Receiver performance
| Receiver performance | |
|---|---|
| Weak-signal measures | Sensitivity, minimum discernible signal (MDS), noise figure |
| Strong-signal measures | Blocking dynamic range, third-order intercept point (IP3), intermodulation dynamic range |
| Close-in limits | Reciprocal mixing from phase noise; roofing filter width |
| Usual units | dBm (decibels relative to 1 milliwatt), microvolts, dB |
| Reference points | |
| Thermal noise | −174 dBm in a 1 Hz bandwidth at room temperature |
| S9 on HF | −73 dBm (50 microvolts into 50 ohms) by IARU convention; real radios vary |
| Where to find figures | ARRL Lab reviews in QST; independent comparison tables |
| Quick fixes for overload | Attenuator on, preamp off, narrower roofing filter |
Receiver performance describes how well a receiver hears weak signals, separates them from their neighbors, and copes with strong signals nearby. The terms below appear in product reviews and in the license question pools, especially Amateur Extra, and most are given in decibels or in dBm, decibels relative to one milliwatt, where −130 dBm is a very weak signal.
Sensitivity, MDS and noise figure
Sensitivity is the weakest signal a receiver can usefully copy, often quoted in microvolts for a stated signal-to-noise ratio.
The minimum discernible signal (MDS), also called the noise floor, is the signal level equal to the receiver's own noise in a stated bandwidth, usually 500 Hz. Lower is better. Physics sets a limit, because thermal noise at room temperature is −174 dBm per hertz of bandwidth:
MDS ≈ −174 + 10 log(bandwidth in Hz) + noise figure
In a 500 Hz bandwidth, 10 log(500) is about 27, giving a theoretical floor of −147 dBm. A receiver with a 10 dB noise figure would have an MDS near −137 dBm.
Noise figure is how much a receiver worsens the signal-to-noise ratio, in decibels, as explained under Preamplifier. On the lower HF bands, atmospheric and man-made noise is usually far stronger than a modern receiver's own noise, so extra sensitivity buys nothing. On VHF and UHF, where band noise is much lower, noise figure matters a great deal.
Selectivity
Selectivity is the ability to pass the wanted signal and reject others close by. It depends on a filter's bandwidth and on how steep its sides, or skirts, are. The shape factor divides the bandwidth at −60 dB by the bandwidth at −6 dB; a filter 2.4 kHz wide at −6 dB and 4.2 kHz wide at −60 dB has a shape factor of 1.75. Closer to 1 is better. A filter protects only the stages that come after it.
Dynamic range
Dynamic range is the span between the weakest signal a receiver can detect and the strongest it can handle without trouble.
Blocking, or desensitization, happens when a strong signal outside the passband overloads an amplifier or mixer and reduces the gain for everything else. Blocking dynamic range is the difference between MDS and the level of an interfering signal that reduces a wanted signal by 1 dB.
Intermodulation (IMD) happens when two strong signals mix in a stage that is not perfectly linear. The most troublesome products, called third-order products, fall at 2f1 − f2 and 2f2 − f1. Stations on 7.200 and 7.210 MHz, for example, create phantom signals on 7.190 and 7.220 MHz. Third-order products grow 3 dB for every 1 dB increase in the two signals. The third-order intercept point (IP3) is the theoretical level where the products would equal the original signals; higher is better. Third-order IMD dynamic range is the difference between MDS and the level of two equal signals whose products just reach MDS. Approximately, IMD dynamic range = (2/3) × (IP3 − MDS), so a receiver with an IP3 of +20 dBm and an MDS of −130 dBm has an IMD dynamic range of about 100 dB.
Reviewers test with signals both widely spaced, often 20 kHz apart, and closely spaced, often 2 kHz apart. Close-in figures matter most on busy bands, and in many older designs they are far worse. IP3 is less meaningful for direct-sampling receivers, whose distortion does not follow the classic pattern; for them, reviewers also report the level at which the analog-to-digital converter overloads.
Reciprocal mixing and roofing filters
No real oscillator is perfectly pure; its output carries faint noise sidebands called phase noise. In reciprocal mixing, a strong signal near the wanted one mixes with the local oscillator's phase noise and spreads noise into the passband. The hiss rises whenever that station transmits, even if its signal is clean. In direct-sampling radios, converter clock jitter does the same.
A roofing filter sits right after the first mixer of a superheterodyne receiver and puts a "roof" over the bandwidth reaching everything that follows. In up-conversion radios, with a first IF of several tens of megahertz, roofing filters were often 15 kHz wide and seldom narrower than a few kilohertz, so strong neighbors could reach later stages. Since the late 2000s, many high-performance radios have used a low first IF instead, allowing roofing filters a few hundred hertz to a few kilohertz wide, matched to the mode.
Reading the numbers
The ARRL Lab measures receivers for the product reviews in QST, and independent comparison tables, such as the one published by Sherwood Engineering, rank receivers largely by close-in performance. Small differences between top radios rarely matter, because band noise usually limits reception first. The figures are most useful for spotting a radio that will struggle in a contest, at a busy Field Day site or near a strong neighbor.