Under Construction: VA7FI is editing this section, please do not edit it until this notice is taken down.
There are three main characteristics of a receiver: sensitivity, selectivity, and stability.
A signal is always accompanied by some sort of noise, and very roughly speaking, if the signal is stronger than the noise, then it can be heard. To quantify this, we use a term called Signal-to-Noise Ratio (SNR or S/N):
\$$\text{SNR} = \frac{\text{Signal}}{\text{Noise}}\$$
Since SNR is a ratio:
Like other ratios, we often express SNR in decibel. Recall that a ratio of 1 = 0 dB, so the above could be stated as:
Now back to the receiver. The sensitivity of a receiver is its ability to pick out weak signals from the noise. That is, it indicates how faint an input signal can be and still be successfully received by the receiver.
For example, here's the specs sheet from the IC-7300:
For example, a receiver with a sensitivity of -123 dBm can pick out a signal of 0.0000000000005 mW.1)
Recall from the intro section that radio signals always take up some bandwidth on the radio spectrum:
Now back to the receiver. The selectivity of a receiver is its ability to pass only the signal of interest and reject everything else.
Sometimes a specific noise or interference is mixed in the signal and we'd like to reduce or eliminate it. One way is to use filters. You can think of filters a little bit like the equalizer on a Hi-Fi stereo.
This picture has three main parts. From top to bottom:
You'll notice that there's a very intense line around 1.5 kHz on the audio spectrum. This line, coming from an interfering carrier signal, sounds like a loud note (a little above \$ \text{F}_{6}^{\text{#}} \$ for the musicians out there) mixed with the voice. To remove this tone, we can “notch” it. We now see a dark spot around 1.5 kHz. If we need to, we can make this spot more or less wide.
This is the ability of a receiver to stay on the right frequency and not drift.