.

The modulator is returned to its normal state, and the digital signal input port is activated by a pattern generator that provides a pseudo-random bit sequence at the nominal bit rate, simulating operational conditions For multiple main bit-streams, such as 2 x 34 Mbit/s, each digital signal input port must be driven by distinct, uncorrelated patterns This process ensures that the output spectrum complies with the defined limits of the specified mask.

Finally, the output spectrum with the alarm indicating signal (A.I.S.) drive is also observed in order to ascertain whether potential interference requirements are met in this case.

The r.f power meter is linked to the transmitter output through an appropriate r.f attenuator Typically, the r.f output power is characterized as the average power of the signal vectors throughout the digital modulation process.

The output power is assessed while the digital signal input is activated through the transmit signal processor, potentially driven by a pattern generator that produces a pseudo-random bit sequence at the nominal bit rate It is crucial, particularly for transmitters using n-QAM modulation, to ensure that all signal states exhibit sufficient randomness by employing a pattern of appropriate length For bit-error-ratio (BER) measurements, the pattern lengths specified in CCITT Recommendation O.151 are recommended.

NOTE – In PSK systems, output power is sometimes specified for the case where no modulation is applied.

The phases and amplitudes of the modulated output carrier in the individual modulation states are represented by the vectors of the signal space diagram.

The phase and amplitude errors, i.e the displacement of these vectors from their nominal position, are basic quality parameters of the digital modulator.

To perform the measurement, the modulator inputs are disconnected from the emission signal processing system, if requested, and are driven with a continuous component transmission by logical signals that sequentially generate the vector sequence in the signal space The amplitude and phase of these vectors are read using a vector voltmeter connected to the modulator's output.

This method does not account for the effects of the transcoder in the entire signal processing chain and the modulation rate, such as unintentional differences in the lengths of the connections to the modulator's inputs It is essential to separately verify the performance of the transcoder.

To avoid the shortcomings of the previously described method, a dynamic approach can be employed In this method, the device under test is subjected to a pseudo-random signal generator, with its output connected to either a network analyzer or a spectrum analyzer This dynamic measurement technique for assessing amplitude and phase errors is currently under investigation.

Présentation des résultats

Les résultats doivent être présentés sous la forme d'une table donnant les lectures d'amplitude et de phase du voltmètre vectoriel pour chaque état de la modulation.

Détails à spécifier

When this measurement is required, it is essential to include the following details in the equipment specifications: a) nominal values of the amplitude and phase of the vector in the signal space; b) allowable phase error; c) allowable amplitude error.

Généralités

Un exemple de récepteur de signaux numériques comprenant un démodulateur de type cohérent est décrit dans la CEI 60835-1-1.

In multi-channel radio frequency beams, it is essential to incorporate the reception filter into the receiver's measurement, even if it is part of the connection device The RF input access of the receiver is considered to be located at the entrance of this filter.

Fréquence de l'oscillateur local

Le fréquencemètre est connecté au point de test de l'oscillateur local, normalement disponible.

S'il n'existe aucun point de test, on peut mesurer séparément la fréquence radioélectrique et la fréquence intermédiaire, et en calculer la différence.

NOTE – Lorsque la fréquence de l'oscillateur local est obtenue par multiplication d'une fréquence fondamentale, la mesure peut aussi être effectuée à cette dernière fréquence.

Signaux parasites

Le but de cette mesure est de vérifier que les signaux parasites issus de l'entrée r.f du récepteur, qui peuvent créer un brouillage, ne dépassent pas une valeur limite spécifiée.

To perform the measurement, the modulator's multiple input terminals are disconnected from the transmit signal processor, if necessary, and are driven by direct current (d.c.) logic levels to sequentially generate the individual vectors of the signal space diagram The phases and amplitudes of these vectors are determined using a vector voltmeter connected to the modulator output.

This method overlooks the impact of the encoder on the transmit signal processor, which can be linked to the actual bit-rate of the input signal For instance, there may be undesirable discrepancies between the paths that drive the multiple input ports of the modulator Therefore, it is essential to independently verify the performance of the encoder.

To address the limitations of the previous method, a dynamic approach is proposed, where the modulator under test is driven by a pattern generator The output from the modulator is then analyzed using a network or spectrum analyzer This dynamic measurement technique for assessing phase and amplitude errors is currently being evaluated.

The results should be given by tabulating the phase and amplitude readings of the vector voltmeter in the individual modulation states.

The detailed equipment specification must include the nominal values of vector phases and amplitudes in the vector space diagram, along with the permitted phase error and the permitted amplitude error.

An example of a digital receiver, including the demodulator of the coherent type, is given in

In multi radio frequency (r.f.) channel systems, the receiving filter, which may be integrated into the r.f branching network, must be considered in the receiver measurements This means that the r.f input port of the receiver is treated as the input to the receiving filter.

The frequency meter is connected to the local oscillator test point, which is normally available.

If, however, no test point is available, it is possible to measure radio frequency and intermediate frequency and calculate the difference.

NOTE – Where the local oscillator is derived by multiplication, it may alternatively be measured at its fundamental frequency.

The purpose of this measurement is to ascertain whether the permitted leakage at the receiver r.f input port, which is a possible cause of r.f interference, is not exceeded.

A spectrum analyzer is connected to the RF input of the coupling device, while the local oscillators of other receivers are disabled The level and frequency of the spectral components of spurious and harmonic signals are displayed on the calibrated screen of the spectrum analyzer, within a specified frequency range that typically encompasses all radio channels of the radio beam.

Caractéristique de c.a.g

Définition et généralités

The characteristic of the automatic gain control (AGC) of the receiver illustrates how its output level in intermediate frequency (IF) changes based on the input level in radio frequency (RF), with both levels expressed in dBm at the nominal frequency of the input signal This static characteristic is determined by manually adjusting the RF input level.

Méthode de mesure

The RF input of the connection device corresponding to the receiver is driven by a generator tuned to the central frequency of the receiver's bandwidth The IF output of the receiver is disconnected from the demodulator and connected to a measurement device for the IF level, with an input impedance equal to the nominal impedance The control voltage of the AGC is measured using a DC voltmeter.

The generator's attenuator is initially adjusted to achieve the specified maximum input level of the receiver The receiver's input level is then gradually decreased to describe the specified dynamics, while recording the indicated value on the level measuring device and the AC voltage on the DC voltmeter.

The characteristics of the automatic gain control (AGC) can be illustrated as follows: the output level in the intermediate frequency (IF) of the receiver ranges from 0 dBm ± 0.5 dB for input levels between –88 dBm and –38 dBm, with the control voltage of the AGC varying from –3 V to –12 V.

La caractéristique peut aussi être présentée sous forme graphique.

When this measurement is required, it is essential to include the following details in the equipment specifications: a) the range of input level in RF, measured in dBm; b) the permissible variation of the output level in the IF of the receiver, expressed in decibels, within the range specified in point a); c) input access.

Sélectivité

Définition et généralités

Selectivity refers to a receiver's ability to differentiate the desired signal from unwanted signals that coexist at different frequencies This distinction is based on the frequency of the signals In digital signal receivers, selectivity typically arises from the combined selective effects of the radio frequency (RF) filter, which operates between the RF input and the intermediate frequency (IF) output, as well as the filters positioned after the demodulator.

A spectrum analyser is linked to the input port of the r.f branching network, with the local oscillators of other receivers turned off It measures the levels and frequencies of unwanted spurious and harmonic spectrum components displayed on the calibrated spectrum analyser within a specified frequency range, typically encompassing all r.f channels of the digital radio-relay system.

The a.g.c characteristic of the receiver is defined by the change in the i.f output level relative to the r.f input level, both measured in dBm at the nominal input frequency This steady-state characteristic is determined by manually adjusting the r.f input level.

The receiver's r.f branching network input port is supplied by an r.f generator set to the center frequency of the receiver's passband The i.f output of the receiver is linked to an i.f level meter, which provides a nominal load impedance, while the a.g.c control voltage is monitored using a d.c voltmeter.

The signal generator's attenuator is first set to achieve the maximum specified input level for the receiver Subsequently, the generator's output is gradually reduced to encompass the designated input level range for the receiver, while recording the output level readings from the intermediate frequency (i.f.) level meter and the automatic gain control (a.g.c.) voltage measurements from the direct current (d.c.) voltmeter.

The a.g.c characteristic should be presented as in the following example: "The receiver i.f output level is within 0 dBm ± 0,5 dB for receiver input levels in the range of –88 dBm to

–38 dBm whilst the a.g.c control voltage covers the range of –3 V to –12 V".

Alternatively, the characteristic may be presented graphically.

The detailed equipment specification must include the following essential items: the range of the receiver's radio frequency (r.f.) input level measured in dBm, the allowable variation of the receiver's intermediate frequency (i.f.) output level in decibels within the specified range, and the designated input connection point.

Selectivity refers to a receiver's capability to distinguish between desired signals and unwanted signals at different frequencies In digital receivers, selectivity encompasses the effectiveness of filters located between the radio frequency (r.f.) input and intermediate frequency (i.f.) output, as well as the filters used after demodulation.

In certain digital signal receivers, the filters positioned after the demodulator play a significant role in affecting unwanted signals These filters are typically not accessible during reception tests However, if necessary, their impact on the overall selectivity of the receiver can be assessed during type testing.

La sélectivité mesurée par la méthode décrite ci-dessous ne prend en compte que l'effet de la partie r.f du récepteur.

Si la partie r.f du récepteur comprend un égaliseur auto-adaptatif, on doit mettre ce dernier hors service.

Méthode de mesure

The selectivity of the RF section of the receiver is measured by applying signals from two generators to the RF input of the connection device using an appropriate combiner, directional coupler, or hybrid, as shown in Figure 2 It is essential to include the connection device in the measurement, as it can influence the results.

Generator number 1 simulates the desired signal and is tuned to the receiver's nominal input frequency Generator number 2, which simulates an unwanted signal, is tuned to other specified frequencies Both generators are unmodulated The output signals corresponding to these inputs are visualized using a spectrum analyzer connected to the receiver's intermediate frequency output.

The measurement method involves several steps First, generator number 2 is turned off while generator number 1 is set to a specified level within the receiver's input range, and the corresponding control voltage of the automatic gain control (AGC) is recorded Next, generator number 2 is activated and tuned to a frequency close to that of generator number 1, ensuring the signals are distinguishable on the spectrum analyzer The resolution of the analyzer is set to its maximum, and the level of generator number 2 is adjusted to just below the desired signal level, ensuring no change in the AGC control voltage The spectral component level displayed on the analyzer, corresponding to this input signal, is noted as the reference for the noise signal level Finally, generator number 2 is tuned to other frequencies within a specified range at the same level, and the selectivity is recorded as the difference in decibels between the noise signal's spectral component and its reference value noted earlier, as displayed on the calibrated spectrum analyzer.

Selectivity is particularly important at frequencies where the input signal can generate an intermediate frequency (IF) that falls within the bandwidth of the IF amplifier This includes the nominal reception frequency (\$f_{ol} + f_{i}\$), the image frequency (\$f_{ol} - f_{i}\$), and the local oscillator frequency (\$f_{ol}\$) Additionally, it encompasses the differences between the received frequency and the local oscillator frequency, represented as \$f_{ol} \pm n \cdot f_{i}\$, where \$n\$ indicates harmonics or sub-harmonics of the intermediate frequency \$f_{i}\$, which can be any frequency within the IF bandwidth.

In digital receivers, post-demodulator filters play a crucial role in rejecting unwanted signals These filters are typically not available for evaluation during acceptance tests, but their impact on the receiver's selectivity can be assessed during type tests if necessary.

The selectivity as measured by the following method takes into account only the effect of the carrier part of the receiver.

An adaptive equalizer, if it is in the carrier part of the receiver, shall be switched off.

The selectivity of the receiver's carrier section is assessed by connecting signals from two signal generators to the r.f branching network input port through an appropriate combiner, like a directional coupler or hybrid It is essential to include the r.f branching network in the testing setup, as it can influence the measurement outcomes.

Generator No 1 is set to simulate the desired signal at the nominal receiver input frequency, while Generator No 2 simulates an unwanted signal at different specified frequencies Both generators operate without modulation The resulting output signal levels from these generators are shown on a spectrum analyzer linked to the receiver's intermediate frequency (i.f.) output.

The measurement procedure involves several key steps First, Generator No 2 is turned off while Generator No 1 is adjusted to a desired input signal level, and the corresponding automatic gain control (a.g.c.) voltage is recorded Next, Generator No 2 is activated and tuned to a frequency close to that of Generator No 1, ensuring it remains resolvable on the spectrum analyser Its level is adjusted to be just below that of the desired signal to maintain a constant a.g.c voltage, with the spectrum line level noted as the reference for the unwanted signal Finally, Generator No 2 is tuned across a specified frequency range at the constant input signal level, and the selectivity, defined as the decibel difference between the unwanted signal spectrum line and its reference value, is measured as a function of frequency on the calibrated spectrum analyser.

The selectivity in specific frequency ranges is crucial, as it determines how the input signal can generate a receiver output frequency that aligns with the intermediate frequency (i.f.) amplifier passband This includes considerations for the local oscillator frequency (\$f_{lo}\$), the image frequency (\$f_{lo} - f_{if}\$), and variations such as \$f_{lo} \pm n \cdot f_{if}\$, where the difference between the incoming and local oscillator frequencies is either a subharmonic or harmonic of the i.f frequency.

Here f if denotes any frequency within the range of the i.f passband.

Habituellement, la mesure est répétée pour plusieurs niveaux d'entrée du signal désiré

(générateur n° 1), dans le domaine spécifié pour les niveaux d'entrée du récepteur.

At point c) of the measurement method outlined below, a frequency-swept generator can be used instead of a manually tuned generator, with the sweep corresponding to the specified frequency range.

Dans ce cas, il convient que la fréquence du balayage soit beaucoup plus faible ou beaucoup plus forte que celle de l'analyseur de spectre.

The results of the high-frequency selectivity measurement of the receiver should be presented in a graph that displays selectivity in decibels as a function of frequency within the specified frequency range Additionally, for measurements taken with a frequency-swept generator, a photograph of the calibrated screen of the spectrum analyzer should be included.

When this requirement is mandated, the specifications for the equipment must include the following details: a) the desired signal input levels; b) the input levels and frequency range of the unwanted signal; c) the specified selectivity template, measured in decibels, within the aforementioned frequency range.

Facteur de bruit

It is essential to measure the noise figure of the digital signal receiver at its output access in the intermediate frequency (IF) with the gain control manually set to the lower limit of the specified input level range Additionally, it is important to specify the access point where the noise generator is connected during the measurement.

Généralités

En plus des mesures décrites aux articles 3 et 4, il est nécessaire d'effectuer des mesures sur l'ensemble émetteur/récepteur pour évaluer les performances de transmission des faisceaux hertziens numériques.

The purpose of these measures is to ensure that the quality of the digital signal transmitted through a microwave link, composed of multiple individually measured transmitters and receivers, meets the overall specifications.

The setup for measuring the transmission quality of a digital transmitter-receiver system is illustrated in Figure 3 The RF connections of the transmitter and receiver branch filters are linked through an RF attenuator, and measurement instruments are connected to the signal processing unit interfaces, as specified in the CCITT Recommendation G.703.

On suppose que les ensembles de traitement de signal sont conformes à leurs spécifications propres.

The measurement is usually repeated at several wanted signal input levels from Generator

No 1 within the specified receiver signal input level range.

In the measurement procedure, a sweep frequency generator can be utilized in step c) as an alternative to a manually tuned signal generator, provided that its sweep range aligns with the specified frequency ranges It is essential that the sweep repetition frequency is significantly lower or higher than the scanning frequency of the spectrum analyzer.

The selectivity measurements of the receiver's carrier part should be graphically represented, displaying the measured selectivity in decibels against frequency across the designated frequency range Additionally, for measurements conducted with a sweep generator, a photograph of the calibrated spectrum analyzer display must be included.

The detailed equipment specification must include the input levels of the desired signal, the input levels and frequency range of the unwanted signal, and the required selectivity in decibels within the specified frequency range, known as the limit mask.

The noise figure of a digital receiver must be assessed at the intermediate frequency (i.f.) output port, utilizing a manual gain control setting that aligns with the minimum specified input level range of the receiver It is essential to specify the connection point for the noise generator during this measurement.

In addition to the measurements given in 3 and 4 above, measurements on a transmit-receive section are also required to assess the transmission performance of the digital radio-relay system.

The goal of these measurements is to determine if the digital signal transmitted via the radio-relay link, consisting of the individually assessed transmitters and receivers, meets the overall performance specifications.

The setup for evaluating the transmission performance of a digital transmit-receive section is illustrated in Figure 3 An r.f attenuator links the r.f ports of the transmitter and receiver branching filters, while measuring instruments can be connected to the signal processor terminals as needed, in accordance with the interface characteristics specified by CCITT.

It is assumed that the signal processors comply with their relevant specifications.

Quality parameters such as error rate and jitter are assessed during reception or type testing The error rate is evaluated under conditions of constant fading and selective fading The outcome of the latter measurement is referred to as the "signature."

Taux d'erreur

The error rate is assessed based on the receiver's input level, specifically regarding the value of non-selective fading, to ensure that the error rate remains below a specified threshold within a defined range of input levels (refer to the measurement setup in Figure 3) When a service channel is transmitted using frequency modulation of the carrier, it is essential that the corresponding modulation input of the transmitter is driven by a signal of specified frequency and level during the error rate measurement.

An example of the error rate characteristic of a digital signal transmitter-receiver system is illustrated in Figure 4 The error rate remains nearly constant at a low residual value in the upper range of the receiver's input levels but increases rapidly in the lower range due to reception noise In this lower range, it is important to also indicate the theoretical curve corresponding to the modulation type used, which shows the error rate as a function of the signal-to-noise ratio C/N, as seen in Figure 4 This theoretical curve only considers the influence of Gaussian noise and is expressed in terms of the ratio of energy per bit to the noise spectral density, calculated as follows:

N o est la densité spectrale de bruit;

P est la puissance d'entrée du récepteur;

T bit est la durée du bit;

F est le facteur de bruit du récepteur; kT o est la densité spectrale de bruit, 4 × 10 –21 W/Hz.

Pour la courbe théorique ci-dessus, il y a lieu d'utiliser une seconde échelle horizontale en

E b /N o calculée au moyen de l'équation (1).

The difference in decibels, measured horizontally and for specified error rates, between the measured curve and the theoretical curve is referred to as the equivalent degradation of the signal-to-noise ratio This metric characterizes the additional noise sources present in the transmitter-receiver system.

Two specific error rate values are particularly significant The first is the short-term error rate, which corresponds to the lowest value within the specified input level range This rate should not exceed \$10^{-3}\$, as any value above this threshold indicates that the connection is unavailable (refer to Recommendations 557 and 594 of the CCIR).

The second value pertains to the long-term residual error rate, which is represented by the horizontal section of the curve A long-term error rate value is recommended for the hypothetical reference circuit mentioned in CCIR Recommendation 556; however, there are currently no subdivision rules in place to define a single jump.

During acceptance or type tests, two key performance parameters are evaluated: the bit-error ratio (BER) and timing jitter The BER is assessed under both flat-fading and selective-fading conditions, with results from selective-fading typically referred to as the signature.

The Bit Error Rate (BER) is evaluated based on the receiver input level, specifically in relation to flat-fading depth, to ensure that the error ratio remains below a defined upper limit within a certain input level range When a service channel is transmitted using frequency modulation of the carrier, the transmitter's service channel input must be loaded with a signal of a specified level and frequency during the BER measurement.

Figure 4 illustrates the Bit Error Rate (BER) characteristics of a digital transmit-receive section, highlighting that the error ratio remains nearly constant at a low value in the upper range of the receiver input levels However, a significant increase in the error ratio occurs in the lower region due to receiver noise In this lower region, the theoretical relationship between BER and the carrier-to-noise (C/N) ratio, based on the modulation format used, is also depicted This theoretical curve is derived under the assumption of Gaussian noise and is expressed as a function of the bit-energy-to-noise-power density ratio, calculated using a specific equation.

N o is the noise power density;

P is the receiver input power;

T bit is the bit interval;

F is the noise figure of the receiver; kT o is the noise spectral power density, 4 × 10 –21 W/Hz.

When plotting the above theoretical curve, a second horizontal scale in units of E b /N o should be employed using equation (1).

The horizontal shift, known as "equivalent carrier-to-noise degradation," represents the decibel difference between measured and theoretical curves at specific Bit Error Rate (BER) values This shift is indicative of the extra noise sources present in the transmit-receive section.

The measured Bit Error Rate (BER) has two key aspects of interest Firstly, the short-term error ratio at the lower limit of the specified receiver input level range must not exceed \$10^{-3}\$ If this threshold is surpassed, the transmission path is deemed unavailable, as outlined by CCIR.

The other point is the residual long-term error ratio within the constant part of the curve.

A long-term error ratio for a hypothetical reference digital path is recommended in CCIR

Recommendation 556, but at present there is no subdivision rule which is applicable to a single hop.

Most digital signal receivers are equipped with an error rate monitoring circuit that triggers an alarm at a specified error rate and initiates a switch to a backup channel for multi-channel radio frequency beams When measuring the error rate curve, it is important to record the input levels corresponding to the transition from normal to alarm state and vice versa Additionally, the appearance of the alarm indication signal (AIS) from the receiver during an alarm should also be noted, if applicable.

Evanouissements sélectifs – Signature

Généralités

Digital high-capacity microwave beams are susceptible to selective fading caused by multipath propagation This results in a higher error rate compared to that associated with a constant fading of the same average value.

To assess the sensitivity of microwave beams to multipath propagation effects during prototype testing, a two-ray fading simulator can be utilized in either frequency or radio frequency circuits The signatures obtained from this simulator enable comparisons of the sensitivity to multipath fading among various microwave beams and adaptive equalizers, as well as predictions of the outage time caused by selective fading in real connections.

In the simplified synoptic diagram of the two-ray fading simulator shown in Figure 5, the input signal (f.i or r.f.) is divided into a direct path and a delayed path, simulating the direct and reflected rays The output signal (f.i or r.f.) is generated by combining the signals from these two paths The voltage transfer function of the simulator, normalized to the gain of the direct path, is expressed as:

The transfer function $ H(f) $ is defined as \$$H(f) = \left[ 1 + b^2 - 2b \cos(2 \pi (f - f_0) \tau) \right]^{1/2}\$$ where $ b $ represents the ratio of the amplitudes of the output signals from the delayed path and the direct path The variable $ \tau $ denotes the delay between these paths, which can be positive when the amplitude of the direct path signal is greater than that of the delayed path signal, or negative in the opposite case, indicating a minimum phase or otherwise.

Cette fonction périodique présente des minima aux fréquences telles que 2π(f – f0) τ = 0, 2π,

4π, et l’écart entre ces fréquences vaut donc 1/τ La différence entre la fréquence centrale du canal et le minimum le plus proche est f 0 (voir figure 6).

To create a single minimum or null in the channel's bandwidth, typically observed during selective fading, a delay is selected that results in a separation between two nulls significantly greater than the channel's bandwidth A common value used for this delay is \$\tau = 6.3 \, \text{ns}\$, which corresponds to a separation of \$\frac{1}{\tau} = 158.4 \, \text{MHz}\$.

Figure 5 illustrates the functional layout of the simulator The depth of the notch and its offset $ f_0 $ from the center of the frequency band can be adjusted to replicate the irregular transfer function of the channel in the presence of selective fading.

The depth of the notch is regulated by adjusting the levels of the direct and/or delayed signals, as well as the frequency offset $ f_0 $ through phase adjustment It is important that the notch adjustment range for the central frequency encompasses the channel bandwidth, which is influenced by both the transmitted bit rate and the type of modulation employed.

Most digital receivers are equipped with an error ratio monitoring circuit that triggers an alarm at a specified error ratio and facilitates a switch-over to a standby channel in multi RF channel systems It is essential to record the input levels that cause the transition to the alarm state and the return to normal operation Additionally, the characteristics of the alarm indicating signal (A.I.S.) emitted by the receiver during the alarm state should be documented when relevant.

Digital radio-relay systems that transmit high bit rates are vulnerable to selective fading caused by multipath propagation, leading to a higher bit error rate (BER) compared to flat fading with the same average value.

To evaluate the sensitivity of radio-relay systems to multipath propagation effects during acceptance or type testing, a "two-ray fading simulator" can be utilized in the intermediate frequency (i.f.) or radio frequency (r.f.) path of the receiver The signatures obtained from this simulator are effective for comparing various digital radio-relay systems and adaptive equalizers regarding their sensitivity to multipath fading, as well as for predicting outage times caused by selective fading in real-world scenarios.

The two-ray fading simulator, illustrated in figure 5, operates by splitting the input signal into direct and delayed paths to replicate both direct and reflected signal components The output signal is generated by combining these two components The voltage transfer function of the simulator, normalized to the gain of the direct path, is a key aspect of its design.

The transfer function $ H(f) $ is defined as \$$H(f) = \sqrt{1 + b^2 - 2b \cos(2\pi(f - f_0)\tau)}\$$ where $ b $ represents the ratio of amplitudes between the delayed and direct path output signals The delay $ \tau $ can be positive, indicating that the direct path signal has a higher amplitude than the delayed path signal, or negative, signifying a lower amplitude, which corresponds to minimum or non-minimum phase conditions, respectively.

This periodic function exhibits minima at frequencies defined by the equation $2\pi(f - f_0)\tau = 0, 2\pi, 4\pi, \ldots$, resulting in a separation of $1/\tau$ between these points Additionally, the nearest minimum is displaced from the channel band center by $f_0$.

To achieve a single minimum value or notch within the channel band, particularly during selective fading conditions, a delay time significantly exceeding the channel bandwidth is selected A typical value for this delay is τ = 6.3 ns, which results in a notch separation of 1/τ = 158.4 MHz.

The simulator's functional arrangement, illustrated in Figure 5, allows for the adjustment of notch depth and frequency displacement (\$f_0\$) to replicate uneven amplitude responses typical of selective fading conditions Notch depth is manipulated by varying the amplitude of either the delayed or direct signal, while \$f_0\$ is adjusted through phase modification It is essential for the notch tuning range to encompass the channel bandwidth, which is influenced by the transmitted bit-rate and the modulation type used.

General

Examples of digital transmitters with r.f and i.f modulators are given in IEC 60835-1-1.

In multi RF channel systems, the transmitting filter, while potentially part of the branching system, must be considered in the transmitter measurements Therefore, the RF output from the transmitter is treated as the output from this filter.

For accurate measurements, connect the instruments to the designated point or an equivalent test point if available Ensure that only the transmitter being tested is operational, while all other transmitters in the multi-channel system are turned off.

Measurements related to the output port of the i.f modulator, if applicable, are not included.

Nevertheless, such measurements may be required for i.f modulators.

Output frequency

The frequency meter is connected via a suitable r.f attenuator to the transmitter output.

Modulation should be disabled to obtain a c.w carrier.

L'analyseur de spectre est branché à la sortie de l'émetteur, par l'intermédiaire d'un atté- nuateur r.f approprié, et l'on effectue trois mesures.

First, modulation is removed to obtain a pure carrier, and the spectral purity of the unmodulated carrier is measured It is essential that the levels of spurious and harmonic spectral components are below specified threshold values within a defined frequency range This is verified on the calibrated screen of a spectrum analyzer.

NOTE – D'autres méthodes sont à l'étude pour le cas ó la modulation ne peut pas être coupée.

The modulation is typically restored, with the digital signal input being driven by a pseudo-random signal generator at the nominal data rate to create a spectrum that aligns with operational conditions When transmitting multiple main digital streams (for example, 2 x 34 Mbit/s), it is essential that each input access is driven separately by sufficiently uncorrelated pseudo-random signals This measurement is conducted to ensure that the output signal spectrum complies with a template defining the specified limits.

Enfin, on observe également le spectre du signal de sortie en présence du signal d'indication d'alarme (S.I.A.), pour s'assurer de la conformité avec d'éventuelles spécifications relatives aux interférences.

The RF wattmeter is connected to the transmitter's output through a suitable attenuator Typically, the RF output power is defined as the time-averaged power of the vectors representing the modulated signal.

When measuring output power, the input of the digital modulator must be driven through the signal processing unit, as required by a pseudo-random signal generator producing a bit sequence at the nominal data rate It is essential to ensure that the signal states are sufficiently random, particularly for MAQ-n modulated transmitters To achieve this, a sufficiently long pseudo-random sequence should be utilized, ideally following the sequence lengths specified in CCITT Recommendation O.151 for error rate measurements.

NOTE – Dans le cas de modulation à sauts de phase, la puissance de sortie est parfois spécifiée en l'absence de modulation.

3.5 Erreurs d'amplitude et de phase

Les amplitudes et les phases des différents états de modulation de la porteuse à la sortie de l'émetteur sont celles du vecteur représentatif de cette modulation dans l'espace des signaux.

Les erreurs d'amplitude et de phase, qui caractérisent l'écart de ces vecteurs de leurs positions nominales, sont des paramètres de base de la qualité d'un modulateur numérique.

R.F output spectrum

The spectrum analyser is connected via a suitable r.f attenuator to the transmitter output and three measurements are carried out.

To ensure spectral purity, the modulation process is initially disabled to measure the continuous wave (c.w.) carrier It is crucial that the levels of unwanted spurious and harmonic components within a defined frequency range remain within specified limits, as indicated by the calibrated spectrum analyzer.

NOTE – In the case where the modulation process is not capable of being disabled, alternative methods of measurement are under consideration.

The modulator is returned to its normal state, and the digital signal input port is activated by a pattern generator that provides a pseudo-random bit sequence at the nominal bit rate, simulating operational conditions For multiple main bit-streams, such as 2 x 34 Mbit/s, each digital signal input port must be driven by distinct, uncorrelated patterns This process ensures that the output spectrum complies with the defined limits of the specified mask.

Finally, the output spectrum with the alarm indicating signal (A.I.S.) drive is also observed in order to ascertain whether potential interference requirements are met in this case.

R.F output power

The r.f power meter is linked to the transmitter output through an appropriate r.f attenuator Typically, the r.f output power is characterized as the average power of the signal vectors throughout the digital modulation process.

The output power is assessed while the digital signal input is activated through the transmit signal processor, potentially driven by a pattern generator that provides a pseudo-random bit sequence at the nominal bit rate It is crucial, particularly for transmitters using n-QAM modulation, to ensure that all signal states exhibit sufficient randomness by employing a pattern of appropriate length For bit-error-ratio (BER) measurements, the pattern lengths specified in CCITT Recommendation O.151 are recommended.

NOTE – In PSK systems, output power is sometimes specified for the case where no modulation is applied.

Phase/amplitude error

Method of measurement

The phases and amplitudes of the modulated output carrier in the individual modulation states are represented by the vectors of the signal space diagram.

The phase and amplitude errors, i.e the displacement of these vectors from their nominal position, are basic quality parameters of the digital modulator.

To perform the measurement, the modulator inputs are disconnected from the emission signal processing unit, if required, and are driven with a continuous component transmission by logical signals that sequentially generate the vector sequence in the signal space The amplitude and phase of these vectors are read using a vector voltmeter connected to the modulator's output.

This method does not account for the effects of the transcoder in the signal processing chain and the modulation rate, such as unintentional differences in the lengths of the connections to the modulator's inputs It is essential to separately verify the performance of the transcoder.

To avoid the shortcomings of the previously described method, a dynamic approach can be employed In this method, the modulator under test is subjected to a pseudo-random signal generator, with its output connected to either a network analyzer or a spectrum analyzer This dynamic measurement technique for assessing amplitude and phase errors is currently under investigation.