I am working on a system which should communicate with a ~100kHz signal, and read that low frequency channels, e.g 10kHz to 200kHz, are especially susceptible to interference.
Why is that? Are they more susceptible than say a 500Hz signal?
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2\$\begingroup\$ If you're talking about radio signals, the quote is probably referring to the fact that there's a lot of man-made noise in that frequency band, primarily harmonics of the power grid produced by nonlinear loads, arcing contacts, etc. 500 Hz is rarely used as a radio frequency. \$\endgroup\$– Dave TweedJan 6, 2013 at 13:13
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\$\begingroup\$ Harmonics from the power grid, got it. Yes, I am talking about radio signals, so your thought make good sense. You should have posted this as an answer \$\endgroup\$– chwiJan 6, 2013 at 13:16
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2 Answers
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If one is trying to send a 10kbit/second data stream modulated at 910.0MHz, the information for every bit of data will be spread out over 91,000 cycle. By contrast, if one were trying to send a 1kbit/second data stream modulated at 100kHz, each bit of data would be spread out over only 10 cycles. A lot of radio interference comes from events like the closing and opening of switches that start and stop significant currents. Each such event may be modeled as clobbering a cycle's worth of communication on all frequencies. At 910MHz, an event which clobbers one cycle's worth of communication would probably go unnoticed, but at 100kHz, it's far more significant. It wouldn't take very many such events happening close together to disrupt a bit on a 100kHz-modulated data stream.
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Higher frequencies allow for more robust modulation techniques. Eg. frequency modulation is a lot less susceptible to noise than amplitude modulation or no modulation at all. The downside of FM modulation with regard to AM modulation is the increased bandwidth required.
Frequency Modulation @ Wikipedia Simple English
Amplitude Modulation @ Wikipedia Simple English
In amplitude modulation or when using no modulation at all, the information is stored in the amplitude of the signal. Interference will introduce variations in amplitude, like spikes, which in turn will be in the received signal.
Higher frequencies tend to use (some sort of) frequency modulation. With FM no information is stored in the amplitude of the signal; all information is stored in the frequency of the carrier wave. When interference is picked up, it may introduce variations in amplitude, but these variations will be discarded by the receiver anyway. The noise introduced on the carrier wave frequency can easily be removed by the receiver, because the receiver can tightly track the carrier wave and disregard changes to it that are too fast to be correct.
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1\$\begingroup\$ I wonder to what extent this is true nowadays. I believe the biggest weakness with amplitude modulation at low frequencies is that it's commonly demodulated with a peak-detector circuit which allows impulse events to be registered much more strongly than they should be; it is increasingly practical today to use digital processing techniques on low-frequency radio signals in ways that can identify and remove noise much more effectively than had been possible in years past. \$\endgroup\$– supercatJan 6, 2013 at 18:04
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\$\begingroup\$ It is simply not true that higher frequencies somehow allow for more robust modulation techniques, whatever "robust" is supposed to mean anyway. The issue you are apparently trying to get to relates to the carrier frequency to bandwidth ratio, not the absolute carrier frequency. \$\endgroup\$ Jan 6, 2013 at 19:23
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\$\begingroup\$ You are right that in the end it is about the carrier frequency to bandwidth ratio. \$\endgroup\$– jippieJan 6, 2013 at 20:01