Introduction
Technology frequently embeds transmitters in a myriad of devices to support an Internet Of Things (IoT) concept from identification to condition-based actions, and this IoT approach includes defense operations. Devices may include multiple transmitters that a few years ago did not include communications of any sort. Inclusion of RF transmitters has prompted a significant demand for antenna emission control. This article looks at RE103, the alternative test method for transmitter antenna port emissions used when the conducted emission CE106 is inappropriate or connection for the test is restricted. Note that alternative means using either CE106 as the preferred test but allows RE103 use as necessary – for example, if the antenna installation prevents a direct connection for a CE106 test.
Normally a spurious emission appears outside of the necessary bandwidth of the intention transmission. Harmonic, parasitic, intermodulation products and frequency conversion products typically fall under the umbrella of spurious emissions – or unwanted emissions associated with a device that normally creates other frequencies.
RE103 does not establish requirements for receivers or transmitters in standby mode. Devices with a designed-in or permanently mounted antenna do not have requirements for antenna port emissions since CE106 is not applicable to this type of device. Therefore, we default to RE102 for emission requirements associated with receive and standby modes of operation.
RE103 can easily become the preferred test method for devices with active antennas. This discussion delves in detection and measurements based on the MIL-STD-461 requirements and methods. Compliance with the MIL-STD-461 RE103 limits may not confirm compliance with commercial and international standards, so the device application may dictate compliance with non-military emission requirements.
Depending on the device, RE103 testing can be rather simple or extremely complex. Planning is critical and in some cases, the need for specialized equipment may present a long lead time to prepare for testing. Many details are involved, and in many cases, it is easy to induce minor factors affecting the results.
This test method has been a part of the MIL-STD-461 test program from the onset appearing under the RE03 designation. Previously, CE06 provided the conducted testing alternative with RE03 being applicable if the average power exceeded 5 kW, the antenna was permanently mounted or if the operating frequency exceeded 1240 MHz. In the early days, testing of tunable devices called for testing with the device tuned to three frequencies per octave within each tuning band including within 5% from each end of the tuning band.
With the release of revision “D” in 1993, the numbering updated to RE103 as the radiated alternative to CE106. At the same time, MIL-STD-462D describing the test method was released to support the many changes. The fundamental process remained, but a few changes emerged:
- The test frequency range provided for testing to 40 GHz instead of the previous version supporting the 20 GHz to 40 GHz as optional. The actual test frequency range is based on the operating frequency of the Equipment Under Test (EUT). Maximum test frequency became 20-times the highest operating frequency up to 40 GHz.
- A more generic test equipment list was provided eliminating call out of specific vendors and models specified in the previous version.
- Calculation details associated with verifying the Effective Radiated Power (ERP) and maximizing the received signal was shortened to a simpler power output measurement with verification once the fundamental frequency measurements were optimized.
- Testing a transmitter at the fundamental frequency ±5% or the necessary bandwidth was not applicable except during the optimizing process.
- Testing of tunable equipment was changed to have the device operating at three frequencies per tuning band (removing the per octave requirement).
- The transmit limit required 80 dBc suppression except for the 2nd and 3rd harmonics of the fundamental operating frequency. The 2nd and 3rd harmonics limit was 50 + 10 log p where p = peak power output in watts but not more than 80 dBc.
- During transmitter testing tune the measurement receiver to the transmit frequency and optimize the bandwidth for the maximum indication. Use this bandwidth for the transmit mode testing.
- Align the transmit and receive antennas azimuth and elevation for maximum indication.
- Signal integrity checks were implemented with the testing to verify the measurement system would correctly measure a known signal through all components in the measurement path.
- Data presentation required graphs with frequency resolution be the lesser of 1% or twice the bandwidth and amplitude resolution of 1 dB.
In 1999 revision ‘E” incorporated a few changes:
- Testing a transmitter at the fundamental frequency ±5% and the necessary bandwidth was not applicable.
Revision “F” was released in 2007 bringing forth a few updates:
- Testing a transmitter at the fundamental frequency ±5% or the transmitted signal bandwidth, whichever is larger was not applicable.
- The transmit limit required 80 dBc suppression except for the 2nd and 3rd harmonics of the fundamental operating frequency. The 2nd and 3rd harmonics limit was set to -20 dBm or 80 dBc whichever required less suppression.
Revision “G” released in 2015 is the current standard and will be used as the basis for the following discussion detailing the test procedures for RE103. Some updates include:
- Additional exclusion was defined for high power transmitters for Navy shipboard applications.
- Established a minimum upper test frequency of 18 GHz for systems that generate or receive frequencies up to 1 GHz. For systems generating or receiving frequencies of 1 GHz or higher, the upper test frequency is 10-times the highest frequency or 40 GHz, whichever is less. Translation if operating at 999 MHz the upper test frequency is 18 GHz and if operating at 1 GHz the upper test frequency is 10 GHz – seems to be unusual from my perspective.
- Updated the suppression requirements for harmonic other than the 2nd and 3rd for some selected applications.
Test Location
Testing in the far-field is required and depending on the operating frequency the transmitter antenna and the measurement system antenna may need a large separation distance. Far-field distance may have some variance depending on the standard definition – for MIL-STD-461 the following parameters are used to establish the separation (measurements in meters).
- If the transmit frequency is equal to or less than 1.24 GHz the greater of:
- R = 2D2/χ or
- R – 3λ
- If the transmit frequency is greater than 1.24 GHz use:
- If 2.5D < d use R = 2D2/λ
- If 2.5D ≥ d use R = (D+d)2/λ
- Where:
- R = distance between transmitter antenna and measurement system antenna
- D = maximum physical dimension of transmitter antenna
- d – maximum physical dimension of measurement system antenna
- λ = wavelength of transmitter frequency
Using the above, an HF transmitter operating at 2 MHz with a 2.5-meter antenna would require a separation of 450-meters. The distance could prompt the need for test personnel at the transmitter and measuring system locations with a communications link to support testing. During test at the 15 MHz frequency, the separation distance would drop to 60-meters. However, the 15 MHz test could still use the 450-meters separation.
Test Configuration
Figure 1 shows a basic test configuration with components that may or may not be required to accomplish RE103 testing depending on the device. Let’s review this configuration to establish a purpose for each component to understand how to configure.
On the EUT side of the configuration a power monitor is installed to verify the rf power being supplied to the transmit antenna. The power verification is to be sure that the EUT is operating at the rated output and serves as the basis for calculating the Effective Radiated Power (ERP) by adding the antenna gain to the power measurement.
The measurement system includes a signal generator that is used to create a known signal for signal integrity checks. Note that the signal generator connection is using a dashed line to indicate a connection only during the signal integrity check process.
The dashed line at the measurement system antenna input is to indicate the presence of an enclosure if appropriate. The test location may need this to prevent interference to neighboring facilities since the transmitter may be pending approval to radiate at the test location. In most cases, the test facility obtains a Special Temporary Authorization (STA) allowing the transmitter to radiate for test purposes. The STA is issued by the Federal Communication Commission in the United States and most countries have something equivalent. Failure to obtain an STA could result in severe penalties if radiation is allowed without the authorization.
The band rejection or high pass filter is used to attenuate the intended fundamental frequency with minimal attenuation of other frequencies being measured. Failing to attenuate the fundamental transmission can result in overload of the measurement system receiver frequently creating spurious signals within the measurement system. Although the standard mentions high pass filter, a low pass filter can be helpful when testing is below the fundamental frequency.
The pre-selector block is another filter to prevent overload conditions. Many receiver systems have a pre-selector option to reduce the amplitude of signals outside the frequency range not under measurement.
Attenuators and pre-amplifiers are frequently used in the test configuration to help reduce high-level signals or amplify signals that have been attenuated improving the sensitivity of the measurement receiver. Note that amplification can include amplification of noise, so a low noise pre-amp is normally specified.
Figure 1: Test Configuration
Signal Integrity Check
Since the release of MIL-STD-461D, signal integrity checks have been a part of the testing process. This step helps us be sure that the measurement system is functioning correctly at the time of testing instead of relying on the last calibration and hoping that measurement system components have not been damaged. In the case of RE103, testing the signal integrity checks are vital to be sure that we have proper correction factors for the various filters, attenuators and amplifies that we incorporate into the rather complex measurement system. In addition, the multiple connections used in the measurement path provide opportunities to induce measurement errors so checking the system is very important.
Signal integrity checks verify that the measurement path is connected and that the system software provides for including the correction factors. The filter curves (see Figure 2) used require that the correction factors are available to the software to make the measurement calculations over the frequency range for a specific filter when installed. It is common practice to change the filter in use for various frequency ranges to minimize attenuation where no high-level signals are present. These changes help maintain the sensitivity of the measurement system.
Figure 2: Filter Curves
Prior to making signal integrity check measurements, we need to establish the limit curve. Although not specified in the MIL-STD, it should be standard practice to apply a signal 6 dB below the applicable limit. The limit is based on suppression relative to the ERP and we normally verify that as part of the EUT testing. We could calculate a limit based on the EUT specification or proceed with the ERP verification portion of the test procedure. Either way will provide information necessary to calculate the complete limit including the limit at the fundamental operating frequency of the EUT.
With the limit available set the signal generator to a mid-band fundamental frequency. The signal generator amplitude setting should compensate for measurement system gains and losses for the measurement path components.
For discussion let’s assume an ERP of 140 dBμV/m, the band rejection filter attenuates 80 dB, a 10 dB attenuator is installed and a pre-amp with 25 dB gain is included in the measurement path. This would require that the signal generator output be set to 27 dBm making a 0.5-W amplifier necessary to attain the 134 dBμV signal (6 dB below the limit) for the signal integrity check. To avoid the need for the amplifier, one could check the measurement path in sections such as checking the band rejection filter independently and then the rest of the measurement path components.
Once the signal generator is set, measure the applied signal and verify that the result is within 3 dB of the applied signal. If not within this tolerance, take corrective action and repeat the process. Repeat the signal integrity check for at least two additional frequencies for the measurement system hardware configuration. The signal integrity check should be accomplished for each hardware configuration to be used for testing.
EUT Testing
Now that the test configuration has been established and signal integrity checks have been successful, testing can proceed. Apply power to EUT, select a desired operating frequency and enable the transmit function. Remain aware that radiation from the antenna is active, so incorporate measures to limit exposure during testing.
Measure the modulated transmit RF power and verify that the power conforms to the transmitter power specification. This ensures that the EUT is operating at the rated output. Convert the power level to dBW and add the antenna gain to obtain the Effective Radiated Power (ERP).
With the EUT transmitting, tune the measurement receiver to maximize the received signal at the transmit frequency. Adjusting the measurement receiver bandwidth and aligning the antennas elevation and azimuth is part of the maximizing the received signal. Measure the maximized received signal and calculate the ERP using:
- ERP = V + 20 log R + AF -135
- V = measurement receiver reading in dBµV
- R = distance between transmit and receive antennas in meters
- AF = antenna factor of the receive antenna in dB (1/m)
Compare this ERP measurement with the ERP measurement obtained from the RF power measurement. If the two ERP measurement differs by more than 3 dB take corrective actions to alleviate the discrepancy. Assuming that the two ERP measurements are within the 3 dB tolerance, the ERP becomes the reference for assessing compliance with spurious and harmonic emission suppression requirements. Verify that the ERP measurements match the RE103 limit and adjust the limit as required based on the ERP reference.
Scan the measurement receiver over the test frequency range keeping the transmit frequency attenuated to prevent measurement receiver overload. This often requires changes in the measurement path components based on their frequency range capability. Maintain the measurement receiver bandwidth used to maximize the fundamental frequency during the ERP verification process. Where spurious or harmonic signals are detected, adjusting the antenna elevation and azimuth should be accomplished to maximize the detected emissions. Verify that the detected emissions are from the EUT and not measurement receiver spurious responses or test site ambient emissions.
Calculate the spurious emission ERP being sure to apply all correction factors for the measurement system components such as cable loss, antenna factors, amplifier gains, attenuator loss and rejection network losses. Remember that testing is specified for at least 3-frequencies per tuning band, so repeat the testing at the other required EUT operating frequencies.
Summary
Planning for transmit mode testing is an absolute necessity to arrange for the measurement system path components with consideration for the test article parameters. Not enough rejection or too much attenuation can easily induce measurement errors that indicate failures or false compliance.
As you can see from our discussion that many components are connected in the measurement path and there are frequent needs to use connector adapters. Each connection includes a certain amount of uncertainty and the cumulative effects can cause a significant measurement error – minimize connections where possible.
The overall test program for RE103 requires many measurement system configurations for transmit mode and testing at several tuned frequencies for the test article so pay attention to the testing.
