A Modulated Signal

To work at the NEM (National Electronics Museum) you must understand the basics of radio communications. Radios communicate through R.F. (radio frequency), which is an invisible wave that travels through the air from transmitter to receiver. The R.F. can be modulated with A.M. (amplitude modulation) or F.M. (frequency modulation) by the transmitter to attach a message onto the R.F. that is decoded by the receiver. Squelch is a receive function that eliminates unwanted noise.

Modulation is essentially a hidden message created by a transmitter that is attached to an R.F. signal, transmitted into the air, and then received and decoded by a receiver. A.M. adjusts the signal strength (height of the sine wave) to attach information that can be transmitted by an A.M transmitter, and received by an A.M. receiver. F.M. adjusts the instantaneous frequency (width of the sine wave) to attach information that can be transmitted by an F.M. transmitter and received by an F.M. receiver. Think of A.M. and F.M. in terms of human speech. A.M. would be analogous to the tone and volume of a voice. You can tell if someone is enthusiastic, flirty, confused, angry, or lonely, by the tone of their voice. If someone says, “I can't wait,” you can perceive if they are genuine or sarcastic by the tone of their voice. F.M. is analogous to a secondary meaning in human speech. If someone says, “my spouse is OCONUS,” you can perceive that a secondary meaning, the fridge is empty, exists. Without modulation, R.F. would be received as noise, and voice communication could not exist through radios. There are other types of modulation other than A.M. and F.M., but these are the two most common.

F.M. and A.M. radios are not always compatible. You should not attempt to establish a link between an F.M. and A.M. radio, unless both of the radios are AM/FM. Your car radio is almost definitely AM/FM, but don't take that for granted, as not all radios are AM/FM. Imagine a large room full of people and two languages are spoken. 90% of the people in the room speak both languages, but 5% of the people speak one language, and 5% the other. If you are among the 5% that only speaks the first language, you cannot speak to the other 5%. In the same way, F.M. and A.M. modulation are incompatible. If you are trying to communicate with an F.M. radio, make sure that you use an F.M. radio, without taking for granted that most radios are AM/FM. 

Squelch is a receive function that eliminates unwanted noise from a transmitted signal. This function is usually only used when a weak signal is being received or when no one is transmitting. Squelch can be adjusted to eliminate unwanted noise from a transmitted signal as well, if the transmitted signal is sending unwanted noise. Be sure not to adjust squelch too high, as you may block out all audio. Receiver sensitivity is another important aspect of squelch. Sometimes receivers are too sensitive, and can blow a speaker from a normal transmitted signal. Receiver sensitivity must be set to an appropriate level so that this does not happen. If the receiver sensitivity is set to an appropriate level, and a receiver receives an excessively strong signal, the fault lies with the transmitter's power output. 

The NEM is not all about radios; there are also fun community events. Tours come through the museum almost every day, and we frequently host field trips. I hope that we have an Easter Egg Hunt this year.