Frequency converters, like many other electronic converters powered from an alternating current with a frequency of 50 Hz, by virtue of their device alone, distort the form of the consumed current: the current does not linearly depend on the voltage, since the rectifier at the input of the device is usually a conventional one, that is, unmanageable. Likewise, the output current and voltage of the frequency converter - they also differ in a distorted shape, the presence of many harmonics due to the operation of the PWM inverter.
As a result, in the process of regular power supply of the motor stator with such a distorted current, its insulation ages faster, the bearings deteriorate, the noise of the motor increases, the probability of thermal and electrical breakdowns of the windings increases. And for the mains supplying, such a state of affairs is always fraught with the presence of interference that can harm other equipment powered from the same mains.
To get rid of the problems described above, additional input and output filters are installed to the frequency converters and motors, which save both the supply network itself and the motor powered by this frequency converter from harmful factors.
The input filters are designed to suppress the noise generated by the rectifier and the PWM inverter of the frequency converter, thus protecting the network, and the output filters are designed to protect the motor itself from noise generated by the PWM inverter of the frequency converter. The input filters are chokes and EMI filters, and the output filters are common mode filters, motor chokes, sine filters and dU / dt filters.
The choke connected between the mains and the frequency converter is, it serves as a kind of buffer. The line choke does not let the higher harmonics (250, 350, 550 Hz and further) from the frequency converter into the network, while protecting the converter itself from voltage surges in the network, from current surges during transient processes in the frequency converter, etc.
The voltage drop across such a choke is about 2%, which is optimal for normal operation of the choke in combination with a frequency converter without the function of regenerating electricity at the time of motor braking.
So, line chokes are installed between the network and the frequency converter under the following conditions: in the presence of noise in the network (for various reasons); with phase imbalance; when powered from a relatively powerful (up to 10 times) transformer; if several frequency converters are powered from one source; if capacitors of the KRM installation are connected to the network.
The line choke provides:
protection of the frequency converter from power surges and phase imbalance;
protection of circuits from high short-circuit currents in the engine;
extending the service life of the frequency converter.
To eliminate radiation, to ensure electromagnetic compatibility with devices sensitive to radiation, an EMP filter is just needed.
The three-phase EMI filter is designed to suppress interference in the range from 150 kHz to 30 MHz according to the Faraday cage principle. The EMI filter is connected as close as possible to the input of the adjustable frequency drive to provide surrounding devices with reliable protection against all PWM interference. Sometimes an EMP filter is already built into the frequency converter.
The so-called dU / dt filter is a three-phase L-shaped low-pass filter consisting of chains of inductors and capacitors. Such a filter is also called a motor choke, and often it may not have capacitors at all, and the inductances will be significant. The filter parameters are such that all interference at frequencies above the switching frequency of the PWM inverter switches of the frequency converter is suppressed.
If the filter contains, then the capacity of each of them is within a few tens of nanofarads, and up to several hundred microhenries. As a result, this filter reduces the peak voltage and pulses at the terminals of a three-phase motor to 500 V / μs, which saves the stator windings from breakdown.
So, if the drive experiences frequent regenerative braking, is not originally designed to work with a frequency converter, has a low insulation class or a short motor cable, is installed in a harsh operating environment or is used at 690 volts, a dU / dt filter between the frequency converter and the motor is recommended. install.
Even though the voltage supplied to the motor from the frequency converter can be in the form of bipolar rectangular pulses, rather than a pure sine wave, the dU / dt filter (with its small capacitance and inductance) acts on the current in such a way that it makes it in the windings engine almost exactly. It is important to understand that if you use a dU / dt filter at a frequency higher than its nominal value, the filter will experience overheating, that is, it will bring unnecessary losses.
A sine-wave filter is similar to a motor choke or a dU / dt filter, however, the difference lies in the fact that the capacitances and inductances are large here, such that the cutoff frequency is less than half the switching frequency of the PWM inverter switches. Thus, a better smoothing of high-frequency interference is achieved, and the shape of the voltage on the motor windings and the shape of the current in them turns out to be much closer to the ideal sinusoidal.
The capacitances of the capacitors in a sine filter are measured in tens and hundreds of microfarads, and the inductances of the coils are measured in units and tens of millihenries. The sine-wave filter is therefore large in size compared to a conventional frequency converter.
The use of a sine filter makes it possible to use in conjunction with a frequency converter even a motor that was originally (according to the specification) not intended for operation with a frequency converter due to poor insulation. In this case, there will be no increased noise, no rapid wear of bearings, no overheating of the windings with high-frequency currents.
It is possible to safely use a long cable between the motor and the frequency converter when they are far apart, while eliminating impulse reflections in the cable that could lead to heat loss in the frequency converter.
it is necessary to reduce noise; if the engine has poor insulation;
experiences frequent regenerative braking;
works in an aggressive environment; connected with a cable longer than 150 meters;
should work for a long time without maintenance;
during engine operation, the voltage increases step by step;
the rated operating voltage of the motor is 690 volts.
It should be remembered that the sine filter cannot be used with a frequency lower than its nominal rating (the maximum permissible frequency deviation downward is 20%), so in the settings of the frequency converter it is necessary to preset the frequency limitation from below. And the frequency above 70 Hz must be used with great care, and in the settings of the converter, if possible, preset the values \u200b\u200bof the capacitance and inductance of the connected sine filter.
Remember that the filter itself can make noise and give off a noticeable amount of body, because it drops about 30 volts even at rated load, so the filter should be installed with proper cooling conditions.
All chokes and filters must be connected in series with the motor with screened cable as short as possible. So, for a 7.5 kW motor, the maximum length of the screened cable should not exceed 2 meters.
Common mode filters are designed to suppress high frequency noise. This filter is a differential transformer on a ferrite ring (more precisely, on an oval), the windings of which are directly three-phase wires connecting the motor to the frequency converter.
This filter is used to reduce the common mode currents generated by discharges in the motor bearings. As a consequence, the common mode filter reduces possible electromagnetic emissions from the motor cable, especially if the cable is not shielded. The three phase wires pass through the core window and the protective earth wire remains outside.
The core is fixed on the cable with a clamp to protect the ferrite from the damaging effects of vibration on the ferrite (the ferrite core vibrates during engine operation). The filter is best installed on the cable from the terminal side of the frequency converter. If the core heats up to more than 70 ° C during operation, this indicates the saturation of the ferrite, which means you need to add cores or shorten the cable. It is better to equip several parallel three-phase cables with its own core.
In industry, a significant part of the consumption of electrical energy falls on ventilation, pumping and compressor installations, conveyors and lifting mechanisms, electric drives of technological installations and machine tools. These mechanisms are most often driven by AC induction motors. To control the operating modes of asynchronous motors, including to reduce their power consumption, the world's largest manufacturers of electrical equipment offer specialized devices - frequency converters. Without a doubt, frequency converters (which are also called frequency converters, inverters, or abbreviated as inverters) are extremely useful devices that can greatly facilitate the starting and operating modes of asynchronous motors. However, in some cases, frequency converters can also have a negative effect on the connected motor.
Due to the design features of the frequency converter, its output voltage and current have a distorted, non-sinusoidal shape with a large number of harmonic components (interference). An uncontrolled rectifier of a frequency converter consumes a nonlinear current that pollutes the power supply network with higher harmonics (5th, 7th, 11th harmonics, etc.). PWM - The frequency converter inverter generates a wide range of higher harmonics at a frequency of 150 kHz-30 MHz. Powering the motor windings with such a distorted non-sinusoidal current leads to such negative consequences as thermal and electrical breakdown of the insulation of the motor windings, an increase in the aging rate of the insulation, an increase in the level of acoustic noise of a running motor, and bearing erosion. In addition, frequency converters can be a powerful source of noise on the mains supply, affecting other electrical equipment connected to the mains. Various filters are used to mitigate the negative impact of harmonic distortions generated by the inverter during operation on the electrical network, the electric motor and the frequency converter itself.
Filters used in conjunction with frequency converters can be conditionally divided into input and output. Input filters are used to suppress the negative influence of the rectifier and PWM inverter, output filters are designed to combat interference created by the PWM inverter and external noise sources. Input filters include line chokes and EMI filters (RF filters), output filters: dU / dt filters, motor chokes, sine filters, high-frequency common mode noise filters.
Line chokes
The mains choke is a two-way buffer between the mains and the frequency converter and protects the mains from higher harmonics of the 5th, 7th, 11th order with a frequency of 250 Hz, 350 Hz, 550 Hz, etc. In addition, mains chokes protect the frequency converter from overvoltage of the mains supply and current surges during transient processes in the mains and the load of the inverter, especially during a sharp surge in the mains voltage, which happens, for example, when powerful induction motors are turned off. Mains chokes with a given voltage drop across the winding resistance of about 2% of the nominal mains voltage are intended for use with frequency converters that do not regenerate the energy released when the engine is braking back into the power supply system. Chokes with a predetermined voltage drop across the windings of about 4% are intended for operation of combinations of converters and autotransformers with the function of regenerating engine braking energy into the power supply system.
- if there is significant interference from other equipment in the power supply network;
- when the supply voltage asymmetry between phases is more than 1.8% of the nominal voltage value;
- when connecting the frequency converter to a power supply network with a very low impedance (for example, when powering the frequency converter from a nearby transformer, the power of which is more than 6-10 times the power of the frequency converter);
- when connecting a large number of frequency converters to one power line;
- when powered from a network to which other non-linear elements are connected that create significant distortion;
- if there are capacitor banks (reactive power compensators) in the power supply circuit that increase the power factor of the network.
Advantages of using line chokes:
- Protect the frequency converter from impulse voltage surges in the network;
- Protect the frequency converter from phase imbalances of the supply voltage;
- Reduce the rate of rise of short-circuit currents in the output circuits of the frequency converter;
- Increase the service life of the capacitor in the DC link of the inverter.
EMP filters
The variable frequency drive (VFD + motor) is a variable load in relation to the mains supply. Together with the inductance of power cables, this leads to high-frequency fluctuations in the mains current and voltage and, consequently, to electromagnetic radiation (EMP) of power cables, which can adversely affect the operation of other electronic devices. EMI filters are necessary to ensure electromagnetic compatibility when installing the inverter in places that are critical to the level of noise in the power supply network.
Design and scope of dU / dt filters
The dU / dt filter is an L-shaped low pass filter consisting of inductors and capacitors. The inductance ratings of the chokes and capacitors are selected in such a way as to ensure suppression of frequencies above the switching frequency of the inverter inverter power switches. The value of the inductance of the filter choke winding dU / dt is in the range from several tens to several hundred μH, the capacitance of the filter capacitors dU / dt is usually in the range of several tens of nF. By using a dU / dt filter, it is possible to reduce the peak voltage and dU / dt ratio of pulses at the motor terminals to about 500 V / μs, thereby protecting the motor winding from electrical breakdown.
- Frequency controlled drive with frequent regenerative braking;
- Drive with a motor that is not designed for operation with a frequency converter and does not comply with the requirements of IEC 600034-25;
- Drive with an old motor (with a low insulation class), or with a general purpose motor that does not meet the requirements of IEC 600034-17;
- Drive with short motor cable (less than 15 meters);
- Variable frequency drive, the motor of which is installed in an aggressive environment or operates at high temperatures;
Since the dU / dt filter has relatively low inductance and capacitance values, the voltage wave on the motor windings still has the form of bipolar rectangular pulses instead of a sinusoid. But the current flowing through the motor windings already has the form of an almost regular sinusoid. The dU / dt filters can be used at a switching frequency below the rated value, but should be avoided at a switching frequency above the rated value as this will cause the filter to overheat. DU / dt filters are sometimes referred to as motor chokes. Most motor chokes are designed with no capacitors and the coil windings have a higher inductance.
Design and scope of sinus filters
The design of sine filters (sine filters) is similar to the design of dU / dt filters with the only difference that they have inductors and capacitors of a larger rating, forming an LC filter with a resonance frequency of less than 50% of the switching frequency (carrier frequency of the PWM inverter). This provides more efficient high frequency damping and suppression and a sinusoidal form of motor phase voltages and currents. The value of the inductances of the sine filter is in the range from hundreds of μH to tens of mH, the capacitance of the capacitors of the sine filter is from units of μF to hundreds of μF. Therefore, the dimensions of the sine filters are large and comparable to the dimensions of the frequency converter to which this filter is connected.
With the use of sine filters, there is no need to use special motors with reinforced insulation, certified for operation with frequency converters. The acoustic noise from the motor and the bearing currents in the motor are also reduced. The heating of the motor windings caused by the presence of high frequency currents is reduced. Sine-wave filters allow longer motor cables to be used in applications where the motor is installed far from the frequency converter. At the same time, a sine filter eliminates impulse reflections in the motor cable, thereby reducing losses in the frequency converter itself.
- When it is required to eliminate acoustic noise from the motor during commutation;
- When starting old motors with worn insulation;
- In the case of operation with frequent regenerative braking and with motors that do not comply with the requirements of IEC 60034-17;
- When the engine is installed in a hostile environment or operates at high temperatures;
- When connecting motors with screened or unscreened cables from 150 to 300 meters long. The use of motor cables longer than 300 meters depends on the specific application.
- If necessary, extend the engine maintenance interval;
- With a stepwise increase in voltage or in other cases when the frequency converter is powered by a transformer;
- With general purpose motors using 690 V.
Sine-wave filters can be used with a switching frequency higher than the nominal value, but they cannot be used with a switching frequency lower than the nominal value (for this filter model) by more than 20%. Therefore, in the settings of the frequency converter, the minimum possible switching frequency should be limited in accordance with the passport data of the filter. In addition, when using a sine filter, it is not recommended to increase the frequency of the inverter's output voltage above 70 Hz. In some case, it is necessary to enter the capacitance and inductance values \u200b\u200bof the sine filter into the inverter.
During operation, the sine filter can emit a large amount of thermal energy (from tens of watts to several kW), so it is recommended to install them in well-ventilated places. Also, the operation of the sine filter can be accompanied by the presence of acoustic noise. At the rated load of the drive, the voltage across the sine filter will drop about 30 V. This must be taken into account when choosing an electric motor. The voltage drop can be partially compensated for by decreasing the field weakening point in the frequency converter settings, and up to this point the correct voltage will be applied to the motor, but the voltage will be reduced at rated speed.
DU / dt chokes, motor chokes and sine-wave filters must be connected to the output of the frequency converter using a shielded cable as short as possible. Maximum recommended cable length between frequency converter and output filter:
- 2 meters with a drive power of up to 7.5 kW;
- 5-10 meters with drive power from 7.5 to 90 kW;
- 10-15 meters with a drive power above 90 kW.
Design and application of high-frequency common mode filters
The high pass common mode filter is a differential transformer with a ferrite core, the "windings" of which are the phase conductors of the motor cable. The high-pass filter reduces high-frequency common-mode currents associated with electrical discharges in the motor bearing and also reduces high frequency emissions from the motor cable, for example, when using unshielded cables. The high-pass common mode filter ferrite beads are oval in shape for easy installation. All three phase conductors of the motor cable are passed through a hole in the ring and connected to the output terminals U, V and W of the frequency converter. It is important to run all three phases of the motor cable through the ring, otherwise it will saturate. It is equally important not to pass the PE conductor, any other grounding conductors or neutral conductors through the ring. Otherwise, the ring will lose its properties. In some applications, it may be necessary to assemble a packet of several rings to avoid saturation.
Ferrite beads can be installed on the motor cable at the output terminals of the frequency converter (terminals U, V, W) or in the junction box of the motor. Installing the HF filter ferrite beads on the terminal side of the frequency converter reduces both the load on the motor bearings and the HF EMI from the motor cable. When installed directly in the motor junction box, the common mode filter only reduces the bearing load and does not interfere with the electromagnetic interference from the motor cable. The required number of rings depends on their geometrical dimensions, the length of the motor cable and the operating voltage of the frequency converter.
In normal use, the temperature of the rings does not exceed 70 ° C. Ring temperatures above 70 ° C indicate ring saturation. In this case, additional rings are required. If the rings continue to go into saturation, it means that the motor cable is too long, there are too many parallel cables, or a cable with a high linear capacitance is being used. Also, do not use a sector cable as a motor cable. Only use cables with round conductors. If the ambient temperature is above 45 - 55 ° C, the derating of the filter becomes very significant.
When using several parallel cables, the total length of these cables must be taken into account when choosing the number of ferrite rings. For example, two cables of 50 m each are equivalent to one cable of 100 m. If many parallel motors are used, a separate set of rings must be installed on each of them. Ferrite beads can vibrate when exposed to an alternating magnetic field. This vibration can lead to wear of the ring or cable insulation material through gradual mechanical abrasion. Therefore, the ferrite beads and cable should be firmly fixed with plastic cable ties (clamps).
When the engine is running, undesirable phenomena are often born, which are called "higher harmonics". They negatively affect cable lines and power grid equipment, and lead to unstable equipment operation. This results in ineffective use of energy, rapid aging of insulation, reduced transmission and generation.
To solve this problem, it is necessary to comply with the requirements for electromagnetic compatibility (EMC), the fulfillment of which will ensure the stability of technical equipment against negative influences. The article makes a small excursion into the field of electrical engineering related to filtering the input and output signals of the frequency converter (FC) and increasing the performance of motors.
What is Electromagnetic Noise?
They arise from literally all metal antennas that collect and emit disorienting energy waves. And cell phones, of course, also induce magnetoelectric waves, so when the plane takes off / lands, the flight attendants are asked to turn off the equipment.
Noises are classified according to the type of their origin, spectrum and characteristic features. Electric and magnetic fields from different sources, due to the presence of commutation connections, create unnecessary potential differences in the cable line, which grow on useful waves.
Noise arising in the wires is called antiphase or in-phase. The latter (they are also called asymmetrical, longitudinal) are formed between the cable and the ground, and act on the insulating properties of the cable.
The most common noise sources are inductive equipment (containing coils) such as inductive motors (AM), relays, generators, etc. Noise can "conflict" with some devices, inducing electric currents in their circuits, causing malfunctions process.
How is noise related to a frequency converter?
Converters for asynchronous motors with a dynamically changing operating mode, having many positive advantages, have a number of disadvantages - their use leads to the occurrence of intense electromagnetic interference and interference, which are formed in devices connected to them via a network or located nearby and exposed to radiation. Often, the AD is placed remotely from the inverter and connected to it with an extended wire, which creates threatening prerequisites for the failure of the electric motor.
Surely someone had to deal with pulses from an electric motor encoder on a controller or with an error when using long wires - all these problems, one way or another, are related to the compatibility of electronic equipment.
Frequency converter filters
To improve the quality of control, weaken the negative influence, a filtering device is used, which is an element with a non-linear function. The frequency range is set, outside which the response begins to weaken. From an electronic point of view, this term is quite often used in signal processing. It defines the limiting conditions for current pulses. The main function of the frequency converter is to generate useful ones, to reduce unwanted vibrations to the level set in the relevant standards.
There are two types of devices, depending on their location in the circuit, referred to as input and output. "Input" and "Output" means that the filter devices are connected to the input and output side of the inverter. The difference between them is determined by their application.
Input are used to reduce noise in the cable power supply line. They also affect devices connected to the same network. The output is intended for noise suppression for devices located near the inverter and using the same ground.
Purpose of filters for a frequency converter
In the process of operation of the frequency converter - an asynchronous motor, unwanted higher harmonics are created, which, together with the inductance of the wires, lead to a weakening of the noise immunity of the system. Due to the generation of radiation, electronic equipment begins to malfunction. Actively functioning provide electromagnetic compatibility. Some equipment has increased requirements for noise immunity.
3-phase filters for the frequency converter allow to minimize the degree of conducted noise in a wide frequency range. As a result, the electric drive fits well into a single network where several equipment is involved. The EMC filters should be placed sufficiently close to the power inputs / outputs of the frequency converter, due to the dependence of the level of interference on the length and method of laying the power cable. In some cases, they are installed.
Filters are required for:
- noise immunity;
- smoothing the amplitude spectrum to obtain pure electric current;
- selection of frequency bands and data recovery.
All models of vector frequency converters are equipped with line filtering. The presence of filtering devices provides the necessary EMC level for the system to work. The built-in device allows you to make minimal interference and noise in electronic equipment, and, therefore, meets the requirements for compatibility.
The lack of a filtering function in a frequency converter often leads to cumulative heating of the supply transformer, impulse changes, distortions in the shape of the supply curve, which causes equipment failure.
Apparatus absolutely necessary to ensure the stability of complex electronics. A buffer is installed between the frequency converter and the power supply to protect the line from higher harmonics. It is able to restrain the etioscillations of waves with a frequency greater than 550 Hz. When a powerful induction motor system is stopped, a voltage surge may occur. At this moment, protection is triggered.
It is recommended to set it to suppress high-frequency harmonics and to correct the system factor. The importance of the installation is to reduce losses in the stators of the electric motor, unwanted heating of the unit.
Line chokes have advantages. Correctly selected inductance of the device allows you to ensure:
- protection of the frequency converter from voltage drops and phase asymmetry;
- the rate of growth of the short-circuit current decreases;
- the duration of the "life" of the capacitors increases.
You can think of a capacitor as a blocker. Therefore, depending on the method of connecting the capacitor, it can act as:
- low-frequency, if you connect it in parallel to the source;
- high-frequency if connected in series with the source.
In practical circuits, a resistor may be required to limit the electron flow and achieve proper cutoff.
2. Filters of electromagnetic radiation (EMI)
Do you use a tea strainer when making tea? It is used to prevent “unwanted! items from logging into your system. In electrical circuits, there are many of these undesirable phenomena that appear at different frequencies.
An electric drive consisting of a frequency converter and an electric motor is considered a variable load. These devices and the inductance of the wires cause the generation of high-frequency voltage fluctuations and, as a result, the electromagnetic radiation of the cables, which negatively affects the functioning of other devices.
It is an inductor with two (or more) windings in which current flows in opposite directions. The use of this device, consisting of a choke and a capacitor, has several advantages. It is more reliable and can be used at the lowest operating temperatures. All this allows to increase the service life of the electric motor. Low inductance and small size are also key features.
They are used in cases when:
- cables up to 15 m long are stretched from the frequency converter to the electric motor;
- there is a possibility of damage to the insulation of the motor windings due to pulsating voltage surges;
- old units are used;
- in systems with frequent braking;
- aggressiveness of the environment.
At fairly high frequencies, the voltage drop is practically zero, and the capacitor behaves like an open circuit. The filter press is made in the form of a voltage divider with a resistor and a capacitor. It is, in fact, used in order to reduce bandwidth, instability, and correct the slew rate Uout.
In simple terms, a conventional choke comes from the word “choke”. And it is still used, because it describes its purpose quite accurately. Think about how the "fist" is clenched around the wire to prevent sudden changes in current.
4. Sinusoidal filters
An alternating electric current is a wave, a kind of combination of sine and cosine. Different sine waves have different frequencies. If you know which frequencies are present, which ones need to be transmitted or removed, then the result is a combination of "useful" waves, that is, without noise. This helps to clean up the current signal to some extent. A sine filter is a combination of capacitive and inductive elements.
One of the measures to ensure electromagnetic compatibility is the use of a sinusoidal device, this is sometimes necessary:
- with a group drive with one converter;
- when operating with a minimum of switching connections with cables (without a shield) of the electric motor (for example, connection in a loop or overhead power supply);
- to reduce losses on long cables.
The purpose of the device is to prevent damage to the motor winding insulators. Due to the almost complete absorption of high pulses, the voltage at the output becomes sinusoidal. Correct installation is an important aspect to reduce the level of interference in the network and therefore radiation. This allows a long wire to be used and helps to reduce the noise level. Low inductance also means smaller size and lower cost. The devices are designed according to the dU / dt filtration method with a larger difference in terms of the element rating.
5. High-frequency common mode filters
If a distorted voltage sine wave behaves like a series of harmonic signals added to the fundamental frequency, then the filtering circuit allows only the fundamental frequency to pass, blocking unnecessary higher harmonics. The input filtering device is designed to suppress high frequency noise.
The devices differ from those discussed above in a more complex design. The most important way to reduce noise is to comply with the required grounding rules in the electrical cabinet.
How to choose the right input and output EMC filter
Their distinctive advantages are their high noise absorption coefficient. EMC is used in devices with switching power supplies. It is worth adhering to the requirements of the instructions for a specific control scheme for asynchronous motors. There are general principles that determine the correct choice.
Please note that the selected model must comply with:
- parameters of the frequency converter and power supply;
- the level of interference reduction to the required limits;
- frequency parameters of electrical circuits and installations;
- features of the operation of electrical equipment;
- the possibilities of wiring the model into the control system, etc.
The easiest way to improve the quality of your electrical network is to take action at the design stage. The most interesting thing is that in case of an unreasonable deviation from design decisions, the fault lies entirely on the shoulders of the electricians.
The correct decision on the choice of the type of frequency converter, together with suitable filter equipment, prevents the occurrence of most problems for the functioning of the power drive.
Ensuring good compatibility is obtained with the correct selection of component parameters. Incorrect use of devices can increase the level of interference. In reality, input and output filters sometimes negatively affect each other. This is especially the case when the input device is built into a frequency converter. The choice of a filtering device for a specific converter is carried out according to technical parameters and, preferably, according to the competent recommendation of a specialist. Professional consultation, perhaps, will bring you significant benefits, since a high-quality inexpensive analogue is always selected for expensive equipment. Or it does not work in the desired frequency range.
Conclusion
Electromagnetic interference affects equipment mainly at high frequencies. This means that the correct operation of the system will only be achieved when the electrical installation rules and production and technical requirements are followed, as well as the requirements for high-frequency equipment (for example, shielding, grounding, filtering) are met.
It is worth noting that measures to increase noise immunity are a set of measures. Using filters alone will not solve the problem. However, this is the most effective way to remove or quite significantly reduce harmful interference for the normal electromagnetic compatibility of electronic equipment. We must also not forget that a particular model is suitable or not for solving a problem - it is determined "on the spot" or by experiment and testing.
Chapter 3
Digital IF overview
Since the 1980s, one of the most significant changes in spectrum analysis has been the use of digital technology to replace instrument assemblies that were previously exclusively analogue. With the advent of high-performance ADCs, new spectrum analyzers are able to digitize the incoming signal much faster than instruments created just a couple of years before. The most dramatic improvements have been made in the IF section of spectrum analyzers. Digital IF 1 has had a dramatic improvement effect in speed, accuracy and ability to measure complex signals through the use of advanced digital signal processing technologies.
Digital filters
Partial digital implementation of IF circuits is found in the Agilent ESA-E Series analyzers. Whereas resolution bandwidths of 1 kHz and wider can usually be achieved with traditional analog LC and on-chip filters, the narrowest resolution bandwidths (1 Hz to 300 Hz) are implemented digitally. As shown in Fig. 3-1, the line-to-line analog signal is downconverted to an 8.5 kHz IF and then passed through a 1 kHz bandpass filter. This IF signal is amplified, then measured at 11.3 kHz and digitized.
Figure 3-1. Digital implementation of 1, 2, 10, 30, 100 and 300 Hz resolution filters in ESA-E instruments
Already in the digitized state, the signal is passed through the Fast Fourier Transform algorithm. To convert a valid signal, the analyzer must be in a fixed state (no sweep). That is, the transformation must be performed on the time domain signal. Therefore, the ESA-E series analyzers use 900 Hz step increments instead of continuous sweep in digital resolution mode. This ramp tuning can be observed on the display, which is updated in 900 Hz increments while digital processing is in progress.
As we will see shortly, other spectrum analyzers such as the PSA series use an all-digital IF, and all of their resolution filters are digital. A key benefit of digital processing with these analyzers is bandwidth selectivity of approximately 4: 1. This selectivity is available on the narrowest filters — the ones we need to separate the closest signals.
In Chapter 2, we calculated the selectivity for two signals spaced 4 kHz apart using a 3 kHz analog filter. Let's repeat this calculation for the digital filtering case. A good model for digital filter selectivity would be a near-Gaussian model:
Where H (Δ f) is the cutoff level of the filter, dB;
Δ f - frequency offset from the center, Hz;
α is a selectivity control parameter. For an ideal Gaussian filter, α \u003d 2. The swept resolution filters used in Agilent analyzers are based on a near-Gaussian model with a parameter α \u003d 2.12, which provides a selectivity of 4.1: 1.
Substituting the values \u200b\u200bfrom our example into this equation, we get:
At 4 kHz offset, the 3 kHz digital filter drops to -24.1 dB, compared to the analog filter, which showed only -14.8 dB. Due to its superior selectivity, the digital filter can distinguish much more closely spaced signals.
Fully digital IF
Agilent's PSA Series spectrum analyzers combine multiple digital technologies for the first time to create a fully digital IF block. A purely digital IF provides a whole bunch of user benefits. The combination of FFT analysis for narrow and swept analysis for wide spans optimizes the sweep to provide the fastest measurements. Architecturally, the ADC has moved closer to the input port, made possible by improvements in analog-to-digital converters and other digital equipment. Let's start by looking at the block diagram of the PSA Series All-Digital IF Analyzer, shown in Fig. 3-2.
Figure 3-2. Block diagram of a fully digital IF in the PSA series
Here, all 160 resolution bands are digitally implemented. Although there are analog circuits in front of the ADC, starting with several down conversion stages and ending with a pair of single-pole prefilters (one LC filter and one on-chip filter). The prefilter helps prevent third-order distortion from entering the downstream circuit, just like in an analog IF implementation. In addition, it makes it possible to expand the dynamic range by automatically switching measurement ranges. The signal from the output of the single-pole pre-filter is routed to the auto-switch detector and to the smoothing filter.
As with any FFT-based IF architecture, an anti-aliasing filter is required to eliminate aliasing (the contribution of out-of-band signals to the ADC data sample). This filter is multi-pole and therefore has a significant group delay. Even a very steeply increasing RF burst carried down to the IF will experience a delay of more than three ADC clocks (30 MHz) when passing through the anti-aliasing filter. The delay allows time to recognize a large incoming signal before it overloads the ADC. The logic circuit that controls the autoranging detector will reduce the gain in front of the ADC before the signal arrives there, thereby preventing clipping. If the signal envelope is kept low for extended periods, the auto-tuning circuit will increase the gain, reducing the effective noise at the input. The digital gain after the ADC is also changed to match the analog gain before the ADC. The result is a floating point ADC with very wide dynamic range with auto-tuning enabled in sweep mode.
Figure 3-3. Auto-tuning keeps ADC noise close to carrier and below LO noise or resolution filter characteristics
In Fig. 3-3 shows the sweep behavior of the PSA. The single-pole pre-filter allows you to increase the gain while the analyzer is tuned away from the carrier frequency. As the carrier approaches the carrier, the gain decreases and the quantization noise of the ADC increases. The noise level will depend on the signal level and its frequency offset from the carrier, so it will appear as step phase noise. But the phase noise is different from this auto-tuning noise. Phase noise cannot be avoided in spectrum analyzers. However, reducing the prefilter width helps to reduce the auto-tuning noise at most carrier frequencies. Since the prefilter bandwidth is approximately 2.5 times the resolution bandwidth, lowering the resolution bandwidth reduces the auto-tuning noise.
Dedicated Signal Processing IC
Let's go back to the block diagram of the digital IF (Figure 3-2). After the ADC gain has been set to match the analog gain and corrected by the digital gain, the ASIC begins sample processing. First, the 30MHz IF samples are split into I and Q pairs in half steps (15 million pairs per second). The I and Q pairs are then high-frequency amplified using a 1-stage digital filter whose gain and phase are roughly opposite to those of an analog single-pole prefilter. The I and Q pairs are then filtered by a linear phase filter with a near-ideal Gaussian frequency response. Gaussian filters have always been the most suitable for frequency sweep analysis due to the optimal trade-off between frequency domain (shape factor) and time domain (fast sweep response) behavior. With the reduced signal bandwidth, the I and Q pairs can now be decimated and sent to the processor for FFT processing or demodulation. Even though the FFT can be implemented for a span segment of up to 10 MHz of the anti-aliasing filter bandwidth, even in a narrower 1 kHz span, with a narrow 1 Hz resolution bandwidth, the FFT would require 20 million data points. Using data decimation for narrower intervals significantly reduces the number of data points required for the FFT, which dramatically speeds up computations.
For swept analysis, the filtered I and Q pairs are converted to amplitude and phase pairs. In traditional sweep analysis, the amplitude signal is filtered over the video strip and sampled by the display's detector circuit. The choice of display mode "logarithmic / linear" and scaling "dB / unit" is done in the processor, so that the result is displayed at any of the scales without repeated measurements.
Additional video processing capabilities
Usually, a video band filter smooths the logarithm of the signal amplitude, but it has a lot of additional features. It can convert the logarithm of the amplitude to a voltage envelope before filtering, and translate it back before detecting the display for consistent readings.
Amplitude filtering on the line voltage scale is desirable for observing the envelopes of pulsed radio signals at zero frequency sweep. A signal with a logarithmic amplitude can also be converted to power (amplitude squared) before filtering and then back again. Power filtering allows the analyzer to give the same average response to noisy signals (digital communications signals) as to continuous waveforms with the same rms voltage. Nowadays, it is increasingly required to measure the total power in a channel or over the entire frequency range. With this measurement, a dot on the display can show the average power over the time that the LO passes through that dot. The video strip filter can be reconfigured to collect data to perform log, voltage, or power averaging.
Frequency Count
Frequency swept spectrum analyzers usually have a frequency counter. It counts the number of zero crossings in the IF signal and tune this count by the known LO offset in the rest of the conversion circuit. If the count goes for 1 second, you can get a frequency resolution of 1 Hz.
Thanks to the digital fusion of the local oscillator and the fully digital implementation of the resolution bandwidth, the inherent frequency accuracy of the PSA series analyzers is quite high (0.1% of the span). In addition, the PSA has a frequency counter that tracks not only zero crossings, but also phase changes. Thus, it can resolve frequencies of tens of millihertz in 0.1 seconds. With this design, the ability to resolve frequency changes is no longer limited by the spectrum analyzer, but rather by the noise level of the signal under investigation.
Other benefits of an all-digital IF
We have already covered a number of features of the PSA Series: Log / Voltage / Power Filtering, High Resolution Frequency Count, Log / Line Switching of Stored Data, Excellent Shape Factors, Dot Averaging Detector Mode, 160 different resolution bands, and of course, frequency sweep or FFT processing mode. In spectrum analysis, resolution filtering introduces error in amplitude and phase measurements, which are functions of sweep speed. For a certain fixed level of such errors, resolution filters of a purely digital IF with linear phase allow higher sweep rates than analog filters. The digital implementation also provides the known compensation for frequency and amplitude data acquisition, thus allowing sweep rates twice as fast as older analyzers, and performs excellently even at quadruple sweep rates.
The digital logarithmic gain is highly accurate. Typical errors typical for the analyzer as a whole are much smaller than the measurement errors, with the help of which the manufacturer estimates the reliability of the logarithm. At the analyzer's input mixer, the log confidence value is specified at ± 0.07 dB for any level down to -20 dBm. The range of logarithmic gain at low levels does not limit the confidence of the logarithm as it would with an analog IF; the range is limited only by noise of the order of -155 dBm at the input mixer. Due to monotone compression in subsequent circuits at higher powers, the confidence parameter degrades to ± 0.13 dB for signal levels down to -10 dBm at the input mixer. In comparison, an analog log amplifier typically has tolerances of the order of ± 1 dB.
Other IF related accuracies also experienced improvements. The IF prefilter is analog and must be tuned like any analog filter so that it is subject to tuning errors. But it's still better than other analog filters. Although it only requires one stage to be made, it can be made much more stable than the 4- and 5-stage filters used in analog IF analyzers. As a result, gain differences between resolution filters can be kept within ± 0.03 dB, which is ten times better than pure analog designs.
The IF bandwidth accuracy is determined by the limitations of the digital filter setting and the calibration uncertainty in the analog prefilter. Again, the prefilter is very stable and introduces only 20% of the error that would be present in an analog implementation of a resolution bandwidth of five such steps. As a result, most resolution bands fall within 2 percent of their stated bandwidth, as opposed to 10-20 percent for analog IF analyzers.
The most important aspect of bandwidth accuracy is minimizing the error in channel power and similar measurements. The resolution filters have even better noise bandwidth than the 2 percent alignment tolerance, and the noise markers and channel power measurements are adjusted to ± 0.5%. Thus, bandwidth errors contribute only ± 0.022 dB to amplitude density and channel power measurements. Finally, in the absence of reference-dependent analog gain stages, there is no “IF gain” error at all. The sum of all these improvements is such that a purely digital IF provides a significant improvement in spectral analysis accuracy. It also becomes possible to change the analyzer settings without any significant impact on the measurement accuracy. We'll talk more about this in the next chapter.
1 Strictly speaking, once a signal is digitized, it is no longer at the intermediate frequency, or IF. From this point on, the signal is represented by digital values. However, we use the term “digital IF” to describe the digital processes that have replaced the analog IF section found in traditional spectrum analyzers.)
