Receiver Dynamic Range: Part 2

The Communications Edge™ Tech-note Author: Robert E. Watson Receiver Dynamic Range: Part 2 Part 1 of this article reviews receiver mea- the receiver can process acceptably. In sim- NF is the receiver noise figure in dB surements which, taken as a group, describe plest terms, it is the difference in dB This dynamic range definition has the receiver dynamic range. Part 2 introduces between the inband 1-dB compression point advantage of being relatively easy to measure comprehensive measurements that attempt and the minimum-receivable signal level. without ambiguity but, unfortunately, it to characterize a receiver’s dynamic range as The compression point is obvious enough; assumes that the receiver has only a single a single number. however, the minimum-receivable signal signal at its input and that the signal is must be identified. desired. For deep-space receivers, this may be COMPREHENSIVE MEASURE- a reasonable assumption, but the terrestrial MENTS There are a number of candidates for minimum-receivable signal level, including: sphere is not usually so benign. For specifi- The following receiver measurements and “minimum-discernable signal” (MDS), tan- cation of general-purpose receivers, some specifications attempt to define overall gential sensitivity, 10-dB SNR, and receiver interfering signals must be assumed, and this receiver dynamic range as a single number noise floor. Both MDS and tangential sensi- is what the other definitions of receiver which can be used both to predict overall tivity are based on subjective judgments of dynamic range do. receiver performance and as a figure of merit signal strength, which differ significantly to compare competing receivers. They DESENSITIZATION DYNAMIC from author to author. They are mentioned include: 1-dB compression dynamic range, RANGE here because of their historical significance, desensitization dynamic range, spur-free but the uncertainty limits their value as a Desensitization dynamic range (DDR) mea- dynamic range, and NPR (noise-power part of receiver dynamic-range specifications. sures the receiver degradation effects due to a ratio) figure of merit (NPRFOM). In gener- A more repeatable measurement is 10-dB single, dominant, out-of-band interferer. In al, they are based on the primary measure- SNR; but this, too, has disadvantages many “real world” signal environments, a ments of receiver performance, but the because of the variations of SNR due to type single, strong signal may be the major source NPRFOM test attempts to simulate the and percentage of modulation. The least of interference due to the effects of receiver actual signal environment in a way that phase noise and out-of-band signal compres- combines all of the receiver dynamic range ambiguous indicator of minimum receivable sion. In this test, a signal that produces an characteristics (see Table 1). This test is pro- signal is probably receiver noise floor. This output SNR of 10 dB is injected at the posed as a practical and realistic measure- can be defined in two ways: noise floor in a receiver input. An interfering sinusoid is ment of receiver dynamic range. 1-Hz bandwidth and total equivalent input noise power in the narrowest receiver band- added to the input at a particular frequency 1-DB COMPRESSION DYNAMIC width. The first is simply -174 dBm plus the offset from the tuned frequency and its mag- RANGE receiver noise figure in dB; while the second nitude is increased until the output SNR has the additional factor of 10 times the log degrades 1 dB. The DDR is then the power The receiver 1-dB compression dynamic of the receiver bandwidth. For most purpos- ratio (in dB) of the undesired signal power range defines the range of signal levels that es, the inclusion of the receiver bandwidth (in dBm) to the receiver noise floor in dBm yields a better estimator of usable dynamic per Hertz. The DDR can be calculated using 100 kHz 10 MHz range. Using this definition, receiver dynam- the equation: 200 kHz 20 MHz ic range can be expressed as: DDR = Pi - NF + 174 500 kHz 50 MHz CDR = Pic + 174 dBm -10 log BW - NF where: 1 MHz* 100 MHz* where: DDR is the desensitization dynamic 2 MHz 200 MHz range in dB CDR is the compression dynamic range Pi is the interfering signal power in 5 MHz* 500 MHz* in dB dBm 1 GHz P is the 1-dB input compression ic NF is the receiver noise figure *Recommended minimum set. power in dBm BW is the narrowest receiver bandwidth DDR is a true measure of dynamic range Table 1. Recommended standard filter frequencies for NPRFOM measurements. in Hz because it includes both noise figure and 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 measurement of overload/ interfering signal effect seen in both the XX-R7000 and XXX- receiver is tuned to center the test signal in power. The use of receiver input attenuation 500 receivers. the IF passband and to produce an audio will improve large signal-handling capability, “beat note” of 1 kHz. The significance of DDR is somewhat but noise figure will be degraded commen- dependent on signal environment. If the If the receiver does not have a “predetection” surately. The DDR, however, is not affected interfering signals have significant phase demodulation mode like “cw” or ssb which by input attenuators. When it is desirable to noise of their own, it is only necessary for uses a bfo frequency conversion to audio, the determine the absolute signal power in dBm the receiver’s phase noise to be better than narrowest available IF output may be used required to cause desensitization for a partic- the interferer’s. Most radio transmitters have with a spectrum analyzer. In this case, the ular receiver configuration, the following significant amounts of phase-noise sideband signal is monitored for a 1-dB amplitude equation can be used: energy at modest offsets from the carrier fre- decrease due to compression, and the noise Pi = DDR + NF - 174 quency. This is especially true for variable floor is monitored for a 1-dB increase due to frequency oscillator (vfo) and most frequen- phase-noise reciprocal mixing. Note that the noise figure must include the cy-synthesized frequency sources. A notable effects of input attenuation as the receiver is The interfering signal generator must have exception, which may have very low levels of intended to be used. phase noise much better than that of the small offset phase noise, is crystal oscillator receiver under test. The tunable bandpass fil- DDR is strongly affected by the frequency signal sources. At large frequency offsets, ter will help eliminate any residual generator offset of the interfering signal. At small fre- many transmitters will have low phase noise phase noise at large frequency offsets. The quency offsets, the DDR is dominated by because of the filtering properties of tuned audio lowpass filter is not required, but the effects of receiver phase noise reciprocal power output stages and narrow antenna serves to minimize the effects of variations in mixing. In this region, the DDR is approxi- bandwidths. For this reason, more attention audio response from receiver-to-receiver. mately 6 dB less than the magnitude of the should be given to obtaining a good DDR at single-sided phase noise spectrum in dB per large frequency offsets. SPUR-FREE DYNAMIC RANGE Hertz below the “carrier” (dBc). For exam- ple, if a receiver’s phase noise at 100 kHz A test setup for DDR measurement is shown Spur-free dynamic range (SFDR), as general- from the tuned (carrier) frequency is -130 in Figure 2. The receiver is tuned to the test ly used, attempts to define receiver dynamic dBc, the DDR at 100 kHz offset will be frequency and set for maximum gain in the range in terms of two undesired interferers about 124 dB. In general, receiver phase narrowest available bandwidth with a bfo and the receiver noise floor. As with the 1- noise improves with frequency offset so that detection mode. In some receivers, it will be dB compression dynamic range, it is based in some receivers, interfering signals well necessary to use the ssb mode to activate the on a mathematical manipulation of the pri- removed from the tuned frequency, will bfo and to achieve narrow bandwidth. The mary measurements of receiver range. In this begin to cause signal compression before the effects of phase noise reciprocal mixing are 190 observed. In this case, the DDR will be 180 worse than 6 dB less than the magnitude of WJ-8615 W/PRE 170 the phase-noise suppression at these frequencies. Because of these frequency effects, it is 160 necessary to specify the DDR at several dif- 150 ferent offset frequencies. The best presenta- 140 GERMAN XXX-500 tion of this data would be in the form of a 130 JAPANESE XX-R7000 graph, as shown in Figure 1. 120 This figure compares the DDR of three pop- 110 TEST FREQUENCY 100 MHz ular vhf/uhf receivers. At small frequency 100 offsets, the DDR is typically dominated by RANGE (dB) DYNAMIC DESENSITIZATION 90 receiver phase noise. At larger frequency off- 10 20 50 100 200 500 1 2 5 10 20 50 100 kHz MHz MHz sets, in receivers with modest signal input fil- INTERFERING FREQUENCY OFFSET tering (rf preselection) 1 dB compression due to signal overload may occur. This is the Figure 1. Desensitization dynamic range (DDR) as a function of frequency offset of the interfering signal. WJ Communications, Inc.

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