Is it Noise or is it Interference?
By our broad definition above, interference is a special type of noise. Interference is noise that is generated by another electrical system. Usually it is generated by electronics of some kind and often is an intended signal. In the last case I only mean that the signal is intended, not the intereference. A common example of that would be the SSB described above where the person transmitting the unwanted signal is unaware that there is another signal on the frequency. But there are other sources of intereference than electronically generated signals. Usually these are bad electrical connections of one type or another. As the connection wavers between connected and disconnected it can make a spark which will create a bit of broad spectrum noise. If it does it a lot then you get a lot of noise.
As for other noise that is not man-made, the two most common sources are atmospheric noise and circuit noise from within your own electronic system. Atmospheric noise is caused by electrical storms and circuit noise is usually the thermal noise that exists inside of electrical components. Thermal noise is quite low and comes into play in the initial receiving stage where the signal is also very small.
Types of Noise
Let's look a little closer at the different types of noise out there.
- Atmospheric Noise - This noise is caused by electrical storms (lightning) and is broadband in nature. It is the source of the "background" noise you hear on HF SSB which sounds like a variable hiss. The reason this doesn't sound like individual pops for each lightning strike is due to a combination of the shear number of strikes (over 30 per second, worldwide) and the multiple paths that each strike take to reach your rig.
- Spark Noise - This is usually caused by electric motors or generators. It is a common problem that must be dealt with in mobile installations and gets the name "alternator whine" based on the sound coming out of the FM transceiver that varies in pitch with the engine RPM's. Especially in mobile installations it is usually picked up on the 12 Volt power wires.
- HF SSB Interference - This is the extremely common situation of having two or more incoming SSB transmissions on very close frequencies. Because of the chang in pitch when you are tuned off frequency, usually one of the two stations sounds like Donald Duck on helium or like a sort of a growling. Modern radios have very fancy filters that minimize this but they can't eliminate it entirely if the frequencies are close enough. And if the two signals happen to be on the same frequency, there is nothing to be done.
- VHF/UHF FM Interference - This is entirely different than SSB Interference. Rather than hearing two distinct signals you will likely hear one that is distorted and with a sort of buzzing underneath it. The stronger of the two signals will be the one you hear...stronger at the repeater that is. If the two signals are nearly equal in strength you will likely hear nothing but incomprehensible buzzing.
- Circuit Noise - This is the thermal noise present in the circuit of your receiver. When you first receive the signal from the antenna, it is very low power. Depending on how noisy your circuit is, it may not be much above the thermal noise. So in each stage of amplification you amplify the noise right along with the signal. I will leave it to Lee in a later session to go into some of the details on how this is dealt with.