These two specifications are a critical part of any radio receiver’s performance characteristics. The reason that we lump them together is that each contributes with the other toward a receiver’s stellar performance. But we are using “selectivity” here in a broad sense so as to be more universal toward a receiver’s ability to notch in only the frequency of interest. Thus, the effect of selectivity may take on different terminology depending on what type of modulation is applicable for the radio of interest.
This article’s intended audience is that amateur new to the hobby. Because 2-meter VHF FM operation is by far the most popular operational mode for newbees, we will center more on specifications relevant to FM but note along the way the relevance to AM and SSB (single-sideband).
Sensitivity
There are two aspects of sensitivity to be aware of. The first is a sensitivity that facilitates an overload or saturation of the signal. The second is its ability to do something with a weak signal.
The job of the RF amplifier is to capture radio frequency (RF) energy from the atmosphere and “zoom in” much as we do with a video camcorder. Even if the transmitter is putting many thousands of Watts of RF energy into the atmosphere, in say for example Packistan, what our antenna brings to the receiver input is too small to do anything with directly. The receiver has a pre-amp that the antenna feeds directly so that the receiver can “zoom in” on the weak signal adding energy to it taken from a battery of wall receptacle in the car or ham shack. That “zooming in” is called “gain.”
There may be times when a particularly strong signal is received such as another ham who is spitting distance away. His or her signal may appear to the receiver so strong owing to being so close that it causes the receiver RF amplifier to saturate with a resulting absolute loss of fidelity. In extreme cases it can be harmful to the receiver causing damage. But “extreme” is usually limited to events that may happen on a test bench rather than from receiving a signal on the air. Some transceivers (such as the famed Icom 706MkII) have an attenuation switch that will radically reduce the receiver gain or sensitivity.
The second, and more relevant aspect of sensitivity, is that relating to weak signals. That same RF amplifier discussed in the paragraph above needs to be able to recognize and do something with weak signals. Sensitivity specifications therefore rate this ability in terms of micro Volts (uV).
The Yaesu DR-2X repeater (the model used by the Chelsea Amateur Radio Club) receiver is specified to recognize and do something with signals coming in off the antenna as low as 0.2uV. If a signal comes in at 1uV the receiver can easily do something useful with it. However, if a weak signal comes in at 0.19uV, it is questionable if the receiver will be able to detect it. But let us compare that to the ancient Hamtronics REP-200 repeater which is specified to recognize signals as small as 0.1uV.
More will be said later about the SINAD (signal to Noise and Distortion) qualification that sensitivity will be applicable to. That starts it getting it complicated when it does not need to be so we will leave it for now at just simply sensitivity being so many micro-Volts.
Smaller is better with receiver sensitivity. But it is not the end of the story because there is another specification that works hand-in-hand with with sensitivity. That is “selectivity.”
Selectivity
This is the measure of how well a receiver can isolate an incoming frequency from adjacent signals. While a receiver may be tuned to 145.450 MHz, it is a matter of its selectivity whether it can block signals coming in at 145.460 MHz. It is impossible to “block” other frequencies no matter what the difference in frequency is. All that a receiver can do is minimize adjacent frequencies to the extent that only the frequency of interest is amplified.
Selectivity measures how many dB (Decibels) down an unwanted signal is. 3dB means a reduction of power by 2.0. 6dB means a reduction in power by 4 or double a double.
Let us once again consider the Chelsea Amateur Radio Club’s Yaesu DR-2X repeater, which is specified to be down 6dB at 12 kHz (which is 0.012 MHz) and 60dB at 28 kHz (which is 0.028 MHz). But let’s apply this to the national calling frequency of 146.520 MHz. Let’s add 0.012 MHz to the calling frequency so that we are talking about 146.532 MHz. The next standard simplex channel above the calling frequency is 146.540 MHz. The question now becomes, how well can the receiver listening on the national calling frequency block a signal that may come in on 146.540 MHz?
If the adjacent channel station’s transmitter is operating properly, it will have a reduced emission at 146.532 MHz (probably something like being 6dB down) which helps our receiver to discriminate. Our receiver is tuned to 146.520 MHz and is at least trying to reject or discriminate against other frequency signals coming in. That incoming signal is already down 6dB and our transmitter cuts it down by another 6dB. If that 12dB brings the signal into the receiver at less than 0.2uV, then we are good to go and will not hear any adjacent channel interference.
What to Take Away
Sensitivity and Selectivity work hand-in-hand. If one is stellar and the other is not-so-stellar, then the resulting performance will be not-so-stellar. Look for both when you compare.
