[Imported] Bandpass filters to reduce out of band noise

[sbeadle]

>>Message imported from previous forum - Category:Trio Data Radios<<
User: joelw, originally posted: 2018-10-16 21:35:49 Id:31
This is a re-posting from the obsoleted (October 2018) "Schneider Electric Telemetry & SCADA" forum.

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**_jweder:_**

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It is fairly common that interference from outside the 900 MHz or 2.4 GHz unlicensed ISM (spread spectrum) bands, or from nearby a licensed 450 MHz frequency, can cause problems to a radio system. This can occur, for example, if a cellular tower operating in the 890-900 MHz range is very close to a site in your system.

The spectrum analyzer built in to a K Series or J Series radio can in some cases detect such interference. It will appear as a much higher than normal noise floor in the lower part of the band, or if very strong possibly all across the band. In some cases a dedicated spectrum anayzer (eg Anritsu) may be needed to detect such interference.

In the licensed 400-500 MHz band similar issues can occur. Radios for this band do not contain a spectrum analyzer but a radio service company will have an analyzer, or the FCC or Industry Canada may help.

If it is determined that an interference source outside the band being used is causing noise inside the band, then often a bandpass filter can help to eliminate the problem. A bandpass filter is designed to only allow a certain range of frequencies to pass through. Above and below that range any signals are attenuated significantly.

One manufacturer whose products are commonly used in this application is Microwave Filter Company. (www.microwavefilter.com) For 900 MHz their model 17965 is quite effective. It has about 1.5 dB of insertion loss, but will have at least 40 dB rejection at 6 MHz above or below the 900 MHz band.

At 2.4 GHz the model 13944 has similar characteristics. If very low insertion loss is necessary you can go with a unit such as the model 16420N, which has only 0.5 dB loss.

In the 400-500 MHz band a simpler bandpass filter can be used, as it does not need to cover a wide range of frequencies. These are physically larger than those for the upper bands as their size is related to the wavelength used. A common model is the Sinclair FP30107. Depending on how narrow the bandpass range, insertion loss will vary between 0.5 and 3 dB.

[sbeadle]

>>Responses imported from previous forum

Reply From User: joelw, posted: 2018-10-17 16:44:21
See the attached documents for samples of available filters.

Attached file: (editor/lt/mrzc29bb4w8m.pdf), 17965 900 MHz Band Filter ($435).pdf File size: 179876

Attached file: (editor/pb/pcl20rqbje44.pdf), ISM (2.4 GHz) Full Band Bandpass Filters.pdf File size: 211354

Attached file: (editor/vv/vaeid6jqklpe.pdf), Sinclair Band Pass Filter FP30107-3-DI.pdf File size: 260801