Interference Testing with Handheld Spectrum Analyzers Manual

Interference Testing with Handheld Spectrum Analyzers Manual
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Summary of Contents

Interference Testing with Handheld Spectrum Analyzers Manual

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    Interference Testing with Handheld Spectrum Analyzers Application Note

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    2Introduction ........................................................................................................................ 3Wireless Systems and Interference .................................................................... 4Radio Duplexing ............................................

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    3Wireless communications systems co-exist across the RF and microwave frequency spectrum and are designed to operate with a limited amount of inter-ference. As wireless systems often share or reuse frequency spectrum, interfer-ence from other users can quickly become an issue. When the amplitude ...

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    4Wireless communications, traditionally known as radio communications, use radio waves operating with carrier frequencies in the range of 3 kHz to 300 GHz. While it appears that this radio spectrum is very wide, practical considerations, such as performance, power, and equipment cost typically li...

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    5Figure 2. Measured spectrum of a discovered in-band interference when the main transmitter was switched offFigure 1. Measured spectrum of a discovered in-band interference when the main transmitter was switched onFigure 2 shows the spectrum of this “in-band” interference with the WLAN transm...

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    6Knowing that the radio spectrum has a limited number of channels and the number of users continues to grow, many radios system are designed to share a single frequency channel by dividing the transmission time among several users. This is called duplexing. For example, a cellular GSM mobile subs...

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    7Example: Duplexing’s role in interference identificationFigure 3 shows an over-the-air measurement across a portion of the NA GSM 850 downlink frequency band. The measurement was taken from the Agilent HSA with an omni-directional antenna, commonly called a “rubber ducky” type antenna, att...

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    8The reason for the breaks in the measured response for the left channel, centered at 870.0 MHz, is that this radio is not transmitting all the time and the spectrum analyzer is measuring the rapid changes in the signal amplitude as the analyzer sweeps across the display. As the analyzer was conf...

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    9Sources of InterferenceInterference becomes a problem when a wireless system no longer operates as expected. Even though regulatory agencies and standards organizations define wireless operation and protocols within each frequency band, interference from intentional and unintentional radiators m...

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    10Figure 4. Backlit keypad and instrument screen make it easy to work in any environmentSources of Interference (continued)Example: Determining the source of interferenceAs a simple experiment to show the impact of unintentional interference, con-sider the effect of fluorescent lighting on radio ...

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    11Hint:Figure 5. Over-the-air measurement of a 915 MHz RF signal in an office environment with the lighting turned offSources of Interference (continued)Center frequencyFrequency spanReference levelMarker10 dBThe analyzer center frequency was set to “915MHz” using the [FREQ] button and the di...

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    12Figure 6. Over-the-air measurement of a 915 MHz RF signal in an office environment with the fluorescent lighting turned on. Shown are the modulation sidebands of the 43 kHz operating frequency of the electronic lighting ballastSources of Interference (continued)Delta markerInterferenceAnother m...

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    13Interference affects a radio system when it enters the receiver and corrupts the detector in the receiver. If the amplitude of the interference is very large, the interference can overpower the receiver’s front-end electronics and reduce system performance. Filters are added to the receiver t...

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    14Hint:Spectrum analyzer functionalityThe block diagram for a spectrum analyzer is similar to the receiver shown in Figure 7 except that a typical spectrum analyzer does not include the front-end BPF. By removing the BPF, the spectrum analyzer is not limited to a specific band of measurement freq...

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    15Power levelsFor example, the high power damage level on the Agilent N9342C HSA is +33 dBm, or 2 watts, for 3 minutes. Measuring signals with power levels above +33 dBm would require the use of an external attenuator or coupler placed at the input to the spectrum analyzer. Also note that the HSA...

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    16Knowing that the vertical scaling is set to 10 dB per box it is easy to see that the amplitude of the interference is approximately 30 dB larger than the main signal. As the interfering signal is the larger of two signals entering the input to the analyzer, it would be the power of interference...

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    17Figure 9. Measurement of a single wideband signal. The adjacent channel interference is still present at the input of the spectrum analyze but not shown on the display due to the narrow frequency span of 20 MHzThe bottom end of the dynamic range is set by the spectrum analyzer noise floor. A si...

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    18Even without a pre-amp, the noise floor of the analyzer can be optimized using the RBW filter. The RBW filter on the Agilent N934xC HSA is adjusted under the [BW] menu and using the {RBW} setting. Often an automatic setting for RBW will provide a sufficient noise floor on the instrument and man...

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    19Figure 12 shows a similar measurement but with a signal having a much wider bandwidth. Once again, the noise floor dropped by 10 dB as the RBW was changed from 100 kHz to 10 kHz, but as the signal’s bandwidth is now wider than the RBW, and therefore appears as noise to the RBW filter, the pea...

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    20Once it has been reported that the system is not operating as expected and it is assumed that the root cause of the problem is interference entering the receiver of the system, a spectrum analyzer should be used to confirm the existence of wireless signals in the frequency channel of operation....

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    21Figure 14. Over-the-air measurement of a GSM 850 downlink transmission using the Agilent N934xC HSA with the trace “Maximum Hold” selectedFor radios that do not provide access between the transceiver and the antenna, the spectrum analyzer can be directly connected to the system antenna or c...

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    22The trace option on the Agilent HSAs allows up to four different traces to be displayed. The multiple traces can include combinations of Max Hold, Min Hold, stored memory, and active measurements with different detection options including the default “positive peak”. Additional information ...

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    23The spectrogram can record 1,500 sets of spectrum data in a single trace file with an update interval that is set by the user. The HSA will automatically create another trace file to save continuously beyond 1,500 sets. For example, on the N9344C HSA sweeping across the full 20 GHz frequency sp...

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    24Interference to radio signals can come from a number of sources including inter-ference created by one’s own radio system or by interference created from other radio systems and unintentional radiators such as nearby electrical equipment and mechanical machinery. It was previously mentioned t...

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    25Figure 16. Measurement of 500 MHz unfiltered transmitter showing second harmonic generated at the outputInterference Classifications and Measurement Examples (continued)Main signal2nd harmonic1000 MHz500 MHzCo-channel interference is one of the most common types of radio interference as system ...

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    26It is important, and often a regulatory requirement, to properly filter out the harmonics of a transmitter so that one wireless system does not affect another system operating in a higher frequency band. When examining harmonics of a wireless transmitter, it is necessary to use a spectrum analy...

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    27Adjacent channel interference This interference is the result of a transmission at the desired frequency channel producing unwanted energy in other channels. Adjacent channel interference is common and primarily created by energy splatter out of the assigned frequency channel and into the surro...

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    28The Agilent HSA was configured to automatically measure the power in the main channel and adjacent channels using the adjacent channel power ratio {ACPR} measurement found under the [MEAS] menu. The table under the spec-trum display lists the total power, in dBm, for the main channel and the ad...

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    29Downlink interference This type of interference corrupts the downlink or forward link communications typically between a BTS and a mobile device. Because of the relatively widely-spaced distribution of mobile devices, downlink interference only impacts a minority of mobile users and has a minim...

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