HUM and NOISE in DUPLEX RADIO SYSTEMS
Mains hum is the curse of audio engineering. It shows up unexpectedly in all types of low frequency equipment as a 50 or 100Hz signal which has been induced into the circuit by magnetic/electrostatic pickup, ground loop currents or power line ripple. Recently I discovered a new way to get mains hum into your circuits - at 400 MHz!
All that is required is a UHF Duplex radio carrier operating in a building or on a site with fluorescent lighting. A fluorescent tube is a column of gas which is being ionized by the 50Hz mains supply, thus it flashes 100 times a second. To a radio signal, this column of gas looks like a conductor (antenna) that appears and disappears at 100Hz.
In-building radio propagation is dominated by signal reflections or, more correctly, passive signal re-radiation. This results in the well-known phenomenon of multipath reception, i.e. the received signal is the amplitude and phase sum of a direct signal plus a multitude of reflected signals. Now for the crunch: in a fluorescent-lit building we have a whole bunch of wavelength-sized reflectors which are appearing and disappearing at 100Hz.
The end result is that at some points in space we find 'hum spots', similar to 'null' spots caused by multipath. These are positions where the 100Hz AM modulated reradiations from the lights cause considerable AM and PM modulation of the received signal. Thus. if vou transmit an unmodulated carrier and move around a fluorescent-lit room with a receiving antenna, vou'll observe a distorted 100Hz signal on the receiver's baseband output. When vou find a hum spot, turn the lights off and the hum goes.
1 . FM is less susceptible than AM. The predominant modulation is 100Hz AM, although a certain amount of narrow band phase modulation (PM) also occurs.
2. FM receivers should have good AM rejection, particularly at high signal levels.
3 . Avoid the need of high signal-to-noise ratios for the decoder to function.
4. Filter the receiver bascband output to take out low frequency signals and use data codes with no low frequency components. The 100Hz hum signal is not sinusodial and has harmnonics at 200Hz, 300Hz, etc.
5. Screen the lights (usually impractical).
6. Use diversity reception (also cures multipath).
7). Avoid CTCSS protected receivers with decode tones of 100Hz etc. ( A Valid Tone ! )
8). Reduce Power levels / field strength, this will reduce multipath reflections.