Transmitter signal output power. Measurement of parameters in radio frequency systems Basic parameters of a radio signal. Modulation

Purpose: to study the instrumental arsenal of the laboratories of the department and the main factors that determine the energy of radio lines.

Lines of satellite communication and broadcasting consist of two sections: a transmitting earth station (ES) - a repeater on an artificial earth satellite (AES) and an AES repeater - a receiving ES. The signal power at the input of the ES receiver can be determined from the formula that is used to calculate any line-of-sight radio links:

where P prd- power at the output of the transmitter of the satellite repeater,

γ prd and γ prm- the transmission coefficients of the paths connecting, respectively, the transmitter output with the transmitting antenna on the satellite and the output of the receiving antenna with the ES receiver,

G prd and G prm- the gains of the transmitting and receiving antennas, respectively,

L o and L add- basic and additional losses of signal energy in the space between the satellite and the ES.

Major losses L o caused by energy dissipation in free space at a distance from the emitter

, (2.2)

where λ is the length of the electromagnetic wave

, (2.3)

f- the frequency of the transmitter signal, c ≈ 3 ∙ 10 8 m / s - the speed of propagation of electromagnetic waves,

d- the distance between the satellite and the ES.

Distance d between the satellite and the ES depends on the height H satellite orbit, which determines the size of the satellite's visibility area.

The satellite's visibility zone is the part of the Earth's surface from which the satellite is visible for a given duration of a communication session at an elevation angle of at least some given angle
.

The instantaneous zone of visibility of the satellite is called the zone of visibility at a certain moment in time, i.e. at zero communication session duration. When the satellite moves, the instantaneous visibility zone moves, therefore the visibility zone during the communication session is always less than the instantaneous one. The size of the instantaneous field of view can be estimated by the arc length
or corners and (Figure 2.1).

Injection is the angular distance of the zone boundary from the sub-satellite point (relative to the center of the Earth), and the angle is equal to half the maximum angular size of the visibility zone relative to the satellite located at the point ... Points and are on the border of the visibility zone and are removed from the satellite at a distance
, called the maximum slant communication range.

For a triangle ∆
the following ratios are true:

, (2.4)

, (2.5)

where R Z= 6400 km - the radius of the Earth.

Additional losses L add caused by the atmosphere, precipitation and other reasons.

Antenna gains when using parabolic reflector antennas with a mirror diameter D is determined from the expression:

. (2.6)

Task 2. Using formulas (2.1) - (2.6), determine the signal power at the input of the receiver of the ES located at the border of the visibility zone. The initial data for the calculation are given in Table 2.1. The variant of the assignment is determined by the teacher.

Table 2.1


f, GHz
R prd, W
γ prd
γ prm
N, thousand km
β min, hail
L add
D prd, m
D prm, m

Using expressions (2.4) - (2.5) determine the distance d between the satellite and the AP.

Substitute the required data into expression (2.1).

Task 3. Determine the signal power at the input of the ES receiver located at the sub-satellite point S (Figure 2.1). The initial data and the calculation procedure are the same as for task 2.

Compare the results obtained in task 2 and task 3.

Report should contain the characteristics and description of the antennas of the department, as well as the results of calculations for tasks 1-3.

WORK IN THE COMPUTER LABORATORY

MODELING

The purpose of the students' work is to acquire programming skills in the MatLab environment.

To enter the MatLab environment, the mouse pointer is brought to the logo of the software system and a double click with the left mouse button (LMB).

Exercise. Building a Simulink-model of the stand.

The transition to the Simulink package can be done in two ways:

after entering the MatLab environment, the simulink command is typed in the command line of the control window opposite the pointer;

using the mouse - one LMB click on the blue-red-black symbol containing the arrow.

After these actions, the library window (Library: Simulink) and the not yet named (untitled) window of the field, on which the model will be assembled, will open. In the seventh version of MatLab, to create such a field after entering Simulink, click LMB on the blank slate symbol.

First, students should become familiar with the sections of the Simulink library: Sources - sources; Sinks - loads, as well as independently find sections containing blocks Abs, F cn, Relational Operator, Mux, etc.

The blocks required for assembling the structural diagram are dragged with the mouse from the sections of the library with the LMB pressed.

Models of assembled stands are shown in Figure 3.1. Figure 3.1a shows a model containing two harmonic signal conditioners. The argument to the sinusoidal functions forms the Ramp block.

To set the parameters of this and other blocks, the block is first selected by clicking the LMB, and then double-clicking opens a window into which the corresponding parameters are entered. The Slope parameter of the Ramp source is set equal to pi / 50 (in the MatLab language, the constant
written as pi).

With the use of the Mux block, the Scope oscilloscope becomes a dual-beam oscilloscope. Students choose parameters of oscilloscopes models on their own. Set the simulation time (Stop time) equal to 100: Simulation - click LMB, Parameters - click LMB, record the time in the Stop time column.

Launching the program for execution is also carried out using the mouse: Simulation - LMB click, Start - LMB click. You can also start the program for execution by clicking LMB on the icon with the image of a triangle.

It is necessary to sketch (print) the structural diagrams of the models and the observed oscillograms.

Figure 3.1b shows a model of a comparator - a device that generates a single signal when the condition specified on the block of the comparator - Relational Operator is met.

By selecting the assembled model and using the Create Subsystem command in Edit mode, you can make the comparator model a Subsystem block. Such a block is shown in Fig. 3.1c, which shows a model of the device for comparing the signal levels of the Sine Wave and Constant sources. In this simulation experiment, the amplitude of the harmonic vibration is 1, the angular frequency is 0.1
with a simulation time of 100.

Sketch (print) the diagram of the model and oscillograms.

Individual tasks are shown in Table 3.1. The structural diagram of the models for all variants is the same. It is obtained from the block diagram shown in Figure 3.1a, if the Fcn 2 block and the Mux block are excluded from the latter. Thus, the output of the Ramp block is connected to the input of the Fcn 1 block, and the input

Scope is connected to the output of the Fcn 1 block.

The simulation time for all variants is 100.

Report for this section should contain:

structural diagrams of the investigated Simulink-models;

oscillograms;

Table 3.1

option	Signal formed by the Fcn block	Parameter value	Block parameters Ramp: Slope; Initial output

SHORT DESCRIPTION

Power Meters Series Anritsu ML2490A are high-speed digitizers and processors of signals coming from power sensors (sensors) connected to them. The Anritsu ML2495A is single channel and supports one sensor, while the Anritsu ML2496A can operate with two different sensors at the same time. Depending on the types of connected sensors, the frequency range can be from 100 kHz to 65 GHz.

Due to the very high digitization speed (time resolution reaches 1 ns), the Anritsu ML2490A series meters can be used for developing and configuring radars, and the bandwidth of these instruments, equal to 65 MHz, allows them to be used at all stages of construction and operation of 3G wireless communication systems. 4G and 5G, including next generation systems based on sophisticated modulation technologies such as OFDM.

In addition to pulse and peak power sensors, the Anritsu ML2490A Series can be connected to a variety of CW (stationary) radio (CW) sensors, making them versatile in use. You can download a full description of all the characteristics of the Anritsu ML2490A series below on this page in the section.

Main characteristics:
Number of channels: 1 (model ML2495A) or 2 (model ML2496A).
Frequency: 100 kHz - 65 GHz (depends on the sensor).
Bandwidth (video bandwidth): 65 MHz.
Typical rise time: 8 ns (with MA2411B pulse generator).
Time resolution: 1 ns. Built-in power calibrator (50 MHz and 1 GHz).
Ideal for radar applications and wireless networks (4G and 5G).
Power measurements: Average, Min, Max, Peak, Crest, PAE (Power Added Efficiency).
Screen 8.9 cm (resolution 320 x 240). Interfaces: Ethernet, IEEE-488 (GPIB), RS-232.
Weight: 3 kg. Dimensions: 213 x 88 x 390 mm. Working temperature: from 0 ° С to + 50 ° С.
Accurate measurement of the strength of any radio signal

DETAILED DESCRIPTION

The Anritsu ML2490A RF Power Meter Series offers the best performance compared to the other two Anritsu RF Power Meter Series (ML2480B and ML2430A). The ML2490A series includes two models: the single-channel ML2495A and the dual-channel ML2496A. Both models work in conjunction with external sensors (sensors). Six series of sensors are compatible with Anritsu ML2490A power meters, which solve a very wide range of tasks in the frequency range from 10 MHz to 50 GHz and in the power range from -70 dBm to +20 dBm.

Depending on the type of sensor connected, the Anritsu ML2490A meters can measure the following signal strength parameters: Average, Min, Max, Peak, Crest, Rise- time (rise time), PAE (Power Added Efficiency), and others. For sensor calibration, Anritsu ML2490A instruments come standard with a built-in power calibrator at two frequencies: 50 MHz and 1 GHz.

This photo shows the Anritsu ML2495A Single Channel RF Power Meter and Anritsu ML2496A Dual Channel RF Power Meter, along with two of the best sensors: Anritsu MA2411 Pulse Sensor (up to 40 GHz) and Anritsu MA2491A Broadband Sensor (up to 18 GHz).

Anritsu ML2495A single-channel meter (top) and Anritsu ML2496A two-channel meter (bottom) along with MA2411 pulse power sensor and MA2491A broadband power sensor.

Anritsu MA2411B pulse power sensor (sensor)

The Anritsu ML2495A and ML2496A Power Meters, together with the Anritsu MA2411B Transmitter, are ideal for measuring pulsed RF signals over the 300 MHz to 40 GHz frequency range. With a typical 8 ns rise time and 1 ns resolution, it is possible to directly measure the characteristics of radar pulses, as well as many other types of signals that have a pulse or burst structure.

This photo shows a screenshot of the Anritsu ML2496A Power Meter screen showing RF edge measurements. The measurements were carried out using an Anritsu MA2411B pulse power sensor. The horizontal axis is scaled 20 ns per division, and the vertical axis is 3 dB per division. The signal coming from the sensor was digitized at a rate of 62.5 MS / s.

This photo shows a screenshot of the Anritsu ML2496A Power Meter screen showing the measurements of four consecutive RF pulses. The horizontal axis is scaled 2 μs per division, and the vertical axis is 5 dB per division. For each pulse, you can measure: rise time, fall time, duration and other parameters, including the pulse repetition interval PRI (Pulse Repetition Interval). The screen also displays the results for a group of pulses: minimum, maximum and average power values.

Measurement of parameters of four consecutive RF pulses.

When measuring strong radio signals, attenuators or couplers are often used. The Anritsu ML2490A Series Power Meters have the ability to automatically account for the value of an external attenuator or coupler so that the readings on the screen match the actual power.

Before using the Anritsu MA2411B Sensor with the ML2490A Series Power Meter, you must calibrate them together. To do this, on the front panel of the power meter there is a reference signal output (Calibrator) with a frequency of 1 GHz and an amplitude of 0 dBm (1 mW). By connecting the sensor to this output and pressing the appropriate menu item, you will calibrate the sensor and zero the errors of the measuring path, which will prepare the device for accurate measurements.

The Anritsu MA2411B is optimized for pulsed and broadband modulated signals, but can be used to accurately measure stationary (CW) and slowly changing RF signals. The corresponding screenshot is shown in this photo.

Anritsu MA2490A and MA2491A Broadband Power Sensors

To measure the parameters of telecommunication signals, as well as some types of pulse signals, two broadband sensors are designed: Anritsu MA2490A (50 MHz to 8 GHz) and Anritsu MA2491A (50 MHz to 18 GHz). Both sensors provide 20 MHz bandwidth (also called video bandwidth or response speed), which is enough to accurately measure rapidly changing signals such as 3G / 4G, WLAN, WiMAX and the pulses of most types of radar systems. The rise time of these sensors in the pulsed measurement mode is 18 ns.

The impulse characteristics of the MA2490A and MA2491A sensors are slightly worse than that of the MA2411B, which was mentioned above, but the minimum measured power is -60 dBm, instead of -20 dBm for the MA2411B. A significant expansion of the lower power threshold is achieved due to the presence of an additional measuring path inside the sensors, which is automatically switched on at low power values.

This photo shows a screenshot of the screen of the Anritsu ML2496A power meter with the results of measurements of the parameters of the GSM signal. The measurements were carried out using an Anritsu MA2491A broadband power sensor. The horizontal axis is scaled 48 µs per division, and the vertical axis is 5 dB per division. The peak power of individual signal fragments reaches 12 dBm.

Measurement of GSM signal parameters using the Anritsu MA2491A broadband sensor.

Anritsu MA2440D Series High Precision Diode Power Sensors

This series of high-precision sensors is designed for radio signals with low rate of change or modulation (eg TDMA), as well as stationary (CW - Continuous Wave) signals. The response speed (video bandwidth) of these sensors is 100 kHz and the rise time is 4 μs. All sensors of the MA2440D series have a built-in 3 dB attenuator, which significantly improves the matching (SWR) of the input radio connector of the sensor. Wide dynamic range of 87 dB and linearity better than 1.8% (up to 18 GHz) and 2.5% (up to 40 GHz) make these sensors ideal for a wide range of applications, including measuring the gain and attenuation of radio devices.

The Anritsu MA2440D sensor series consists of three models with different upper frequency range and input connector type: Model MA2442D (10 MHz to 18 GHz, connector N (m)), model MA2444D (10 MHz to 40 GHz, connector K (m)) and model MA2445D (10 MHz to 50 GHz, connector V (m)). As an example, this photo shows an Anritsu MA2444D sensor with a K (m) connector.

Anritsu MA24000A Series High Precision Thermal Power Sensors

This series of high-precision sensors is designed for stationary (CW - Continuous Wave) and slowly changing radio signals. The rise time for these sensors is 15 ms. The principle of operation of the sensors of this series is based on the thermoelectric effect, which makes it possible to accurately measure the average (average) power of any radio signal, regardless of its structure or type of modulation. The dynamic range of these sensors is 50 dB and the linearity is better than 1.8% (up to 18 GHz) and 2.5% (up to 50 GHz).

The Anritsu MA24000A sensor series consists of three models with different upper frequency range and input connector type: Model MA24002A (10 MHz to 18 GHz, connector N (m)), model MA24004A (10 MHz to 40 GHz, connector K (m)) and model MA24005A (10 MHz to 50 GHz, connector V (m)). All three Anritsu MA24000A Series sensors are shown in this photo.

Principle of Operation and Internals of Anritsu ML2490A Series Power Meters

Power sensors connected to the Anritsu ML2490A Series provide the function of converting the high frequency signal to be measured into a low frequency signal. This low-frequency signal is fed from the sensor to the input of the ML2490A series meter, digitized using the built-in ADC, processed by a digital signal processor and displayed on the instrument display.

This figure shows the block diagram of the ML2495A single channel model. In this block diagram, two ADCs (analog-to-digital converters) are highlighted in green, with the help of which the low-frequency signal from the power sensor connected to the meter is digitized. If an Anritsu MA2440D diode sensor or Anritsu MA24000A thermoelectric sensor is connected, then digitization is performed using a 16-bit ADC. And if an Anritsu MA2411B pulse generator or Anritsu MA2490A or MA2491A broadband sensors are connected, then digitization is performed using a high-speed 14-bit ADC.

Block diagram of the Anritsu ML2495A single-channel power meter.

And this is how the internal structure of the Anritsu ML2490A series power meter looks like. In the center there is a small rectangular board of the built-in calibrator for 50 MHz and 1 GHz, the high-frequency cable with which is connected to the N connector on the front panel. Under the calibrator board is a large measurement board containing the analog part, an ADC, and an array of programmable logic arrays. Immediately below the measurement board is a second large digital processing and control board containing a DSP (digital signal processor), a microcontroller, and digital display and control units.

All Anritsu ML2490A Series Power Meters come with PC Remote Control Software Anritsu PowerMax... This program runs on a Windows compatible personal computer and allows you to remotely control the operation of an Anritsu ML2495A single channel instrument or Anritsu ML2496A dual channel instrument. Taking measurements with PowerMax simplifies initial instrument setup, speeds up measurement processing, and makes it easy to document and store results.

An example of the Anritsu PowerMax main window is shown in this screenshot. In this case, the two-channel Anritsu ML2496A model is controlled, to the first channel of which the Anritsu MA2411B pulse power sensor is connected, and the Anritsu MA2491A broadband power sensor is connected to the second channel. To enlarge the image, click on the photo.

Anritsu ML2490A Series Power Meters come with Anritsu PowerMax software.
Click on the photo to enlarge the image.

Anritsu ML2490A Meters and Power Sensors Specifications

Below is a list of the key specifications for the Anritsu ML2490A Series Power Meters. For detailed specifications of the meters, see the section below on this page.

Main technical characteristics of Anritsu ML2490A series power meters.

Below is a list of the main specifications for the various types of power sensors (power sensors) that are compatible with the Anritsu ML2490A Series Meters. For detailed technical characteristics of sensors, see below on this page in the section.

Key Features of Anritsu ML2490A Series Compatible Power Sensors.

Anritsu ML2490A Series Power Meters Package

Name	Short description
Anritsu ML2495A	Single-channel power meter for pulsed, modulated and stationary radio signals
or
Anritsu ML2496A	Two-channel power meter for pulsed, modulated and stationary radio signals
a plus:
2000-1537-R	1.5 meter cable for sensor connection (1 pc. For each channel)
-	Power cord
-	Optical disc with documentation and PowerMax software
-	Calibration certificate
-	1 year warranty (it is possible to extend the warranty period up to 3 and 5 years)

Anritsu ML2490A Series Power Meter Options and Accessories

Main options:
- option 760-209 (rigid transport case for transporting the device and accessories).
- option D41310(soft bag for transporting the device with a shoulder strap).
- option 2400-82 (set for rack mounting one meter).
- option 2400-83 (kit for rack mounting two meters).
- option 2000-1535 (protective cover for the front panel).
- option 2000-1536-R(0.3 meter cable for connecting the measuring sensor).
- option 2000-1537-R(1.5 meter cable for connecting the measuring sensor).
- option 2000-1544 (RS-232 cable for flashing the device).

Compatible power sensors (sensors):
- sensor Anritsu MA2411B(pulse sensor from 300 MHz to 40 GHz, from -20 dBm to +20 dBm).
- sensor Anritsu MA2490A(broadband sensor 50 MHz to 8 GHz, -60 dBm to +20 dBm).
- sensor Anritsu MA2491A(broadband sensor from 50 MHz to 18 GHz, from -60 dBm to +20 dBm).
- sensor Anritsu MA2472D(standard diode sensor from 10 MHz to 18 GHz, from -70 dBm to +20 dBm).
- sensor Anritsu MA2473D(standard diode sensor from 10 MHz to 32 GHz, from -70 dBm to +20 dBm).
- sensor Anritsu MA2474D(standard diode sensor from 10 MHz to 40 GHz, from -70 dBm to +20 dBm).
- sensor Anritsu MA2475D(standard diode sensor from 10 MHz to 50 GHz, from -70 dBm to +20 dBm).
- sensor Anritsu MA2442D(high-precision diode sensor from 10 MHz to 18 GHz, from -67 dBm to +20 dBm).
- sensor Anritsu MA2444D(high-precision diode sensor from 10 MHz to 40 GHz, from -67 dBm to +20 dBm).
- sensor Anritsu MA2445D(high-precision diode sensor from 10 MHz to 50 GHz, from -67 dBm to +20 dBm).
- sensor Anritsu MA2481D(universal sensor from 10 MHz to 6 GHz, from -60 dBm to +20 dBm).
- sensor Anritsu MA2482D(universal sensor from 10 MHz to 18 GHz, from -60 dBm to +20 dBm).
- sensor Anritsu MA24002A(thermoelectric sensor from 10 MHz to 18 GHz, from -30 dBm to +20 dBm).
- sensor Anritsu MA24004A(thermoelectric sensor from 10 MHz to 40 GHz, from -30 dBm to +20 dBm).
- sensor Anritsu MA24005A(thermoelectric sensor from 10 MHz to 50 GHz, from -30 dBm to +20 dBm).

Documentation

This PDF document contains the most complete description of the Anritsu ML2490A Series power meters, specifications and modes of operation:

Description of Anritsu ML2490A Power Meters and Sensors (in English) (12 pages; 7 MB)

Specifications of Anritsu ML2490A Meters and Sensors (in English) (12 pages; 1 MB)

Anritsu ML2490A Power Meters User Manual (English) (224 pp .; 3 MB)

Anritsu ML2490A Meter Programming Guide (English) (278 pp .; 3 MB)

Brief information about devices for measuring the strength of radio signals (in English) (4 pages; 2 MB)

And here you can find our tips and other useful information on this topic:

Anritsu RF Meter Series at a Glance

Anritsu Handheld RF Analyzer Series at a Glance

How to buy equipment cheaper - discounts, special prices, demo and used devices

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7.9 Measurement of parameters in radio frequency systems Measurement of the BER (C / N) function

In the modern BER measurement technique, various schemes are used, of which two main ones can be distinguished.

Rice. 7.16. Diagram of the tunable attenuator method.

In this method, a tunable attenuator is connected to the radio frequency path of the receiver, with the help of which additional attenuation is introduced, and the stability of the reception signal is assumed constant during the entire measurement time. Signal and noise levels are measured using a power meter, while measuring the noise in the intermediate frequency path of the receiver without filtering gives a value that is greater than the real noise power in the operating band of the path. Therefore, when measuring power, additional filters are used, tuned to the operating frequency band.

The BER is measured by a digital channel analyzer.

The main disadvantage of the method is the assumption of a constant useful signal power throughout the entire measurement period. In real conditions, the level of the wanted signal undergoes significant fluctuations due to multipath propagation of radio waves and changes in propagation conditions. For this reason, the C / N ratio can also change, whereby even a 1 dB change in C / N can cause an order of magnitude change in BER. Thus, this method does not provide the required measurement accuracy, especially of low BER values.

2.Interference method for measuring BER (C / AT), the diagram of which is shown in Fig. 7.17, uses a special device - analyzer / simulator of the C / N parameter, which measures the power level of the useful signal C when introducing a given noise level N, which ensures high accuracy in determining the C / N parameter. In this method, the analyzer / simulator automatically adjusts the level of introduced noise, while the measurement accuracy of the BER (C / AT) characteristic can reach values of ~ 1СГ12. In conclusion of this consideration of the BER (CIN) function, we note the following.

(1) Comparison of theoretical and practical BER / N dependences) show that practical dependences differ from theoretical ones in that for practical BER values a larger C / N ratio is required. This is due to various reasons for the deterioration of the parameter in the intermediate and radio frequency paths.

(2) In practice, the contributions of the radio and intermediate frequency paths are comparable with each other, while for systems for transmitting digital information with a speed of up to 90 Mbit / s, the following values of the levels of degradation of the BER parameter are observed.

Rice. 7.17. Diagram of the interference method for measuring BER (C / N)

Deterioration in the IF IF path:

Modulator phase and amplitude errors - OD dB;

Intersymbol interference associated with the operation of filters - 1.0 dB;

The presence of phase noise - 0.1 dB;

Differential encoding / decoding procedures - 0.3 dB;

Jitter (phase jitter) - 0.1 dB;

Excess noise bandwidth of the demodulator - 0.5 dB;

Other reasons (aging effect, temperature instability) - 0.4 dB.

So, in total in the IF path, the degradation in BER can be as much as 2.5 dB. Decreased BER in the RF Path:

Nonlinearity effects - 1.5 dB;

Impairments associated with channel bandwidth limitation and group delay - 0.3 dB;

Interference in adjacent channels - 1.0 dB;

Impairments associated with fading effects and echo appearance - 0.2 dB. In total, in the RF path, the BER degradation will be 3 dB, that is, in total in the system

The transmission BER degradation can reach -5.5 dB.

It should be noted that in the diagrams in Fig. 7.16, 7.17, the assignment of equalizers in digital radio paths was not considered.

Frequency and power measurements in RF paths.

Frequency and power measurements of the useful radio signal are implemented in practice by the following methods:

1) frequency counters and power meters are used,

2) spectrum analyzers with marker measurement functions are used.

In the second method, the marker provides movement along the spectral characteristic with simultaneous display of the values of the frequency and power of the useful radio signal.

To expand the capabilities of power measurements, modern spectrum analyzers provide spectrum smoothing, noise filtering, etc.

Analysis of the work of equalizers.

Compared to cable systems, radio air, as a transmission medium for radio signals, has characteristics that randomly change over time. Due to the widespread use of digital radio communication systems and the increased requirements for the accuracy of their transmission, equalizers are turned on in receiving devices, which sharply reduce the effect of multipath propagation (signal equalization) and group delay time (signal auto-tuning). When using digital methods for modulating high-frequency signals, developers have encountered difficulties in fine tuning modems and other channel-forming devices as part of a radio frequency path. In this case, equalizers also act as elements of compensation for possible nonlinearities in the devices of the radio frequency transmission path. In modern radio frequency information transmission systems, there are two main types of attenuation associated with the factors of radio signal propagation along the radio frequency path.

1) Linear attenuation, which is a frequency-independent uniform decrease in the signal amplitude from the signal distribution factors. Linear attenuation is usually caused by natural factors in the propagation of electromagnetic waves:

With end-to-end distribution in woodlands;

When propagating in the atmosphere in the presence of hydrometeors (rain, snow).

2) Attenuation due to multipath propagation of radio signals.

These two factors change the amplitude of the useful signal, leading to a change in the value of the C / N ratio, which ultimately affects the BER error parameter. The changes in the signal structure associated with these two attenuations are compensated for by equalizers. As you know, the operation of any equalizer is based on the use of a narrow-band notch filter to eliminate the nonlinearity of the useful signal. The main measurement parameter is the dependence of the filtering depth on the frequency at a given BER parameter, which has been called the M curve or the W curve in various reviews (Fig. 7.18).

Rice. 7.18. Curves M for cases of absence and presence of an equalizer.

To obtain the M curve, various signal transmission conditions are usually simulated, which are compensated by the equalizer and in the process of compensation, the M curve is built.The measurement circuit is shown in Fig. 7.19.

As a result of measurements, diagrams are obtained in the form of two-sided curves M, of which one is hysteretic (showing the ability of the equalizer filter to provide a filtering depth at a given frequency, sufficient to level the structure of the useful signal) and the other is hysteresis (showing the performance of the filter during its real operation, if necessary first increase and then decrease the filter depth parameter). In practice, both types of curves are essential for analyzing EQ performance.

Rice. 7.19. Measurement scheme of curves M

Measurements of the parameters of uneven phase-frequency characteristics and group delay time.

The non-uniformity of the phase-frequency characteristic (PFC) of the radio-frequency path is determined by the group delay time (GDT) from the formula:

Direct measurement of the dependence of the phase shift on the frequency f (w) and the subsequent differentiation of the obtained dependence is carried out, as a rule, for systems with a low level of phase noise, however, for radio communication systems, phase noise in the channel is present, which leads to non-uniformity of the phase response and a change in the group delay. Usually, GVZ measurements are carried out during acceptance tests of radio systems and take into account possible deviations in the operation of the transmitter, receiver, antenna devices and the conditions of radio signal propagation. The paper describes two methods for measuring the group delay based on the use of composite radio signals.

Measurements of immunity parameters to linear attenuation and attenuation associated with multipath propagation of radio signals

The parameters of radio signals are altered by linear attenuation and attenuation caused by multipath propagation of radio signals. During factory testing, the acceptable linear attenuation limit is not exceeded 50 dB for BER = 10 ~ 3. To compensate for linear attenuation, equalizers are used as part of the transmitter / receiver. The performance of the linear attenuation equalizer can be measured using tunable attenuators.

When measuring the parameters of resistance to attenuation associated with multipath propagation of radio signals, it is possible to use a state diagram and an eye diagram, which display:

State diagram - crosstalk between signals I and Q is displayed as ellipses,

Eye diagram - the phenomenon of multipath is displayed by the displacement of the centers of the "eyes" from the center to the edges.

However, both the state diagram and the eye diagram do not provide all the required measurement specification. For practical measurements of the efficiency of compensation for the multipath phenomenon, methods are used that are consistent with the compensation methods. Since it is practically impossible to predict the appearance of a multipath propagation factor, the impact of this factor is taken into account by stress methods, that is, by simulating the phenomenon of multipath propagation of a signal. As noted in the work, two models are used to simulate multipath signal propagation.

1.Dual-beam model. The modeling principle is reduced to a theoretically grounded assumption that the attenuation is associated with two-beam interference, and the interfering beam has a time delay (for the reflected beam). From the characteristics of the non-uniformity of the frequency response (amplitude-frequency characteristic) and GVZ for two-beam propagation of a radio signal, it follows:

Decrease in amplitude with change in frequency;

Change in the group delay and frequency response in the case of a minimum phase (when the main radio beam has a large amplitude);

Change in frequency response and group delay in the case of a non-minimal phase (when the resulting beam after the interference of two beams exceeds the main signal in amplitude).

2. Three-beam model. Since the two-beam model does not describe the phenomenon of amplitude modulation and the appearance of weak beat patterns within the operating frequency range, as a result of which the amplitude of the useful signal deviates within the operating range, even if the beat unit is outside the operating range, then a three-beam model is used, which makes it possible to take into account amplitude shift effect. Typically, a two-beam model is used for quality measurements, and a three-beam model is used for accurate measurements.

Analysis of intermodulation interference.

When radio signals propagate in the path, intermodulation interactions of signals occur during multiplexing and demultiplexing, as well as when the nonlinearities of channel forming devices in the path are influenced. Usually, intermodulation distortion is quite low - less than 40 dB relative to the level of the wanted signal. Nevertheless, the control of intermodulation distortion and elimination of their causes provide in some cases a solution to the problem of interference in adjacent channels. Spectrum analyzers are used for intermodulation analysis.

Measurements of characteristics of channel-forming radio-frequency paths.

In addition to complex measurements, measurements of the characteristics of channel-forming radio-frequency paths are widely used in practice, knowledge of which is necessary in the design and operation of radio-technical information transmission systems. In addition to measuring frequency and power in the service area, it becomes necessary to measure antenna systems, the level of thermal noise, frequency stability of master oscillators, phase jitter, parameters of modems and amplifying paths together with filtering devices.

Antenna system measurements.

Antenna-feeder devices in the RF path play an extremely important role. The main parameters: radiation power, radiation pattern in the corresponding planes, gain, impedance, etc., are usually calculated and measured at the stage of antenna production. During operation, important parameters are

Traveling wave ratio (KBV): KBV = Umin / Umax, (7.38)

Standing Wave Ratio (SWR): SWR = 1 / KBV, (7.39)

The level of return loss from the antenna input, where Umin and Umax are the minimum and maximum voltage in the feeder line.

In the case of ideal matching of the path: transmitter output - feeder - antenna input, KBV = 1 (since all the energy from the transmitter output is directed to the antenna and at the same time £ / min = Umax), in the case of Umin = O, SWR = oo KBV = 0 - a standing wave mode appears in the feeder, which is unacceptable.

In a real case, the VSWR can take values 1.1 ... 2, that is, KBV = 0.5 ... 0.9. In radio paths of digital information transmission systems with digital types of modulation, a low level of return loss is required, that is, a minimum VSWR value of -1.1, when the mode in the feed line is close to a high degree of matching.

For example, for microwave links using 64 QAM modulation, the recommended antenna return loss rejection level is 25 dB or more. To measure return loss, the circuit shown in Fig. 7.20.

From the microwave oscillator, a signal is fed to the antenna through a passive directional coupler. In the presence of a wave reflected from the input, electromagnetic oscillations through a directional coupler enter the spectrum analyzer (or selective receiver), where the reflected power level is measured. To reduce the reflected power level, the antenna-feeder path is matched. When used in practice, instead of a spectrum analyzer, a power meter, the measurement accuracy decreases, since, together with the reflected signal, the power meter takes into account the noise level associated with external influences on the radio channel in a given operating frequency range.

Measurements of the level of intrinsic thermal noise of the elements of the radio frequency path.

With an increase in the noise level, the intersymbol distortion of digital signals sharply increases and the BER value increases. In state and eye diagrams, this translates into larger state display points and a “closing the eyes” effect. Measurement of the noise of various devices in the radio frequency path is performed at the operational stage to localize the point of increased noise level. Considering that the intrinsic noise of various devices in the radio frequency path is small, differential methods are used for measurements. For this, an interfering single-frequency signal is mixed into the test signal, and then the noise is measured by the difference between the interfering signal and the noise. This method is used when measuring low power noise. As an example, Fig. 7.21 shows the results of noise measurements against the background of an interfering single-frequency signal for 16 QAM modulation at a signal-to-noise ratio С / I = 15 dB, while, as can be seen from the figure, an increase in the noise level leads to an increase in the size of points in the state diagram and the effect of "closing the eye "On the eye diagram.

Rice. 7.21. Examples of state and eye diagrams for noise measurements at C / 1 = 15 dB.

Phase jitter measurements.

An important parameter for measurements of digital modulated radio frequency transmission systems is the jitter of the signal of the master oscillators of the receiver / transmitter, the so-called jitter. For jitter analysis, a state diagram is effectively used, since the eye diagram is not sensitive to it. If a phase jitter of the signal occurs in the path, then, as follows from

Rice. 7.22, there is an increase in the size of the points of the state diagram. To eliminate the problems associated with the measurement of jitter associated with the presence of jitter, additional measurements of the operating parameters of the master oscillators are usually made and the malfunctions are eliminated.

Measurements of the parameters of modems.

To measure the parameters of the modem, analyzers are usually used that provide measurements of signals in the form of state diagrams and eye diagrams, which provide the most complete information about the structure and changes in digital modulation parameters. In fig. 7.23 shows the state diagram and eye diagram for the case of quadrature amplitude modulation with 16 states of 16 QAM as an example, from which follows:

The blurring of the points of the state diagram indicates the influence of noise;

Distortion of the "eye" size indicates possible disturbances in the operation of the digital channel (for example, the occurrence of intersymbol distortions).

Rice. 7.23. Example state and eye diagram for AM case with 16 states 16 QAM

Consider the following types of modem malfunctions and their corresponding diagrams.

1. Loss of synchronization in the digital channel.

Global malfunction / disconnection of the demodulator or loss of phase alignment can lead to a violation of the coordination between the modulator and the demodulator and loss of signal in the transmission system. In this case, the state diagram is a random distribution of signals over the corresponding modulation levels, the "eye" of the eye diagram is completely closed (Fig. 7.24).

Rice. 7.24. An example of a loss of synchronization in a digital channel: the state diagram is a random distribution of signals at the corresponding modulation levels, the "eye" of the eye diagram is completely closed.

2. Violation of the setting of the parameters of the modulation / demodulation level.

In fig. 7.25 shows a state diagram, from which it follows that when the levels of modulation / demodulation were set, there was an imbalance in the signal amplitude. Changes in the state diagram may indicate modulator nonlinearities or DAC malfunction.

Rice. 7.25. An example of violation of the setting of the parameters of the modulation / demodulation level.

3. Violation of the orthogonality of the I and Q vectors of the demodulator.

One of the common malfunctions in the operation of the modem is the malfunction of the demodulator, when the vectors I and Q of the polar coordinates of the demodulator are not strictly orthogonal. This leads to a state mismatch with the orthogonal grid of coordinates in the state diagram (Figure 7.26).

This fault may or may not be accompanied by a phase alignment error in the carrier recovery loop. In the absence of an error, the effect of this malfunction on the eye diagram is reduced to closing the "eye" in the diagram by signal I and the absence of any change in the Q diagram. In the presence of an error, the "eyes" of both diagrams will be closed. It should be noted that the analysis of the eye diagram alone does not allow to establish the cause of the malfunction, since this diagram completely coincides with the eye diagram in the presence of a high level of additive noise in the channel. A reliable determination of the cause of the malfunction in this case can only be given by the state diagram. Elimination of the described malfunction requires adjustment of the demodulator in terms of the orthogonality of the I and Q signals. In the state diagram in Fig. 7.27, the presence of a phase synchronization error of 2.3 degrees was noted.

Rice. 7.27. An example of a phase alignment error.

Measurements of operating parameters of amplifiers as part of a radio frequency path.

The main measured parameters of the amplifiers as part of the radio frequency path are:

Noise introduced by amplifiers;

Parameters of non-linearity of amplifying sections.

An overload in amplitude can lead to a transition of the amplifier to a non-linear mode and, as a consequence, a sharp increase in the probability of error in a digital transmission system. The use of state diagrams and eye diagrams allows one to assess the reasons for the decrease in the quality of radio communication (nonlinear distortions lead to blurring of the points of the state diagram and closing the "eye" of the eye diagram).

Basic parameters of the radio signal. Modulation

§ Signal strength

§ Specific signal energy

§ Signal duration T determines the time interval during which the signal exists (nonzero);

§ Dynamic range is the ratio of the highest instantaneous signal power to the lowest:

§ Signal spectrum width F - frequency band within which the main signal energy is concentrated;

§ The base of the signal is the product of the signal duration and the width of its spectrum. It should be noted that there is an inversely proportional relationship between the spectrum width and the signal duration: the shorter the spectrum, the longer the signal duration. Thus, the size of the base remains practically unchanged;

§ The signal-to-noise ratio is equal to the ratio of the useful signal power to the noise power (S / N or SNR);

§ The volume of transmitted information characterizes the bandwidth of the communication channel required for signal transmission. It is defined as the product of the signal spectrum width by its duration and dynamic range.

§ Energy efficiency (potential noise immunity) characterizes the reliability of the transmitted data when the signal is exposed to additive white Gaussian noise, provided that the sequence of symbols is restored by an ideal demodulator. It is determined by the minimum signal-to-noise ratio (E b / N 0), which is necessary for data transmission through the channel with an error probability not exceeding a given one. Energy efficiency defines the minimum transmitter power required for acceptable performance. The characteristic of the modulation method is the energy efficiency curve - the dependence of the error probability of an ideal demodulator on the signal-to-noise ratio (E b / N 0).

§ Spectral efficiency - the ratio of the data transmission rate to the used bandwidth of the radio channel.

AMPS: 0.83
NMT: 0.46
GSM: 1.35

§ Resistance to the effects of the transmission channel characterizes the reliability of the transmitted data when the signal is affected by specific distortions: fading due to multipath propagation, band limitation, frequency or time-centered interference, Doppler effect, etc.

§ Requirements for linearity of amplifiers. To amplify signals with some types of modulation, nonlinear class C amplifiers can be used, which can significantly reduce the power consumption of the transmitter, while the level of out-of-band radiation does not exceed the permissible limits. This factor is especially important for mobile communication systems.

Modulation(lat. modulatio - regularity, rhythm) - the process of changing one or more parameters of a high-frequency carrier oscillation according to the law of a low-frequency information signal (message).

The transmitted information is embedded in the control (modulating) signal, and the role of the information carrier is performed by a high-frequency vibration, called the carrier. Modulation, thus, is the process of "landing" of the information waveform on a known carrier.

As a result of modulation, the spectrum of the low-frequency control signal is transferred to the high-frequency region. This allows, when organizing broadcasting, to tune the functioning of all transmitting and receiving devices at different frequencies so that they do not "interfere" with each other.

Vibrations of various shapes (rectangular, triangular, etc.) can be used as a carrier, but harmonic vibrations are most often used. Depending on which of the parameters of the carrier oscillation changes, the type of modulation is distinguished (amplitude, frequency, phase, etc.). Modulation with a discrete signal is called digital modulation or keying.