In this White paper Electric Magnetic (EM) Field Generation in the 1 to 6 GHz frequency range is addressed. Looking to conventional methods of EM field generation and in Electric Magnetic Field Generation with the a electric field generator. The RF immunity standard EN-61000-4-3 states that, a uniform field must be generated at a distance of three meters. The three meter distance distinctly starts from the tip of the antenna. The name of this area is the homogeneous area or quite zone. The Device Under Test (DUT) is tested with respect to its immunity to the applied uniform field. In this case the area of uniform, illumination is 1.5 * 1.5 meters, to ensure that also 1 meter of cable to the DUT is in the field. The field has to be uniform with 75% of 12 points in this area comply with the 0 to +6dB rule.
In RF immunity systems using a traditional design generally need a large number of individual specifications of the system components. For integration into the test system all components need to be evaluated accordingly. This white paper explains how to simplify the configuration of RF immunity test systems compared to the conventional designs.
Power or Field?
Quite often many engineering hours are needed to analyze amplifier power, antenna gain, H/V antenna beam width and cable losses. This is to establish whether the combination of all these individual components offer the overall required compliancy towards the standard with respect to uniform area and field level. The RadiField offers the user a guaranteed EM field in an anechoic test environment as well as compliancy to the uniform field area. The user does not have to worry about amplifier power levels, 1dB compression points, cable losses nor the illumination area’s. simply a field level to test his products. This white paper will show some significantly considerations which need to be taken into account when working with conventional systems:
- Class A versus Class AB with respect to reflected power handing and 1dB compression power capability
- Power versus Field
- Complexity of the design
The Conventional Setup
Figure 1 shows a Conventional Chamber and System Setup system. In a traditional RF Immunity setup, the Amplifier-rack (Single or Dual band) is preferably located outside the test chamber together with all the other supporting equipment like signal generator, power meters, couplers and control PC. Critical items influencing the final field level of are the following coaxial cables and connection joints:
- From the amplifier output to the input of the Dual Directional Coupler (DDC).
- Between the DDC output and feed through panel on the chamber wall.
- From the chamber feed through panel to the antenna input.
The purple coaxial cables are crucial and when designing EMC test systems all the frequency dependent losses of these red cable sections are important. Losses ranging from 1 to 2 dB are typical and not out of the ordinary. Not embedded in the loss figure are also the insertion losses of the coaxial connections, total 6 pieces. Taking all losses into account the overall loss may mount up to 2.5-3.0 dB! An alternative, yet less preferable, solution is having the RF immunity rack inside the test-chamber. The reasons to do this are clear, however it is not compliant with the standard! And although this setup
reduces the length of the cable between antenna and system output but undeniably losses will never be zero! Another disadvantage of this method occurs when immunity test racks are inside chambers is that this will influence the measurement. Simultaneously test equipment which come in a combination inside a rack must be able to withstand the field-levels inside the test chamber (self-influence). Furthermore it is a time consuming aspect of a conventional system is the process of moving setup’s inside the chamber and moving it out again.
Antenna and Amplifier considerations
In EMC applications loads having VSWR ratios > 1:6 are quite common. A 1:6 VSWR means that 50% of the output power is reflected and therefore returned to the amplifiers final stage. In the following sections some application examples in the 20 to 1000 MHz (and above) frequency range. The effects of the VSWR and how these effects have to be taken into account with respect to selecting the correct amplifier with respect to it’s class of operation.
Electric Magnetic Field Generation below 80 MHz
High VSWR ratios are mainly found when testing is performed at low frequencies between 20 and 80 MHz. In this frequency range wavelengths are long and compact EMC antennas are thus a compromise between matching, efficiency and size. An example of such an antenna is the Biconical antenna of which one can be seen in figure 3. The Biconical antenna and similar ‘small’ sized antennas have compromise designs that thus can fit into the anechoic test rooms.
Higher frequencies | up to 1 GHz
For increasing frequencies, wavelengths are getting shorter and due to this the size of these antennas is much closer to the respective wavelengths. Therefore causing the antennas to be a much better match to the RF amplifier. Antenna types in the higher frequency regions are in general Log Periodic Dipole Arrays (LPDA’s), Horns and Ridged structures that have VSWR ratios well below 1:3
Higher frequencies | 1 GHz and above
Some examples of these antennas and their corresponding VSWR curves. The bent elements at the backside of this antenna consequently create a shorter an distance between the antennas phase center and the DUT.
This results in a better efficiency between antenna input power and thus the Electric Magnetic Field Generation. The graph of the power versus field (@ 3m) for this antenna as can be seen in figure 5. For frequencies above 1 GHz the most common type of antenna is in general the (stacked) LPDA or Horn antenna. In general these type of antennas are capable of providing a very good 50Ω match to the amplifiers output.
RF Power considerations 80 – 1000 MHz with an LPDA
Reading the graph we can conclude as follows for the lowest frequency, 80 MHz:
- Power to create 10 V/m non-modulated CW 50 Watt CW
- Peak Envelope Power to create 10 V/m + 80% AM = 161.5 Watt PEP* (+ 5.1 dB to the CW power level)
- Average Power with 80% modulation 66 Watt Average ( + 1.2 dB to the CW power level)
*The definition of PEP power in AM modulated signals is the power-level when the modulation is at its maximum.
CW versus modulated
In the graph can be seen that the AM Modulation envelope is symmetrical to the peak voltage of the CW carrier. This means that the average voltage of the modulated signal is the same as the average voltage level of the CW signal.
This does not apply to the power of the signal and therefore the signal power is a function of the power of two of the RMS voltage. The average power of the AM modulated signal is slightly (1.2 dB) higher when comparing it to the power value of the unmodulated Power.
What is needed?
150 Watt amplifiers are quite normal when powering these type of antennas. The questions is, is there need for a class A amplifier in this situation? Or is Class AB sufficient? Next to the power requirements, it’s important to look at power reflection and distortion requirements accordingly.
At 80 MHz the required forward (PEP) power is 162 Watts PEP. However, we are still running the amplifier at 66 Watts average as the AM modulation is symmetrical around the CW power level. This is the power level generating the heat in the final stages.
The average reflected power resulting from the antenna VSWR is 50% (1:6) of the forward power, i.e. 33 Watts. Thus this means that this antenna can easily use a 160 Watts (P1dB) Class AB amplifier having a max VSWR spec of 1:3 (maximum reflected power is 25% of the forward power i.e. 40 Watts)
Moreover when designing a conventional EMC immunity system the two main parameters to take into account are:
- The 1 dB compression point, to ensure an undistributed test signal in the peaks of the modulation
- The VSWR handling capability. This occurs when the antenna VSWR mismatch is high and results in much the transmitted power to return to the amplifier.
The 1 GHz to 6 GHz band
For the 1 to 6 GHz frequency range mainly two different antenna are available, a LogPeriodic Antenna (LPDA) or a Horn type antenna. The main difference between the two antennas is the gain figure.
Gain and area of illumination
An LPDA generally have a gain of 7 to 8 dB. While horn antennas (have much) higher gains which increase along with frequency.Also the aperture size increases for the higher frequencies. If the gain increases above the 12dBi level, and compliancy with EN-61000-4-3 will not be possible anymore.
High gain of the horn antenna sounds appealing but also have a negative side effect. Due of the larger gain the illuminated area is smaller in size. In other words by using the higher gain, the more difficult it becomes to stay compliant to the EN61000-4-3. To stay in compliance to the 1.5 * 1.5 meter uniform field area, (as stated in the EN61000-4-3) the level of gain needs to be taken in to account. With the lower gain and large -6dB angle of the LPDA, the illumination of the area is no issue. However, the lower gain requires more net antenna power into the LPDA to create the required field level.
VSWR at higher frequencies
The VSWR of these high frequency antennas in general is much lower than 1:3. For this reason Class A is not a real need and Class AB is mostly a good solution.
Selection of the RF power amplifier
Class A or AB amplifier?
In the EMC immunity test world, the class of operation of the amplifier is often a point of discussion. A Class A amplifier is often recommended for EMC immunity testing in order to ensure that the power that is driven into the load, i.e. the antenna, is not influenced by an increasing VSWR mismatch. Before drawing conclusions about which amplifier Class should be used, firstly the differences between these two operation classes need to be assest. What are the design criteria and what are the operational limitations of each Class.
Class A amplifier
The main specification influenced by the Class of operation of any amplifier is the overall efficiency of the amplifier and more specifically the final stages that are driving the load. An example of a simple Class A amplifier, as shown in the figure 7, is the most linear type available. Generally these amplifiers produce signal currents and voltages that are well within and much smaller than the set points of the bias values. On the final stage however, these amplifiers can only achieve a maximum (theoretical) efficiency of 50%. In reality efficiencies from 20% to 30% are realistic. In the single-ended Class-A amplifier the transistor amplifies the full cycle of the signal without any or very low distortion
Class B amplifier
When the bias current is reduced to zero, the steady state current through the transistor will stop. This also reduces the power dissipation. The input signal is only amplified on its positive cycle, as can be seen in the figure 8 below. The input signal needs to ‘open’ the base-emitter diode of the transistor to get the output current flowing and create amplification. This effect immediately increases the efficiency of this amplifier, but the signal distortion in the single ended stage is enormous as only half of the original signal is available at the output of this stage.
By adding the other half of the signal in a so called Push-Pull design. a Push-Pull design can be an improvement on th the output signal. In this case the two transistors act together and each one of the pair takes care about one half of the signal. See figure 9.
As the bias current in the Class B amplifier is close to zero, each half of the output signal is not an exact copy of the input signal. Each of the two transistors Base-Emitter junction must be ‘opened’ by the input signal causing the signal to look like the waveform as can be seen in figure 10. Each transistor just starts to conduct after the signal voltage crosses the BE diode voltage of approximately 0.7 Volts to start the collector currents. The point where the current flow switches between the transistors is the crossover point. The associated distortion has the suiteble name the crossover distortion. If we look to the overall distortion performance of this stage, the crossover distortion has a larger effect at low power levels
Moving to AB
By inserting a DC voltage of twice the value of a BE junction between the base inputs (see figure 11) of the two transistors both start to conduct, removing the unwanted behavior around the zero volt line. As we have increased the bias current in the final stage, going from full Class B towards Class A, we call this efficient amplifier a Class AB amplifier.
Class A versus Class AB in EMC applications
The use of Class A versus AB amplifiers has always given rise to an interesting and lively discussion in the world of EMC testing. Let’s analyse test applications and the real requirement for either of the classes of Amplifiers. If we go back to the Class A output stage, we have seen that the signal current is much smaller than the steady state current. Even with no signal the final stage will produce a lot of or heat which has to desipate out of the device through heat sinking.
When the Class A amplifier drives the amplified signal into a matched load, we still see the high power dissipation level in the final stage. When there is NO LOAD, or even a SHORT, the final stage is subject to 100% reflection of the power of the output signal. All this power returns back into the final stage and is heat. When designing transistor amplifiers the developer looks to the SOAR (Safe Operating Area) to ensure that the total power dissipation of the device is within the SOAR
area for a trouble free and long life operation. In a solid-state Class A design the output transistors operate well in their (SOAR) and all the reflected power causing additional heat. This heat is no problem for the amplifier’s output stage without causing damage. This area has to do with maximum device voltages, current ratio’s (power) and the thermal power dissipation limts. See figure 12 for this graph.
From figure 11 is becomes clear that the device has limits for maximum collector current and maximum collector voltage. The thermal dissipation limit (which is the product of voltage and current, minus the net power delivered to the load) shows how far you can go with heating up the device.
In Class A designs (theoretical) 50% of the power is heat, in Class AB designs just 12.5% (theoretical) . A device set in the Class AB mode has a limited steady state power dissipation with a quiescent current that varies with the drive power. The same transistor used in the Class A design can deliver less power when used in a Class AB mode due to the thermal constraints under normal load conditions. The Class AB design has smaller heat-sink requirements resulting from the increased efficiency.
The efficiency of the AB type amplifier is best at its high power output levels, which in practice provide up to 60 % practical efficiency approximately. But, what happens if the VSWR of the load increases. Again reflected power from the load returns back into the output devices and is converted in additional heat that can increase the device temperature beyond the SOAR curve. When protection measures are not in place, the device will get damage due to this extra heat. For this reason, class AB amplifiers have a VSWR protection system that lowers the drive to the final stage and reduces the (average) power that must dissipate.
dB compression point
Differences in compression characteristics between Class A and AB are not so large as long as the load has a good VSWR performance. As such, the 1dB compression point performance is quite comparable between these two Classes
The RadiField® – Electric Field Generator (Triple A)
With the RadiField Triple A (AAA stands for Active Antenna Array) Raditeq sets a complete new standard for immunity testing. RadiField Triple A offers:
- A simplified system approach
- Guaranteed EM field level
- High level of integration
- No loss of expensive RF power
- Low cost of ownership
The two setups in Figure 13 show that the complexity isn’t nessesary in the case if the RadiField solution (see Figure 14). The system is build up with a RadiCentre modular system. In the RadiCentre the following modules are inserted for this application:
- RadiGen 6 GHz Signal Generator
- RadiSense 10 GHz Field Probe Laser card
- RadiField Power Supply and Control Card
This system uses just two coaxial cables. Only one coaxial cable runs from the RadiCentre RadiField power supply card to the RadiField Triple A carrying:
- Control / Communication
- Driving RF signal
The second cable runs from the output of the RadiGen to the RF input of the RadiField power supply card in which a special Bias-Tee combines DC power, Control and driving for the RadiField All cable losses lapse. All generated RF power is directly injected in the antenna and converted into an EM field. Where in the conventional setup also Low-Loss coaxial cables are a must to ensure that as much as possible of the expensively generated RF power is transported from the output of the system to the antenna many meters away.
Reducing Complexity of the System
Comparing the traditional setup of an RF immunity test system with the RadiField Triple A it becomes clear that the RadiField Triple A, with its high level of integration, is the answer how to reduce space and costs.
The conventional approaches
Looking to the basic setup of a conventional system, generally the following system components can be recognized:
- Signal Generator
- RF Power Amplifier(s), single or dual-band
- Dual Directional Coupler(s), one or more bands
- 2 RF power meters (forward and reflected power)
- RF Switches (not needed with a single band amplifier)
Next to the system components in the Single Band Solution, 5 coaxial, and at least 4 control cables are needed. When a Single Band amplifier is not available, the complexity drastically increases to 12 coaxial cables and 5 control cables in case of the Dual Band approach.
Minimum installation time
The design concept of the RadiField ensures a very short installation time for the user. With the RadiCentre (loaded with a Signal Generator, RadiField Card en Laser Driven Field Probe system), located in the engineers control room only two coaxial cables (from RadiCentre to the anechoic chamber’s entry panel and from the entry panel to the RadiField) are sufficient to start the generation of an EM field!
Three Meter Equivalent (TME) field
With the RadiField the determination and definition of the required field level is easier than ever before. A newly defined parameter, the Three Meter Equivalent or TME allows easy recalculation of field strengths at different distances with respect to the value at 3 meter. The formula for recalculating fields at different distances is:
3 * TME / d
Thus given a system with a TME of 10V/m, an easy recalculation, shows that this system will generate:
- 10 V/m * 3 / 10 equals 3.0 V/m @ 10 meters
- 10 V/m * 3 / 1 equals 30.0 V/m @ 1 meter.
No more worry about amplifier power, antenna gain and gain calibrations at various distances, cable losses versus frequency etc. Just establish your required field level, distance and frequency range, and select the RadiField that covers your needs for the required test distance and field level.
|RFS2006A||800 MHz – 6 Ghz||9||3||0,9|
|RFS2006B||800 MHz – 6 Ghz||30||10||3|
Accuracy and reliability
Radiated Immunity testing is based on placing the DUT in a pre-calibrated uniform field. This calibration is performed by a field-probe placed in the 16 equidistant spaced positions of the uniform area. During execution of the test, the field probe is removed and the field illuminates the DUT with the pre-calibrated values.
During the field calibration process, relationships are recorded between the measured field levels and the power levels measured on the output ports of the directional couplers positioned behind the RF power amplifiers. During the test, after the field probe is removed from the chamber, the control software “replays” the power levels using the directional coupler’s forward and reflected power data.
From the signal generator up to the directional couplers, everything is measured. However any change in signal attenuation (change of field) due to changes in cable loss, switch-loss and connector loss, in the cables marked RED marked are in figure 16, is not measured and thus not corrected. The operator is under the impression that the test runs correctly, but there is no guarantee whatsoever that this test is the same as during the calibration run. With the design of the RadiField all these sources of errors are completely removed. The radiating antennas are integrated together with the RF power amplifiers and couplers in one single system removing any possible source of errors in these parts.
Electric Magnetic Field Generation & Cost of Ownership
Last but not least the total cost of ownership of the RadiField is very attractive in comparison with atraditional setup. For this we have made a comparison table to compare the RadiField with a number of other combinations of amplifiers and antennas. Not only the base price of the unit is approximately 50% lower than for conventional setups also maintenance is lower due to less wear and tear and no need for separate calibration of cable losses, antenna gain, coupler and RF power meters. Less easy to value is the time needed for installation and re-installation (e.g. antenna switch) and mistakes that can easily be
made due to the complexity of conventional systems. First comparable setups are defined as follows:
Conventional Setup contains:
- Signal generator
- Power amplifier
- Forward power meter
- Reflected power meter
- Field Sensor
- Cables and connectors (6 sets)
RadiField Electric Field generator setup contains:
- RadiCentre including
- RadiGen Signal generator
- RadiSense Field Sensor
- RadiField Triple A
- Cables and connectors (2 sets)
List of Abbreviations
DEF – Distance Equivalent Field level
DUT – Device under Test
DDC – Dual Directional Coupler
EM – Electro Magnetic
LPDA – Log Periodic Dipole Array
PEP – Peak Envelope Power
Triple A – Active Antenna Array
Accredited Test Laboratory Results.
We have tested the RadiField in anechoic test chambers of KIWA DARE Services to obtain a validation of this test system towards the proposed field levels and the uniformity of the illuminated area. The results of this test can be found in the downloadable PDF.