Tips for Performing Precision Interference Measurements in the Field

Adjacent channel interference – is the result of transmissions in a given frequency channel generating unwanted energy in other adjacent channels, usually within the same system. Adjacent channel interference is usually caused by energy leaking from the given frequency channel into surrounding higher or lower channels. This energy leakage is caused by modulation, switching transients, and intermodulation distortion. In the power amplifier of a wireless transmitter, intermodulation distortion, or spectral regrowth, is often generated due to nonlinear effects of the power electronics. For more information on intermodulation distortion testing, refer to the Keysight product guide, Optimizing Dynamic Range for Distortion Measurements.

An example of two channel-related measurements is shown in Figure 7. The signal under test is a 17.725 GHz modulated signal similar to Cable Television Relay Service (CARS). CARS is licensed to operate in the 17.7 to 19.7 GHz band. The CARS channel spacing in this band is specified to be 10 MHz. The recorded measurements are then played back and used to determine the channel power and adjacent channel power. Figure 7a shows that the measured channel power is -19.6 dBm over a 10 MHz bandwidth. It is clear from Figure 7a that this signal is radiating unwanted energy into nearby channels. Figure 7b shows the adjacent channel power in dBc relative to the main signal power. The two channels above and below the main channel in this example are also shown. The largest unwanted adjacent channel power occurs on the two channels immediately to the side of the main channel, with a relative level of approximately -23 dBc. FieldFox also provides occupied bandwidth measurements in the Channel Measurements menu, which can be made using live or recorded signals.

Figure 7a. Channel power measurement results

Figure 7b. Adjacent channel power

While adjacent channel interference is usually associated with active components in transmitters, passive components (including antennas, cables, and connectors) can also generate unwanted interference, such as intermodulation interference [11]. This type of interference is often referred to as passive intermodulation (PIM) interference and is primarily generated in passive components that use two or more high-power signals. The resulting PIM can generate interfering signals in the receive channel of a communication system, degrading receiver performance. Intermodulation interference is an important consideration in modern communication systems that use multi-carrier modulation, including mobile wireless systems, satellites, space probes, and shipborne systems [12, 13]. For more information on specific PIM test equipment, see the Keysight application note, "Innovative Passive Intermodulation (PIM) and S-Constant Measurement Solutions Using ENA."

Figure 7 shows the results of measuring the channel power characteristics of a 17.725 GHz modulated transmit signal using FieldFox.

Downlink Interference

Downlink interference may typically disrupt downlink communications between a base station and a mobile device. Due to the relatively wide separation distances between mobile devices, downlink interference will only affect a small number of mobile users and have little impact on the overall system communication quality. In most cases, downlink interference manifests itself as co-channel interference, which has a significant impact on the quality of service.

Uplink Interference

Uplink interference, also known as reverse link interference, can affect the base station's receiver and related communications from mobile devices to the base station. Once a base station is interfered with, the performance of the entire service area of ​​the cell site will be degraded. Uplink interference determines the capacity of each cell site.

Interference Measurement Technology

When a system is not operating as expected, assuming some form of wireless interference is the source of the problem, a spectrum analyzer should be used to determine the presence of unwanted signals in the operating frequency channels. This discovery process may involve determining the type of signal, including transmission time, number of occurrences, carrier frequency and bandwidth, and possibly the geographic location of the interfering transmitter. If the system is operating in full-duplex mode, it may also be necessary to check the uplink and downlink frequency channels for the interfering signal.

Measuring interference—particularly over-the-air interference—usually requires the use of a spectrum analyzer with a very low noise floor, or DANL. DANL is determined by the resolution bandwidth (RBW) setting, and the lower the DANL, the lower the noise. Typically, reducing the RBW by a factor of 10 will reduce the noise floor by 10 dB [15]. As mentioned earlier, the analyzer’s measurement sweep time is an inverse function of the RBW, so the smaller the RBW setting, the longer the sweep time required. Since the ability to quickly measure and display low-level signals is directly related to the signal-to-noise ratio (SNR) of the analyzer’s detector, reducing the analyzer’s input attenuation can improve signal levels. Lower input attenuation values ​​(usually as low as 0 dB) increase the RBW, resulting in shorter sweep times. Using an internal or external preamplifier can also improve the measured signal level at the detector. FieldFox has a DANL specification of -138 dBm at 2.4 GHz without a preamplifier and -154 dBm with the internal preamplifier.

Special attention should be paid to the analyzer when reducing input attenuation and measuring large amplitude signals. Large amplitude signals can overdrive the analyzer front end, causing internally generated distortion or instrument damage. The analyzer can display internally generated distortion as if it were coming from the signal of interest. Under these conditions, the attenuator setting should be optimized to achieve the highest dynamic range. FieldFox includes a 30 dB attenuator that is adjustable in 5 dB steps to optimize the dynamic range of the measurement. For additional information on dynamic range and DANL, see the Keysight application note, Fundamentals of Signal Analysis Measurements with Spectrum Analyzers.

Equipment Requirements

When selecting an analyzer, measurement accuracy, sweep speed, and analyzer portability are extremely important requirements, as field testing is often performed in extremely harsh conditions in marine, aerospace, and automotive applications, including high altitudes (such as outdoor tower and mast installations) and confined spaces. When performing interference testing in the field, many key features of the measurement equipment need to be considered, including the need for a rugged spectrum analyzer with long battery life and quick replacement, fast recovery from pause, built-in GPS, DC blocks, and DC voltage sources. The DC voltage source is ideal for powering low noise blocks (LNBs) in satellite applications when used with an external bias tee. High-performance FieldFox analyzers with a maximum frequency of up to 26.5 GHz meet all requirements for field testing in all environmental conditions.

FieldFox not only has the capabilities of a benchtop spectrum analyzer, but also includes a unique feature called InstAlign that, once enabled, immediately provides better amplitude accuracy across the entire RF and microwave frequency range and temperature range of -10 to +55°C. The InstAlign feature is based on a very stable internal continuous wave (CW) amplitude reference that is characterized over the entire frequency range of the instrument. Any deviation between the amplitude measurement and the characterized value of this reference is used as correction data during the measurement of the test signal. When the internal sensor detects that the temperature of the instrument has changed by approximately 2°C, FieldFox can perform the amplitude correction through a background process without user intervention. As a result, FieldFox can provide an overall absolute amplitude accuracy of typically less than ±0.6 dB over the frequency range up to 26.5 GHz and the temperature range of -10 to +55°C without warm-up.

Figure 8. Over-the-air measurements comparing the signal received using an omnidirectional antenna (blue trace) and a high-gain antenna (yellow trace).

In addition to a high-performance spectrum analyzer, it is also necessary to use high-quality test cables to establish connections between the analyzer and the system test ports or test antennas. Proper maintenance of the cables—protecting and cleaning the connectors on the analyzer and the cables—is essential for making accurate, repeatable measurements. Most coaxial cables are rated for a "minimum bend radius," and if the cable is stored with a bend radius smaller than this, it is possible for the cable to break internally, resulting in intermittent measurements.

The test antenna is another important part of the interference test component. It should be designed to cover the frequency range of interest while being lightweight and portable. The antenna can be connected directly to the spectrum analyzer using the FieldFox top-mounted N-type female 50 ohm connector. Although the N-type connector is more durable when conducting field tests, FieldFox also offers an APC-3.5 port connector option. Ideally, the antenna characteristics should be similar to the test antenna used for the wireless system under investigation. If the system antenna is a low-gain omnidirectional antenna with vertical polarization, the antenna connected to the spectrum analyzer should be the same.

When monitoring spectrum over a wide frequency range, a typical narrowband system antenna can be used instead of a broadband whip antenna. There are many broadband antennas available on the market, including the Keysight N9311x-500 and N9311x-501 (covering the frequency ranges of 70 MHz to 1000 MHz and 700 MHz to 2500 MHz, respectively). When measuring extremely weak signals or direction finding unlicensed transmitters, a high-gain directional antenna should be connected to the analyzer. Keysight offers several models of directional antennas, including the N9311x-504, 508, and 518, with gains of 4 to 5 dBi and frequency ranges up to 4, 8, and 18 GHz, respectively.