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Receiver Dynamic Range: Part 1

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Receiver Dynamic Range: Part 1 The Communications Edge™ Tech-note Author: Robert E. Watson Receiver Dynamic Range: Part 1 The task of the radio receiver has always only by linear amplification, frequency trans- measures of receiver linearity, dominate the been to “get the signal.” However, with the lation, and bandwidth. Because noise figure signal overload end of receiver dynamic- proliferation of high-powered transmitters degrades with each successive stage of the range specifications. It is tempting to define and the burgeoning growth of electronic receiver, the most desirable measurement receiver dynamic range in terms of noise noise pollution, often weak-signal reception port is the audio output. Measurement at floor and overload level alone. However, is difficult, if not impossible. Receiver this port can be accomplished in either the measurement of second- and third-order dynamic range is the measure of a receiver’s cw or ssb mode because both of these are intercept is somewhat more problematic ability to handle a range of signal strengths, pre-detection modes. Note, however, that than measurement of noise figure. from the weakest to the strongest. Because of some receivers may not use a true product Nonetheless, these measurements can be the severe dynamic range requirements detector for cw detection, and the apparent used to predict a wide range of receiver per- placed on modern receivers, it is imperative noise figure will be degraded. In the case of formance. to define rational criteria for evaluating receivers that do not have pre-detected audio The second-order input intercept point receiver performance. This two-part article outputs, the IF output may be used for (IIP2) is the receiver input level at which the provides a tutorial review of receiver dynam- noise-figure measurements. The most rigor- curves of linear output and second-order dis- ic-range specifications and measurements. It ous measurement will require the selection tortion intersect. The third-order input discusses the limits and applicability of the of the narrowest available IF bandwidth intercept point (IIP3) is, similarly, the receiv- various measurements, highlighting potential because it is under this condition that the er input level at which the curves of linear errors and misleading specifications. largest number of receiver stages are in the output and third-order distortion intersect Procedures for estimating and measuring signal path. (see Figure 1). true receiver performance are recommended. In general, the noise figure of most radios In the most common measurement of these will vary both with receiver temperature and PRIMARY MEASUREMENTS parameters, two equal-amplitude sinusoids tuned frequency. Because of this, it is useful are linearly combined and applied to the Primary measurements that affect receiver to note the frequency and temperatures over receiver input. The distortion products dynamic range include: noise figure, second- which the receiver data is valid. appear at the output as new frequency com- order intercept, third-order intercept, 1-dB ponents whose relative magnitudes are mea- compression, phase noise, internal spurs and SECOND- AND THIRD-ORDER sured and compared to the original inputs. A bandwidth. This group of receiver measure- INTERCEPT single set of data is then used to extrapolate ments is considered primary because most Second- and third-order intercept, which are the curves of receiver distortion. For conve- other receiver dynamic-range measurements can be predicted from them. NOISE FIGURE SECOND-ORDER The most common expression of noise fig- THIRD-ORDER INTERCEPT INTERCEPT POINT ure is the ratio (in dB) of the effective receiv- POINT er input noise power with respect to -174 dBm/Hz. This single number dominates those receiver characteristics which are gen- erally described as sensitivity. It also describes OUTPUT dB THIRD-ORDER DISTORTION the “noise floor” of most dynamic-range SLOPE = 3 LINEAR OUTPUT measurements. SECOND-ORDER DISTORTION SLOPE = 2 To determine noise figure accurately, it should be measured at a pre-detected output INPUT dB of the receiver: that is, at any output which is a version of the received input modified Figure 1. Receiver distortion vs. input power intercept point extrapolation (theoretical). WJ Communications, Inc. • 401 River Oaks Parkway • San Jose, CA 95134-1918 • Phone: 1-800-WJ1-4401 • Fax: 408-577-6620 • e-mail: [email protected] • Web site: www.wj.com The Communications Edge™ Tech-note Author: Robert E. Watson nience, these curves can be completely speci- not agree, the validity of the intercept speci- the input filter can also produce distortion fied by the intercept points. fication is in doubt. products (see Figure 2). The filtering in a typical receiver is produced by the cascade of Several problems exist with intercept specifi- Another problem of intercept measurements several different parts of the receiver. For cations. The first problem is that the inter- is their frequency dependence. Second-order example, the filtering is provided by the cept points are not directly measurable. distortion produces distortion components input (rf) filter, followed by a narrower first Because the intercept points are mathemati- at twice the frequency of a single input sig- IF filter, and finally, a still narrower final IF cally extrapolated, their accuracy depends on nal (second harmonic distortion) and at the filter. Because as signals pass through the the assumption that the curves of second- sum and difference frequencies of two input receiver they are increasingly distorted, it is and third-order distortion are described by signals. It can be shown that a band-pass fil- necessary to specify at what frequencies the ter at the receiver input can suppress the straight lines with slope values of two and unwanted signals occur with respect to the undesired signal that would otherwise pro- three, respectively. To be useful, this assump- tuned frequency. Typically, third-order dis- duce second-order distortion products at the tion must be valid over the usable dynamic tortion, due to signals at frequencies within receiver’s tuned frequency. Because realizable range of the receiver. Unfortunately, there the final IF passband, is worse than that due filters are not ideal, rejection of second-order are two potential errors associated with this to signals in the first IF passband, but out- distortion will typically vary with the fre- assumption. First, as the receiver approaches side the final IF. Distortion from signals in quency of the undesired signals as well as overload compression, the actual distortion the input filter passband, but outside the with the receiver’s tuned frequency. curves are no longer straight lines. This first IF, is even less, and distortion from sig- effect can be avoided by measuring the dis- Third-order distortion effects are even more nals with frequencies outside the input filter tortion products at relatively low input lev- frequency dependent than second-order dis- is the least. For this reason, measurements of els. Typically, the intercept measurements tortion. This is because third-order distor- receiver third-order intercept can vary radi- will be most accurate if measured at input tion can produce distortion components cally depending on the frequencies of the levels where the distortion products are 60 from unwanted signals that a receiver input test signals with respect to the receiver’s dB less than the input signals. Second, cer- filter cannot remove. In this case, the distor- tuned frequency (see Figure 3). It is typical tain nonlinear radio components do not tion components from two input sinusoids for receiver manufacturers to specify the seem to produce distortion curves of the occur at frequencies of twice the first fre- third-order intercept point for test tones at appropriate slope. Examples of this are fer- quency minus the second. While a receiver frequencies in the first IF. Another common rite and GaAs FET components. This effect input filter can remove most of the unde- specification is for test-tone frequencies out- can be detected by measuring the intercept sired signals that can produce third-order side the first IF, but inside the input filter. In point at two different input levels and com- distortion products at the tuned frequency, this case, the manufacturer should specify paring the results for agreement. If they do the signals which are within the passband of the frequencies of the test signals with OUTSIDE SIGNALS PRESELECTOR FINAL IF FILTER IN PRE- AMPLITUDE SELECTOR IN FIRST IF IN FINAL IF DISTORTION DISTORTION PRODUCT PRODUCT THIRD-ORDER INTERCEPT (2f1-f2)f1 f2 (2f2-f1) TUNED f ∆f ∆f ∆f FREQUENCY Figure 2. Third-order distortion products from two signals inside the Figure 3. Third-order intercept as a function of test-tone frequency relative to receiver input filter. receiver-tuned frequency. WJ Communications, Inc. • 401 River Oaks Parkway • San Jose, CA 95134-1918 • Phone: 1-800-WJ1-4401 • Fax: 408-577-6620 • e-mail: [email protected] • Web site: www.wj.com The Communications Edge™ Tech-note Author: Robert E. Watson respect to the tuned frequency. Like second- receiver gain setting. The receiver-gain effect undesired signals near the tuned frequency order distortion, third-order distortion can occurs because many receivers, as part of will interact with the receiver phase noise also vary with tuned frequency, so a proper their gain-control scheme, attenuate signals and degrade the SNR of the desired signal. specification should list a worst-case value or early in the receiver signal path. If a receiver Receiver second- and third-order intercept specify the frequency range of validity. were to control its gain by rf attenuation points are typically much greater than the alone, its 1-dB compression point could the- 1-dB compression points, but there is no 1-DB COMPRESSION oretically be unlimited. For this reason, the reliable method for predicting one value 1-dB compression point is best used to The 1-dB compression point is the measure from the others.
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