Receiver Signal Discrimination (T7A04)

The 2022-2026 Technician License question pool includes a question about the ability of a receiver to discriminate between multiple signals. Let's pick this one apart for better understanding:

T7A04: Which term describes the ability of a receiver to discriminate between multiple signals?

A. Discrimination ratio B. Sensitivity C. Selectivity D. Harmonic Distortion

Operations on the VHF and UHF bands typically utilize simplex or repeater FM channels adhering to local band plans that identify channels by specific carrier frequency designations. These channels are selected by the local frequency coordinator to provide sufficient bandwidth between channels for the (nominally) 10 to 15 kHz FM signals and to avoid interference that would otherwise occur if one signal partially or wholly “overlapped” with a second signal within the band.

Outside of VHF/UHF FM operations, channelization is not common, and operators may tune to any frequency selectable by the resolution of the transceiver VFO, commonly down to 10 Hz steps. As a result, interfering signals within the same receive bandwidth are a common problem with SSB or CW and termed “QRM.”

However, on the HF bands where single sideband (SSB) operations and continuous wave (CW) operations are very popular, signal frequencies are not coordinated as distinct and separated channels. Rather, the approximately 3 kHz of a SSB signal may be abutted right up against, or even overlapping substantially with, another SSB signal. Same goes for CW signals, although they are of much narrower bandwidth than SSB. The contiguous and unchannelized tuning in the HF bands and in some portions of the VHF and UHF bands increases the likelihood of multiple signals occupying the same frequency segments. That is, multiple signals will overlap in the same “frequency space” (within the tuned “receive band”) and create QRM interference for opposing operators.

Given this inevitable interference of QRM, the ability of a receiver to discriminate between multiple signals is quite an important characteristic. A receiver should have the ability to isolate a desired signal among other signals immediately adjacent or partly overlapping with that desired signal in the tuned receive band.

The receiver filter bandwidth is a primary factor in determining the receiver’s ability to isolate desired signals from undesired signals. The receive filter bandwidth determines the range of frequencies that will be allowed to pass on through the demodulation sequence and ultimately drive an audio signal to be heard. With common receiver types such as the heterodyne receiver, this filtering takes place at the intermediate frequency (IF) stage of signal processing. The IF is an unchanging frequency value, of significantly lower frequency than initially received RF signals, to which the receiver circuits shift the tuned RF receive band in the demodulation sequence of steps. (See The Heterodyne Receiver article.)

Since the receive band at the IF stage is a predetermined, engineered range of frequencies, filters can be precisely engineered to be very sharp and effective at passing only the desired passband of signals. (See our SSB Receive Filters article.) Modern digital signal processing (DSP) is very effective at creating easily selectable ranges of passband filtering, thereby allowing the operator to vary the receive bandwidth dynamically while receiving and monitoring resultant audio. In this way an operator may select as wide or as narrow a passband as desired to minimize undesired adjacent received signals. (See our Digital Signal Processing article.)

The lower frequency (black) signal is the desired receive signal, but a higher frequency (blue) signal is interfering when a full width 3 kHz receive band is used. The receive band is filtered with a narrower receive filter (red), reducing the received signal bandwidth to only 2.4 kHz. The interfering portion of the undesired higher frequency signal is attenuated, while the bulk of the desired signal is passed to audio processing.

A receiving operator usually selects a filter that is somewhat narrower in bandwidth than the desired receive signal bandwidth, as illustrated here. The filter is positioned to cut out the higher end interference (from the blue signal). The red dashed line illustrates the “passed band” by the receive filter, and it will attenuate the power of the interfering blue signal. While some frequencies of the desired signal are also attenuated, there is still sufficient bandwidth to produce intelligible desired signal audio, and the bulk of that pesky interfering signal in your receive audio is diminished to near nothing!

By using filtering techniques to isolate a desired signal, and by amplifying the receive passband signals among other noise, a receiver selects the signal desired by the operator from among other interfering signals. The better the filtering, noise reduction, and signal amplification of the receiver, the better its selectivity, or its signal isolating ability.

The answer to Technician Class question T7A04, “Which term describes the ability of a receiver to discriminate between multiple signals?” is “C. Selectivity.”

-- Stu WØSTU