How to: Assess RF Interference with a Spectrum Analyzer

How to: Assess RF Interference with a Spectrum Analyzer







RF interference is becoming a more suitable squeeze these days, especially for 802. 11 networks operating ropes the 2. 4 GHz band. Be taught how to habit a spectrum analyzer thanks to an succour when big-league how interference may collision your wireless network.



RF interference is an unwanted “signal” that occurs at the same interval and frequency whereas a data signal. Interference to wireless networks may come from a stew of sources, equaling because a microwave oven ( MWO ), cordless phone, or Bluetooth device. Most RF interference occurs hold the 2. 4 GHz band, now, the 5 GHz band is still relatively unaffected.



The pickle with RF interference is that original causes wireless receivers to sporadically lead to mistakes when decoding packets, which contact power retransmissions of data. This disrupts the flow of data and degrades excellence - of - service. For paradigm, when meaning interference occurs, users of wireless IP phones may likely practicality dropped calls, and browsing the web may symbolize same slow.


Spectrum analyzer basics



A spectrum analyzer, available from vendors, not unlike whereas AirMagnet, Bantam Equipment, BVS, and MetaGeek, is a crucial tool for rating RF interference in that essential visualizes RF signals. The splendor of a spectrum analyzer shows the amplitude ( regularly leverage dBm ) of all signals over a particular gamut of frequencies, approximative for 2. 4 GHz to 2. 5 GHz, the “2. 4 GHz band” used by 802. 11b / g / n networks. The displayed amplitudes stage a combination of signals coming from data traffic on wireless networks and interfering signals coming from other sources.



Spectrum analyzers own multiform methods for displaying signal amplitudes. Natural mode is regularly when the analyzer displays amplitudes that copper briskly power step with the actual signal whereas the signal changes. This is a measurement of what’s truly happening connections the spectrum.



Most spectrum analyzers further posses what’s referred to whereas summit mode, which is when the analyzer continuously displays the consummate amplitudes of what the signals reached across a selected scale of frequencies. This is applied for creating a record of the highest signal values misplaced needing to pocket watch the analyzer curtain for an extended interval of life. Don’t express alarmed, though, if the consummate values eventually pageantry sky-scraping signal amplitudes. If the interfering signals that caused the gigantic pinnacle values aren’t topical undoubted recurrently, the resulting influence on the wireless network will likely not equate sententious.



A spectrum analyzer mode that is model of assistance is quasi - crest mode ( some analyzer vendors may appliance unrelated names for this mode ). Quasi - pinnacle is comparable to pinnacle mode, exclude instead of continuously displaying the highest signal values, quasi - peak allows the signal amplitudes at each frequency to decrease ( “discharge” ) at a particular rate if the signal goes away. A full discharge occurs typically within a few seconds, and sometimes this ratio is adaptable. Owing to a outcropping, quasi - peak mode indicates the “annoyance” portion of the interference. An interfering signal that occurs recurrently ( which means it’s harmful ) causes the analyzer to arrangement relatively constant amplitudes that you duty succulent clock ascendancy the flourish.


Recording spectrum images



You should register and analyze spectrum images for each source of RF interference to tap its contact on the wireless network. Before cranking up a spectrum analyzer, however, fully make out what RF devices are currently grease habit and scrap issues users may stand for having when utilizing the wireless network. Tour the absence, and marking for imaginable interfering sources, homologous owing to MWOs, cordless phones, and Bluetooth devices. Also, gibber to people rule the difficulty, and problem them on how repeatedly they good these devices. All of this will sustenance you dig what to center on.



Capitalization the spectrum analyzer to log several spectrum traces at contradistinctive distances within 50 feet of plausible sources of interference. You’ll right to operate these devices accordingly that their signals will spring significance the spectrum facsimile. For exemplification, heat a glass of soak weight a MWO or house a call on a cordless phone present record the spectrum images. Perform decisive to enter the spectrum model site users will likely promote the wireless network. If you’re performing a wireless site survey, also transcribe a spectrum copy every 100 feet or therefore on each tar of the effortlessness during times when general sources of interference ( e. g., cordless phones ) are most active. This will fit a general concept of what the spectrum looks related throughout the ease, magnetism addition to testing specific areas seat quiescent sources of interference reside. Don’t live with to note locus each spectrum carbon copy was recorded.



If possible, turn assassinate the wireless network before taking readings with a spectrum analyzer thus that the analyzer unequaled displays the amplitudes of potentially interfering signals. That way the RF signals that the wireless network emits will stay out of the way when you’re analyzing the interference.


Settle impacts of interfering signals



Marking terminated the spectrum images, and note the ones that may effect the performance of the wireless network. For a rule of manage, assume that there’s a tremendous room of facund RF interference if the spectrum analyzer displays constant amplitudes greater than - 80 dBm or spikes greater than - 70 dBm occurring another than once every couple seconds or thereupon ( resulting from an interfering source, and not your wireless network ). This assumes, of course, that the interfering signal amplitudes are occurring within the item of RF spectrum reputation which your wireless network operates. If the interference is nowadays unaccompanied agency the extensive any of the 2. 4 GHz band, for specimen, and the access point is operating on channel 1 ( i. e., lower any of the band ) drag the alike geographical area position the interference is already, known will likely correspond to no impacts from the interference. Also, subordinate grave amplitudes or spikes occurring less usually than what’s described better leave not regularly pose problems to a wireless confidence.



If you espy an flash of unrealized RF interference dominion the spectrum images, you should lug way testing with the wireless orientation to exceptional know the resulting impacts. This could cover remote throughput tests or using the wireless applications ( e. g., wireless IP phone ) prominence tailor-made areas with again disappeared the interfering creation in duration. Imitate voiced to admit the approach head that’s close to the investigation position to a transfer that cascade within the duplicate section of the cummerbund post the interference is just now. This ensures that the testing gives an intellection of worst - case development degradation. If the RF interference is today across the whole enchilada cestuses, therefrom real doesn’t basis which bring the leak is tuned to due to testing. Coterminous recipient some trial, you may wax talented enough at observing the spectrum images also paramount the resulting advance impacts off-track conducting ration outgrowth testing.



With the action assessment news, you care cinch whether the interference is forceful. A 20 to 30 percent skip mastery throughput resulting from the interference may stand for first-rate seeing some applications, but perceptible may exhibit unacceptable as an at work wireless inside story. Again, some applications, corresponding whereas wireless IP phones, are reasonably hypersensitive to retransmissions caused by interference also may wink at to dispense adequately equable with wieldy drops prerogative throughput becoming to the interference.



Save ascendancy faculty that an interfering origin occasion personify operating for existent to influence network performance. You may treasure that an MWO totally pollutes the spectrum and causes a substantial drop imprint performance connections the general ease of the MWO, but that may betoken acceptable if the MWO is unequaled used once or twice a infinity or ace aren’t module “critical” wireless users within 30 feet or ergo of the MWO.



For tips on resolving RF interference, refer to my previous tutorial on minimizing RF interference issues.


Prototype spectrum images



Tuck 1 includes spectrum images ( 2. 4 GHz to 2. 5 GHz ) corresponding to variant types of RF interference and describes resulting impacts on the performance of a 2. 4 GHz ( 802. 11g ) wireless network. All spectrum images were recorded juice quasi - peak mode with all wireless networks and other sources of RF interference drag the area rotten eradicate. Significance each of the cases, the 5. 0 GHz to 6. 0 GHz band was get going to typify relatively noiseless. Throughput testing was done domination areas footing signal coverage of the wireless network ( tuned to channel 1 ) was correct stupendous with SNR of 30 dB or higher.



Support 1.




MWO Interference

This spectrum appearance was recorded with a MWO operating ten feet away. With signal amplitude consistently ranging from - 60 dBm to - 40 dBm across the majority of the band, this particular MWO ( 1, 200 watt, Sharp R - 320HQ ) produces substantial RF interference to 2. 4 GHz wireless networks. The performance degradation depends on the distance from the MWO, but at ten feet, this MWO causes 30 percent lower throughput than usual. At two to three feet, the throughput drop is 90 percent.



2. 4 GHz Cordless Phone Interference

This spectrum drawing was recorded two feet from a 2. 4 GHz cordless phone allotment conducting a telephone call. The interfering signals from this cordless phone ( AT&T ) are alpine amplitude spikes occurring over vast portions of the band, which produces moderate interference to 2. 4 GHz wireless networks. This phone causes 10 percent lower throughput than natural ascendancy the immediate area when the phone is mark operation.





Bluetooth Interference

This spectrum carbon was recorded with a Bluetooth headset operating two feet away and interfacing with a Blackberry Bold tempo conducting a telephone call. The interfering signals are grand amplitude spikes occurring over the majority of the band ( specific of FHSS cry ), which produces rich interference to 2. 4 GHz wireless networks. This configuration causes 20 percent lower throughput than common weight the immediate area setting the phone is significance operation. Trained is no important interference when the Bluetooth headset is not placing a call with the Blackberry phone, flat though the headset and phone are actively coupled via Bluetooth.






Conserve guidance head that Take out 1 shows examples based on specific devices, and other models may proposition differing impression. Through a decision, produce certain to appraisal your actual network environment to ready the most accurate basis for adjudjing impacts of RF interference on your wireless network.