filterinmg RF analog signals

I am writing the control software for a system that uses RF (clocked at 72 MHz) as a major component. None of our signals have harwdare filters and software I am sampling 100 samples at 1 k Hz. I have on average these samples and update every 250ms.

Using a language I can understand, can someone help me understand how RF may influence our signals and what options I have to filter this noise (harwdare and software). Or at least point me to a good resource.

Doug,

Here's the ME version of RF interference. The combination of transmitter/antenna radio creates electromagnetic waves in the space around the antenna. Think of dropping a stone in a pool of water. The wave analogy is reasonable for the basic understanding. A leaf floating on the pond goes up and down with the wave. If you put a float on the water and connect to the Mainland by a gearbox, you can extract energy from the waves.

In your DAQ system two things are necessary to produce interference. 1. the RF wave must be coupled to the system. 2. the RF energy must reach a point where a non-linear device converts the sine waves of very high frequency in a form which allows to measure the low frequency DAQ system. (2.a. There are a more complicated way interference can occur without a nonlinearity - subsampling).

How the coupling can occur? The wavelength of an electromagnetic signal in free space is the speed of light, divided by the frequency. For 72 MHz the wavelength is 4.17 m. A wavelength of antenna 1/4 long can couple to electromagnetic waves very effectively. Y at - it wire in your system about 1 meter long? USB cable? It is likely that you have some very nice antennas. If the voltage from one of these antennas reached about 0.5 V, it will be sufficient to cause the internal diode junctions in any device of semiconductor (such as amplifiers, analog/digital converters and multiplexers) of lead. This conduction is non-linear and produce a DC voltage out to the RF input. This tension Gets the sum (with unknown polarity) with the signals you want to measure, prodcuing errors.

What can you do about it? Disable the transmitter will not solve the problem, but because it is needed for other purposes, is not an option. The signals of interest being very low frequencies, it should be fairly easy to eliminate parasites. Three general principles apply. First is to eliminate the source, which has already been ruled out. However, check if the transmitter power can be reduced without compromising the performance of the RF System. Second is to reduce the coupling. If possible, keep the transmitting antenna as much as possible to the source of your slow signals and threads of connection to your data acquisition system. The orientation of the antenna and son can help too. There are too many combinations to try even give general guidelines on this without more information on the physical configuration of your system. Armor is a big part of the principle of coupling reduction and works well at 72 MHz. All the sons of your signal sources for the acquisition of data and data acquisition to the computer must be shielded. Should be based on the shields. Just make sure that you don't create ground loops in the process that can lead to other problems. Connect everything to a pattern with a 'star' grounded to System. The third principle is to reduce the signals interfere by filtering. I would try to put a ceramic capacitor of 100 nF for each signal line to the mass input on the DAQ hardware, keeping the capacitor leads as short as possible. If your sources of signals have a very high impedance or inductive, you might have an EE assess suitable filtering. The benefits of the capacitors are: cheap!, simple to install, will reduce RF interference over a wide bandwidth, require no development and are generally quite effective.

The problems are depending on the severity, it can be quite simple eliminate your interference or it might require a major overhaul of the entire system.

Lynn

ADDRA Consulting, LLC

Tags: NI Hardware

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filterinmg RF analog signals

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