Power supplies for radio monitoring equipment.

Power supplies for radio monitoring equipment..

BELYAKOV Andrey Leonidovich,
GLADKIKH Aleksandr Viktorovich

POWER SUPPLIES FOR RADIO CONTROL EQUIPMENT

The options for power supply of radio monitoring equipment, its features in terms of meeting the requirements of electromagnetic compatibility with power sources, as well as the possibility and necessity of using pulse power sources are briefly considered.

1. General characteristics of radio monitoring equipment

Radio monitoring equipment (RME) covers a wide range of technical means, which are usually based on a radio receiving device (RRU). In addition to the RRU, the RRU usually includes one or more antenna-feeder devices (AFD) and a device for processing/displaying information. Regardless of the purpose of a specific RRU kit, it includes a power supply (PS) in one form or another. This can be a voltage stabilizer built into the RRU for power supply from a chemical current source (CCS), a power supply for operation from an AC network, external or internal, a universal power supply with the ability to operate from a backup battery, etc.

It should be noted that the reliability of the ARC, as well as other electronic equipment (REA), is largely determined by the reliability of the power source, since in the event of malfunctions or failure of the power source, the equipment becomes completely unsuitable for operation. At the same time, in the event of defects or failures in other units, or in the presence of software errors, the equipment can function without external manifestations of a malfunction for the user until he uses certain resources

The most significant requirement for power sources of the ARC equipment, given its ultra-wide — from units of kilohertz to tens of gigahertz — range of operating frequencies, should be considered electromagnetic compatibility (EMC). Strict requirements for the level of radio emissions stem from the fact that the ARC power supply is directly part of the equipment designed to receive and analyze radio signals, which fully include interference and parasitic emissions of various electrical and radio equipment. It should be noted that due to the strict requirements for the level of interference in a significant part of the measuring and communication equipment, the use of high-frequency pulse stabilizers and converters of supply voltage is still severely limited, despite their advantage in efficiency and parameters derived from it.

The ARC, like any other functionally complex radio equipment, includes units and blocks for various purposes, the supply voltage of which, the power consumption and the requirements for the quality of the supply voltage vary greatly. That is, the ARC usually has a kind of hierarchical structure of power sources with a power range from a few milliwatts to (in some cases) hundreds of watts. It is often necessary to use thermostable micropower voltage stabilizers to stabilize the operating point, for example, field-effect microwave transistors of the input stages of the RPU or cascades of automatic gain control. Another extreme is stabilizing voltage converters for PCs included in the ARC complexes, which are located on vehicles. In these power supplies, the main thing is not the accuracy of voltage stabilization within fractions of a percent, but maintaining the output voltage with a fairly large tolerance (up to 20%) with fluctuations in the on-board voltage of up to 50% and a temporary loss of the latter. Onboard power supplies usually provide emergency power supply from the ARC complex battery and its automatic recharging from the onboard network.

2. Primary sources of electricity for powering the ARC

Radio control equipment, like any radio electronic equipment, receives energy for operation from a primary source (it should be taken into account that the secondary power supply (SPS) in the product can be structurally separated from the consumer). Primary sources of electrical energy are usually understood to be those in which it is obtained directly from some other type of energy. This can be chemical, thermal, mechanical and other energy. In our case, the meaning of the term is somewhat violated. For example, a 220 V AC network is not primary by definition, since this voltage is usually obtained as a result of several transformations of electrical energy into electrical energy (increasing and decreasing voltage by transformers). On the other hand, AC power grids for the end user are a primary source of electrical energy, like a regular salt element, due to the certainty of parameters and prevalence.

In the ARC, one can note a wide variety of primary power sources used, which is due to a wide range of tasks and application conditions. Let's take a brief look at the most frequently used ones.

2.1. Alternating current network.

It is usually used to supply power to permanently installed equipment. The tolerance for the effective value is usually taken to be ±10%, although for reliable operation of the equipment from real power grids, a tolerance of ±20% for long-term deviations from the nominal value should be included in the technical specification for the development of the power supply. In public power grids, there are always consumers that create powerful impulse interference. For example, a regular household refrigerator, having a power consumption in the stationary mode of only 100 — 150 W, consumes up to 4 — 10 kW when starting the compressor. A decrease in the network voltage at the moment of switching on has the nature of a pulse with a level of up to 100 V. When switching to the stationary mode and when switching off the compressor, damped oscillatory processes of various frequencies and durations with a peak voltage of tens and hundreds of volts occur in the network. Short-term voltage drops are also possible when turning on televisions, computers and other electronic equipment (charging the input capacitors of the power source and operation of the kinescope demagnetization loop, etc.).

It follows from the above that when developing secondary power sources operating from an alternating current network, it is necessary to take into account not only the change in the effective or amplitude value of the voltage, but also the presence of pulse interference with high energy.

2.2. On-board network of 12 V vehicles.

Historically established world standard of on-board voltage of passenger cars and trucks (small-capacity). It arose in connection with the use of lead batteries in motor vehicles, which, with low cost, relative simplicity of design and easy maintenance, have good electrical parameters. The main advantage of lead batteries in motor vehicles is the high short-term permissible current required to start the engine with an electric starter. The conventional figure 12 V indicates the nominal voltage of a lead battery of six elements connected in series under load. It is also close to the sum of the potentials (12.6 V) of six elements in the most electrochemically balanced state.

When the car engine is running, the voltage of the on-board network is determined mainly by the characteristics of the generator used in the power unit. The maximum voltage of the on-board network is determined by the equilibrium potential of a fully charged battery and is 14?14.5 V. That is, the output voltage of the generator is limited by the built-in regulator by this value, in some cases automatically adjusted with a change in the ambient temperature. If the battery was deeply discharged, then at the initial stage of its charge from the generator, the voltage of the on-board network can be limited (up to 10?12 V) by the value of the battery potential and its low internal resistance. A voltage on-board voltage reduced to 10?12 V can be observed with the generator running in case of increased load. For example, at night and in bad weather, when many regular consumers of electricity are turned on in the car (heater radiator blower, windshield wiper, headlights, rear window defroster, etc.). In such conditions, partial operation of on-board consumers from the battery and some discharge of it are possible.

The operation of the 12 V on-board network with the engine off is determined by the characteristics of the battery, the resistance of the on-board circuits and the amount of current consumed. Maximum voltages: 14?14.5 V — with a fully charged battery and the first minutes after disconnecting the charger, 9?10 V — full discharge (potential without load, or with a small load), 7?8 V — briefly, when starting the engine.

The voltage of the on-board network can go beyond the specified limits only if the electrical equipment of the car is faulty. In addition, as in any wired networks, pulse sags and voltage surges are possible from the operation of various consumers, for example, brake lights.

2.3. On-board network of 24 V cars.

In principle, it does not differ from the 12 V on-board network in any way except for the doubled voltage. It is used to reduce the current in the circuits of heavy-duty trucks and heavy special equipment, where there are more consumers of electricity and their consumed power is higher. It should be noted that in modern passenger car manufacturing, a transition to an on-board voltage of 36 V has begun. Increasing the voltage allows to significantly reduce the weight of copper wires in the car by reducing the current consumption at the same power. In serially produced cars with this on-board network voltage, an auxiliary network of the 12 V standard is installed for the transition period, intended mainly for additional devices such as radios, etc.

2.4. On-board network of an aircraft (LA) = 27 V/~ 115 V, 400 Hz.

According to GOST 19705-89, the DC voltage of the aircraft on-board network may be in the range from 21 V to 31.5 V (except for emergency mode). Short-term voltage drops of up to 13 V during autonomous engine start and surges of up to 53.5 V during switching of power equipment (duration up to 200 ms) are separately stipulated. For time intervals of about 10 — 100 ms, voltage surges and drops may have even greater values. For alternating voltage with nominal parameters of 115 V (effective) and a frequency of 400 Hz, the steady-state voltage value should be in the range of 100-127 V.

2.5. Chemical current sources.

A significant part of radio control equipment is designed for autonomous operation from built-in or external chemical current sources included in the ARC. In this class of devices, rechargeable batteries are usually used. The main share of the used chemical current sources is lead maintenance-free batteries with gel-like acid electrolyte. They have a number of advantages over other types of batteries, the main ones being their low cost and ability to deliver high current without loss of capacity. This property allows you to easily use a small-capacity battery as an emergency buffer when powering equipment with high energy consumption. They are especially often used as part of uninterruptible power supplies for personal computers, when, in the event of a loss of mains voltage, it is necessary to maintain the system's operability for at least a few minutes for emergency data storage. Lead batteries, in addition, do not have a memory effect for the charge-discharge mode and allow for long-term cyclic battery operation without complex circuit solutions.

Until recently, sealed nickel-cadmium batteries were predominantly used in autonomous equipment with a relatively low power consumption (0.1 — 5 W), which is due to the technological difficulties of manufacturing small-sized lead batteries. Now more compact and energy-intensive nickel-metal hydride and lithium-ion batteries have become available for use. The latter, in addition to the high density of stored energy, are preferable for use due to the absence of a memory effect to the charge-discharge cycle.

3. Secondary power sources

In this article, by secondary power supply sources we mean all types of converters, stabilizers and voltage regulators that provide the components of radio equipment with electrical energy with standardized characteristics. Below we consider the main classes of secondary power supply sources, their features, advantages and disadvantages.

3.1. Linear voltage regulators and stabilizers in ARC

Almost no radio engineering device, including radio control equipment, can do without using a linear voltage regulator or stabilizer. Moreover, in the ARC, linear voltage regulators are in a privileged position, since they fundamentally do not create electromagnetic interference. Let's consider the positive aspects of using linear voltage regulators:

• absence of electromagnetic radiation, as indicated above, a fundamental and main advantage over pulse and other types of regulators;
• absence of internal interference in power supply circuits (input and output);
• simplicity of circuit and design solutions;
• small dimensions of the power supply on integrated microcircuits (IMS), without taking into account, however, heat-dissipating elements (radiators, fans, etc.);
• low cost;
• minimal time and money spent on developing the power supply electronics.

At the same time, linear stabilizers and voltage regulators also have disadvantages, the main one of which is low efficiency, especially with large tolerances on the input supply voltage.

The second of the significant disadvantages is the fundamental impossibility of increasing the voltage and galvanic isolation of the output circuits from the input ones.

The first negative property of linear power supplies directly results in design and technological problems in terms of heat removal from the regulating element and ensuring maximum operating time from an autonomous current source.

For example, when powering equipment from a high-voltage power supply, the voltage at the terminals of which changes by 30% as it discharges (typical value), the operating time of the product may be less than 50% of what is theoretically possible with a lossless stabilizer. This is due not only to direct energy losses in the form of heat on the regulating element of the linear regulator, but also to the dependence of the capacity of the high-voltage power supply on the value of the consumption current. For many electrochemical systems, the drop in capacity with a doubling of the current can be up to 20%.

3.2. High-frequency pulse stabilizers and voltage converters

As noted in the first section, the main factor restraining the use of pulse power supplies (UPS) in the ARC is the interference they generate in the radio frequency range. This interference is directly emitted by the circuit elements into the air, and is also transmitted from the power module to other units of the product and to the power supply network via power wires and control circuits. The interference spectrum from modern UPSs operating at frequencies of up to 1 MHz can extend to 200 — 300 MHz. Its envelope usually has a classic form for a pulse signal, approximately as follows (Fig. 1):

Fig. 1.
A — amplitude of spectral components;
F — frequency;
f1, f2, f3,… fN — frequencies of interference harmonics.

However, the increase in the conversion frequency in UPSs of recent years of implementation has led to a paradoxical fact at first glance, which consists in some simplification of the fight against interference. The first reason for this is the simplification of the fight against the magnetic component of interference. With an increase in the conversion frequency, the penetration of the electromagnetic field into the screen material decreases due to the skin effect. As a result, thick-walled permalloy screens are not required to suppress interference radiation, as for UPSs operating at frequencies of about 100 — 10,000 Hz. The interference spectrum from the UPS in a stable operating mode does not contain subharmonics and begins with the main conversion frequency (usually 50 — 100 kHz), spreading exclusively upward in frequency. Increasing the energy conversion frequency also made it possible to reduce the dimensions of the chokes and filters in the input and output circuits of the UPS without reducing the quality of filtration.

Considering that the UPS element base in modern microelectronics has been widely developed, and also in connection with the need to create equipment with reduced energy consumption (especially autonomous), it would be unreasonable to neglect the above fact. In this regard, the developers have relied on the use of UPS in all units of the produced ARC.

As an example of practical implementation of a low-power UPS, we can cite the panoramic-technical analysis (PTA) unit. It is a path for amplifying and filtering the intermediate frequency with its transfer. Internal supply voltages: +5 V and -5 V at a consumption current of 100 mA. External power supply 8?16 V, unstabilized. The path bandwidth is 2 MHz, the central frequency at the input is 10.7 MHz, at the output — 1.6 MHz. The input sensitivity in the panorama mode is 0.3 μV (by noise level). In this product, the presence of a pulse power supply is not determined in the spectrum with a shorted input. The appearance of the unit is shown in Photo 1. The spectrum of the received (from the G4-164 generator) signal with an effective value of -50 dBV is shown in Figs. 2 and 3. Fig. 2 shows the spectrum of the signal when the power supply screens are not installed, and Fig. 3 – spectrum of the same signal with the board fully mounted.

Photo 1. Panoramic technical analysis unit.

The elements of the pulse power supply are visible on the left in the background. The screen is not installed

Fig. 2. Spectrum of unmodulated signal with frequency of 10.76 MHz and level of -50 dBV, supplied to the input of PTA unit. Power supply unit screen is not installed. Regular spectral components of interference from power supply unit with step of ~ 69 kHz are clearly visible

Fig. 3. Spectrum of unmodulated signal with frequency of 10.76 MHz and level of -50 dBV, supplied to the input of PTA unit with fully mounted board

An example of a different scale is the universal power supply unit ARK-UBP designed to power a complex of radio-electronic equipment for various purposes, in particular the helicopter complex SKRK-V. The power supply unit is a multi-channel device that can provide stabilized voltage to up to 9 loads and has 9 outputs of non-stabilized through power supply, switched by electronic keys. The power supply unit has a connector for connecting a battery and automatically switches to backup power in the event of failures or loss of the main power supply network. The power supply unit includes an automatic charger to ensure the operability of the battery. To protect against false on/off switching in conditions of increased vibration, software protection of the “BOARD NETWORK” key from accidental pressing is provided. The power supply unit has an audible alarm for incorrect polarity of the input voltage.

The ARC-UBP consists of the power supply unit itself, a remote control, an emergency siren, and cables for connecting the remote control and siren.

Main technical parameters of the ARC-UBP:

The appearance of the power supply from the side of the cells is shown in photo 2.

Photo 2. External view of the ARK-BP power supply unit
for the helicopter complex

The above examples do not mean a complete rejection of the use of linear power supplies. For example, it makes no sense to build a stabilized UPS into an antenna amplifier located directly at the vibrators of the receiving antenna, although it consumes a current of tens of milliamps. The share of this current in the total consumption is small, and the use of a UPS will not only create interference directly next to the sensitive element, but can also introduce distortions into the antenna pattern.

In some cases, especially if the product is powered exclusively from an alternating current network and the power consumption does not exceed 1 — 2 W, the use of a linear integral voltage stabilizer can significantly reduce the cost of the product.

However, it is sometimes quite problematic to do without using a UPS. In the ARK-MK1M mobile complex based on a Gazelle class vehicle, the nominal voltage of the on-board network is 12.6 V, the real voltage is from 10 to 14 V with surges under normal conditions up to 16 — 18 V, and when switching electromagnetic devices of electrical equipment (when starting the engine) — with sags up to 7 — 8 V. Under such conditions, it is almost impossible to ensure stable operation of the device with an internal supply voltage of 9 V, using a linear voltage stabilizer. If the power consumed by the product (unit) is about 20 W, another 10 W dissipated by the voltage stabilizer is added to the useful power consumption. With strict climatic requirements for equipment, the use of a linear stabilizer, as we see, creates design problems with the removal of additional heat.

The following example is no less illustrative. The on-board network is 24 V. The ARC consists of six products with a nominal supply voltage of 12 V. The power consumption of each of them is 20 W. The on-board voltage can fluctuate between 18 and 28 V. Given the good reserve for the input voltage, a linear voltage stabilizer can be used, and there will be no problems with interference. But as a result, the current consumption from the on-board network will have a value of about 10 A regardless of the voltage, which in itself is a considerable value (this is no more than 10 hours of autonomous operation with a battery capacity of 100 A? h). In addition, the heat dissipation on the voltage stabilizer will be: 10 A? (28 V 12 V) = 160 W (peak value). Without forced airflow, a radiator with an area of ​​0.8 m2 with a temperature difference of 20 ° C will be required to dissipate such power. At the same time, the use of a step-down UPS with an efficiency of 90% in this case will allow:

• to almost halve the current consumed from the on-board network;
• to reduce the power dissipated by the stabilizer to approximately 12 W at any permissible input voltage;
• to increase the battery life by approximately two times.

In this example, which is quite typical for ARC complexes, the advantages of the UPS are clear. Note that pulse step-down voltage stabilizers that meet the requirements of this example are quite widely represented by specialized microcircuits. Integrated microcircuits for this purpose can have minimal strapping with discrete elements. Ten years of experience using UPS in ARC has shown that the key to successfully combating parasitic radiation is, first of all, the correct approach to the design of printed circuit boards of the product and the design of its screens, following from the general physical laws of electromagnetic energy propagation.

It is interesting that interference from pulse power supplies is significant for equipment operating in the frequency range up to 100 — 200 MHz. For ARC of higher frequency ranges, more problems are caused by radiation from the digital part. This includes analog-to-digital conversion (ADC) circuits, all types of signal and control processors, programmable and hard logic microcircuits, as well as display and processing devices, such as a personal computer. All these components of the ARC operate at clock frequencies from units to hundreds of megahertz, and the spectrum of harmonics and noise generated by them extends to several gigahertz. It is more difficult to combat interference from the digital part than from UPS, despite the smaller amplitude of voltages and currents of the first harmonic. This is due to the very high switching speed of modern digital microcircuits, which is orders of magnitude higher than that of UPS power elements.

As an illustration, Fig. 4 shows the spectra of radio signals received by the digital RPU ARK-CT1 in the frequency range of 20 — 1620 MHz. Reception was carried out by an antenna in the form of a 7 cm long piece of wire inserted directly into the antenna socket of the DRP. To reduce external industrial interference, measurements were carried out in a laboratory on the outskirts of the city on a Sunday. The power supply of the control PC was carried out from batteries. The purpose of the measurements was to refine the product in terms of its own radiation from the power supply and the digital part.

The analysis shows that the consecutive actions (four) on shielding and grounding the design elements led to a noticeable decrease in the level of intrinsic emissions, in which the components of the digital part of the product in the range of 200 — 800 and 1300 — 1450 MHz clearly predominated. Almost all the spectral components visible in the fourth «sketch» belong to the communication and broadcasting systems of the region. There are no components from the UPS in all spectra, which are usually located below 100 MHz. It is impossible to carry out such measurements in an unshielded room on a working day, since the level of interference from operating computers, especially from their power supplies, is very high.

Fig. 4. 

4. Conclusion

Achieving small dimensions and reducing power consumption are closely interrelated with each other and with the consumer qualities of radio monitoring equipment. Both are impossible without using a UPS, which follows from the general trends in the development of electronic equipment. Problems with electromagnetic compatibility create certain difficulties in creating ARC products that include a UPS. However, as experience shows, the correct choice of circuitry and design solutions allows for effective interference control. The gain in operating time of products from autonomous power sources that is achieved in this case is difficult, and sometimes impossible, to achieve in other ways. With high power consumption of the equipment, weight reduction from using a UPS is achieved not only by possibly reducing the battery capacity, but also by reducing the size of heat-dissipating elements.

The greatest difficulties arise when creating an ARC whose operating frequency range includes the main frequency of the UPS and its first harmonics, especially at low conversion frequencies. In these cases, it is more difficult to implement good shielding of the magnetic component of interference. The transition to conversion frequencies above 100 kHz (up to megahertz units) allows us to overcome these difficulties to a significant extent.

The rapid development of the UPS element base also contributes to their successful implementation in the ARC, as well as in all other electronic products. This allows us to expect further improvement in the consumer properties of the equipment, determined by the parameters of the power sources.