Dynamic range

The dynamic range of a radio receiver in the range from the minimum discernible signal to the maximum allowable signal (measured in decibels, dB). Although this simplistic definition is conceptually easy to understand, in the concrete its a ïttJe more complex. SeveraJ definitions of dynamic range are used (Dyer, 1993).

One definition of dynamic range is that it is the input signal difference between the sensitivity figure (e.g., 0.5 puV for 10 dB S + N/N) and the level that drives the receiver far enough into saturation to create a certain amount of distortion in the output. This definition was common on consumer broadcast band receivers at one time (especially automobile radios, where dynamic range was somewhat more important because of mobility). A related definition takes the range as the distance in decibels from the sensitivity level and the - L-dB compression point. Still another definition, the blocking dynamic range, which is the range of signals from the sensitivity level to the blocking level (see below).

A problem with these definitions is that they represent single-signal cases so they do not address the receiver's dynamic fcharacteristics. Dyer (1993) provides both a "loose'1 and a more formal definition that is somewhat more useful and is at least standardized. The loose version is that dynamic range is the range of signals over which dynamic effects (e.g., intermodulation, etc,) do not exceed the noise floor of the receiver. Dyer's recommendation for HF receivers is that the dynamic range is two-thirds of the difference between the noise floor and the third-order intercept point in a 3-kHz bandwidth. Dyer also states an alternative,* dynamic range is the difference between the fundamental response input signal level and the third-order intercept point along the noise floor, measured with a 3-kHz bandwidth. For practical reasons, this measurement is sometimes made not at the actual noise floor, (which is sometimes hard to ascertain) but rather at 3 dB above the noise floor.

Magne and Sherwood (1987) provide a measurement procedure that produces similar results (the same method is used for many amateur radio magazine product reviews). Two equal-strength signals are input to the receiver at the same time. The frequency difference has traditionally been 20 kHz for HF and 30 to 50 kHz for VHF receivers (modern band crowding might indicate a need for a specification at 5-kHz separation on HF). The amplitudes of these signals are raised until the third-order distortion products are raised to the noise floor level.

For 20-kHz spacing, using the two-signal approach, anything over 90 dB is an excellent receiver, and anything over 80 dB is at least decent.

The difference between the siligle-to-signal and two-signal (dynamic) performance is not merely an academic exercise. Besides the fact that the same receiver can show as much as a 40-dB difference between the two measures (favoring the single-to-signal measurement), the most severe effects of poor dynamic range show-up most in the dynamic performance.

Blocking

The blocking specification refers to the ability of the receiver to withstand very strong off-tune signals that are at least 20 kHz (Dyer, 1993) away front the desired signal, although some use 100-kHz separation (Magne and Sherwood, 19S7). When very strong signals appear at the input terminals of a receiver, they can desensitize the receiver (i.e., reduce the apparent strength of desired signals over what they would be if the interfering signal were not present).

Figure 9-20 shows the blocking behavior. When a strong signal is present, it takes up more of the receiver's resources than normal, so there is not enough of the output power budget to accommodate the weaker desired signals. But if the strong

Strong undesired signal

Desired signal n

Normal output level

Output when undesired signal present

9-20 The blocking, or "desensiUaatioiC phenomenon.

undesired signal is turned off then the weaker signals receive a full measure of the unit's power budget.

The usual way to measure blocking behavior is to input two signals, a desired signal at 60 dBjiV and another signal 20 (or 100) kHz away at a much stronger level. The strong signal is increased to the point where blocking desensitization causes a 3-dB drop in the output level of the desired signal. A good receiver will show ^90 dB|*,V, with many being considerably better. An interesting note about modern receivers is that the blocking performance is so good that it's often necessary to specify the input level difference (dB) that causes a 1-dB drop, rather than a 3-dB drop, of the desired signal's amplitude (Dyer, 1993).

The phenomenon of blocking leads to an effect that is often seen as paradoxical on first blush. Many receivers are equipped with front-end attenuators that permit fixed attenuations of 1, 3,6,12, or 20 dB (or some subset of same) to be inserted into the signal path ahead of the active stages. When a strong signal that is capable of causing ¿©sensitization is present, adding attenuation often increases the level of the desired signals in the output—even though overall gain is reduced. This occurs because the overall signal that the receiver front end is asked to handle is below the threshold where desensitization occurs.

Cross-modulation

Cross-modulation is an effect in which amplitude modulation (AM) from a strong undesired signal is transferred to a weaker desired signal. Testing is usually done (in HF receivers) with a 20-kHz spacing between the desired and undesired signals, a 3-kHz IF bandwidth on the receiver, and the desired signal set to 1000 pVemf (-53 dBm). The undesired signal (20 kHz away) is amplitude-modulated to the 30% leveL This undesired AM signal is increased in strength until an unwanted AM output 20 dB below the desired signal is produced.

A cross-modulation specification a 100 dB would be considered decent performance. This figure is often not given for modern HF receivers, but if the receiver has a good third-order intercept point then it is likely to also have good cross-modulation performance.

Cross-modulation is also said to occur naturally—especially in transpolar and North Atlantic radio paths, where the effects of the aurora are strong. According to one (possibly apocryphal) legend there was something called the "Radio Luxembourg Effect" discovered in the 1930s. Modulation from very strong broadcasters appeared on the Radio Luxembourg signal received in North America, This effect was said to be an ionospheric cross-modulation phenomenon. If anyone has any direct experience with this effect, or a literature citation, then 1 would be interested in hearing from them.

Reciprocal mixing

Reciprocal mixing occurs when noise sidebands from the local oscillator (LO) signal in a superheterodyne receiver mix with a strong undesired signal that is close to the desired signal. Every oscillator signal produces noise, arid that noise tends to amplitude modulate the oscillator's output signal. It will thus form sidebands on either side of the LO signal. The production of phase noise in all LOs is well-known, but in more-recent designs the digitally produced synthesized LOs are prone to additional noise elements as well. The noise is usually measured in —dBc (decibels below carrier, or, in this case, dB below the LO output level).

In a superheterodyne receiver, the LO beats with the desired signal to produce an intermediate frequency (LF) equal to either the sum (LO + RF) or difference (LO - RF). If a strong unwanted signal is present then it might mix with the noise sidebands of the LO to reproduce the noise spectrum at the IF frequency (see Fig. 9-21). In the usual test scenario, the reciprocal mixbig is defined as the level of the unwanted signal (dB) at 20 kHz required to produce noise sidebands 20 down from the desired IF signal in a specified bandwidth (usually 3 kHz on HF receivers). Figures of 90 dBc or better are considered good.

The importance of the reciprocal mixing specification is that it can seriously deteriorate the observed selectivity of the receiver, yet is not detected in the normal static measurements made of selectivity (it is a "dynamic selectivity" problem). When the LO noise sidebands appear in the IF the distant frequexicy attenuation (>20 kHz off-center of a 3-kHz bandwidth filter) can deteriorate 20 to 40 dB.

The reciprocal mixing performance of receivers can be improved by eliminating the noise from the oscillator signal. Although this sounds simple, in practice it is

Reciprocal Mixing

9*31 Reciproca] mixing phenomenon; (A) LO with noise sidebands, desired signal, and a strong undesired signal; (B) transfer of noise sidebands id IF signal.

often quite difficult. A tactic that will work well, at least for those designing their own receiver, is to add high-Q filtering between the LO output and the mixer input. The narrow bandwidth of the high-Q filteT prevents excessive noise sidebands from getting to the mixer. Although this sounds like quite the easy solution, as they say "the devil's in the details."

Other important specifications IF notch rejection

If two signals fall within the passband of a receiver they will both compete to be heard. They will also heterodyne together in the detector stage, producing an audio tone equal to their carrier frequency difference. For example, suppose we have an AM receiver with a 5-kHz bandwidth and a 455 kHz IF. If two signals appear on the band such that one appears at an IF of 456 kHz and the other is at 454 kHz, then both are within the receiver passband and both will be heard in the output. However, the 2-kHz difference in their carrier frequency will produce a 2-kHz heterodyne audio tone difference signal in the output of the AM detector

In some receivers, a tunable, high-Q (narrow and deep) notch filter is in the IF amplifier circuit. This tunable filter caxi be turned on and then adjusted to attenuate the unwanted interfering signal, reducing the irritating heterodyne. Attenuation figures for good receivers vary from —35 to —65 dB or so (the more negative the better).

There are some tradeoffs in notch filter design. First, the notch filter Q is more easily achieved at low-IF frequencies (such as 50 to 500 kHz) than at high-IF frequencies (e,g., 9 MHz and up). Also, the higher the Q the better the attenuation of the undesired squeal but the touchier it is to time. Some happy middle ground between the irritating squeal and the touchy tune is mandated here.

Some receivers use audio filters rather than IF filters to help reduce the het-enidyne squeal. In the AM broadcast band, channel spacing is typically 10 kHz, and the transmitted audio bandwidth (hence the sidebands) are 5 kHz. Designers of AM BOB receivers usually insert an RC low-pass filter with a -3-dB point just above 4 or 5 kHz right after the detector in order to suppress the 10-kHz heterodyne, This RC filter is called a t weet fitter in the slang of the electronic service/ repair trade.

Another audio approach is to sharply limit the bandpass of the audio amplifiers. For AM BCB reception, a 5-kHz bandpass is sufficient, so the frequencies higher can be rolled off at a fast rate in order to produce only a small response an octave lugher (10 kHz), In shortwave receivers, this option is weaker because the station channels are typically 5 kHz, and many don't bother to honor the official channels anyway. And on the amateur radio bands frequency selection is a perpetually changing ad-hocracy> at best. Although the shortwave bands typically only need 3-kHz bandwidth for communications, and 5 kHz for broadcast, the tweet filter and audio roll-off might not be sufficient. In receivers that lack an effective IF notch filter, an audio notch filter can be provided. This accessory can even be added after the fact (as an outboard accessory) once you own the receiver.

Internal spurii

All receivers produce a number of internal spurious signals that sometimes in terfere with the operation. Both old and modern receivers have spurious signals from assorted high-order mixer products, power-supply harmonics, parasitic oscillations, and a host of other sources. Newer receivers with either (or both) synthesized local oscillators and digital frequency readouts produce noise and spurious signals in abundance. (Note: low-power digital chips with slower rise times—CMOS, NMOS, etc .—-are generally much cleaner than higher-power, fast-rise-time chips like TTL devices).

With appropriate filtering and shielding, it is possible to hold the " spurs" down to -100 dB relative to the main maximum signal output or within about 3 dB or the noise floor, whichever is lower

A writer of shortwave books (Helms, 1994) has several high-quality receivers, including tube models and modern synthesized models. His comparisons of basic spur/noise level was something of a surprise. A high-quality tube receiver from the 1960s appeared to have a lower noise floor than the modem receivers. Helms attributed the difference to the internal spurs from the digital circuitry used in the modem receivers.

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