2-10
Reception
Reception occurs when a transmitted electromagnetic wave passes through the receiver antenna and
induces a voltage in the antenna.
Selection
Selection is the ability of the receiver to select a particular frequency of a station from all other
station frequencies appearing at the antenna of the receiver.
Detection
Detection is the action of separating the low (audio) frequency intelligence from the high (radio)
frequency carrier. A detector circuit is used to accomplish this action.
Reproduction
Reproduction is the action of converting the electrical signals to sound waves, which can then be
interpreted by your ear as speech, music, and the like. An example of this might be the stereo speakers in
your car.
RECEIVER CHARACTERISTICS
Sensitivity, noise, selectivity, and fidelity are important receiver characteristics. These characteristics
will be useful to you when performing receiver tests. They can help you to determine whether a receiver
is working or not or in comparing one receiver to another.
Sensitivity
The ability of a receiver to reproduce weak signals is a function of the sensitivity of a receiver. The
weaker a signal that can be applied to a receiver and still produce a certain value of signal output, the
better the sensitivity rating. Sensitivity of a receiver is measured under standardized conditions. It is
expressed in terms of the signal voltage, usually in the microvolts that must be applied to the antenna
input terminals to give an established level of the output. The output may be an ac or dc voltage measured
at the detector output or a power measurement (measured in decibels or watts) at the loudspeaker or
headphone terminals.
Noise
All receivers generate a certain amount of noise, which you must take into account when measuring
sensitivity. Receiver noise may originate from the atmosphere (lightning) or from internal components
(transistors, tubes). Noise is the limiting factor of sensitivity. You will find sensitivity is the value of input
carrier voltage (in microvolts) that must be applied from the signal generator to the receiver input to
develop a specified output power.
Selectivity
Selectivity is the degree of distinction made by the receiver between the desired signal and unwanted
signals. You will find the better the ability of the receiver to reject unwanted signals, the better its
selectivity. The degree of selection is determined by the sharpness of resonance to which the frequency-
determining circuits have been engineered and tuned. You usually measure selectivity by taking a series
of sensitivity readings. As you take the readings, you step the input signal along a band of frequencies
above and below the circuit resonance of the receiver; for example, 100 kilohertz below to 100 kilohertz
