Minimizing Selective Fading Distortion


Fig. 1   Shortwave voice signal in the 25-meter band, two AVC settings


Fig. 2   Shortwave voice signal in the 31-meter band, two AVC settings

Fig. 1 shows two audio waveforms, each 11 seconds long. This is a voice signal from an AM broadcast station in the 25-meter shortwave band. One waveform was received using fast AVC and the other with slow; other receiver settings were identical. Fig. 2 shows a similar comparison for a signal received in the 31-meter band; these waveforms are six seconds long. Can you tell which AVC rate applies to which waveform?

Automatic volume control is a receiver feedback system intended to minimize variation in demodulated audio level. AVC controls the gain of the RF and IF stages (and sometimes the converter stage). An RC filter extracts a signal proportional to the AM carrier level from the detector output. AVC applies this signal to the tube control grids. The RC time constant determines how quickly the AVC system responds to changes in signal level. Receivers with a fixed time constant invariably use fast AVC (generally 100 to 200 ms) for AM reception. This lets the receiver track rapid variation in signal strength with little variation in audio level. Knowing this much, it's easy to guess that the top waveform in each figure, which shows less level variation, corresponds to fast AVC.

Selective Fading


Fig. 3   Digital shortwave broadcast signal. 20 kHz span, 10 dB/div.

What I haven't told you is that the shortwave signals exhibited selective fading. Selective fading is nonuniform signal-strength variation across the receive passband. Fig. 3 shows some examples of selective fading. These are spectrum analyzer traces of a digital shortwave broadcast signal on 17.620 MHz in the late afternoon. The signal spectrum is 10 kHz wide, apparently and presumably flat. This montage shows the variety of ways selective fading can alter a spectrum.


Fig. 4   HF radar signal. First column, 0.5 ms/div; others, 0.2 ms/div.

Fig. 4 shows the cause of selective fading: multipath propagation. The signal is a uniform pulse train emitted by an HF radar at 17.450 MHz. The radar tracks meteor ionization trails for scientific study. I recorded these traces from a spectrum analyzer in nonscan mode using 30-kHz bandwidth and linear amplitude. In these images the horizontal axis is time, not frequency. The first column shows the pulse train early in the day when a slightly delayed pulse was just starting to appear. This extra pulse may be due to the extraordinary ray, a second ionospheric path caused by earth's magnetic field. The remaining images are closeups taken later in the day when each pulse had become an ever-changing ensemble of many. Mentally superimpose an AM carrier for each pulse you see, recall that time delay is phase shift, and you'll get some idea of the self-interference multipath propagation can cause.

Selective fading can change an AM signal's tonal character by altering the shape of the sidebands, and it can change the audio level by altering the sideband or carrier amplitudes. When it attenuates the carrier, selective fading can cause distortion by raising the effective modulation level beyond 100%. When it changes the relative carrier and sideband phase, it can cause distortion by altering the signal envelope. Distortion from these two mechanisms can be severe.

Now return to figs. 1 and 2. In fact, the upper waveform in each figure was received using very slow AVC (a time constant of about 5 seconds), while the lower trace used fast AVC (about 300 ms). It's clear from the traces that slow AVC was much more effective at stabilizing audio level. What the traces don't show is that slow AVC greatly reduced audio distortion.

Selective fading fakes out an AVC system by giving a false indication that the audio level has changed. When the carrier level decreases but the sideband level remains the same, AVC applies less negative bias to the control grids to increase receiver gain. This causes the audio level to rise when it doesn't need to. Similarly, the audio level will drop when the carrier level alone rises. The AVC does the opposite of what it's designed to do. The problem occurs because carrier and sideband levels are not well correlated during selective fading.

Distortion

Unwarranted volume variation during selective fading isn't the worst of it though. A misbehaving AVC increases audio level just when it is most likely to be distorted. When a selective fade greatly reduces the level of the carrier with respect to the sidebands, it's as though the transmitter were severely overmodulated. A diode detector can cope well with upward modulation exceeding 100%, but downward modulation so large causes the demodulated waveform to clip and fold. With little or no carrier, which is common during severe selective fades, the audio sounds raucous, irritating, and sometimes unintelligible. Most of the high-amplitude sections in the lower traces of figs. 1 and 2 sound highly distorted.

What's interesting is that when the carrier fades and the audio distorts, the audio level generally drops a few dB if the receiver gain is held constant. This may happen because the selective fade is not so selective and attenuates nearby sidebands along with the carrier. It may also be due to the carrier phase rotating with respect to the sidebands. When the carrier phase is off 90°, the output of an envelope detector will drop 14 dB (and consist of pure second harmonic, which may account for some of the audible distortion). It's unlikely that the phase of the carrier and all of the sidebands will align so exactly, but smaller phase rotations still reduce the detection level. In general, the audio level drops during a selective fade if the AVC system is prevented from raising it. You can take advantage of this natural distortion-suppression mechanism by not undoing it with fast AVC.

Carrier loss during a selective fade often is quite brief. If the AVC time constant is very long, the AVC voltage can ride through brief periods of carrier loss with little change. Such an AVC system measures the long-term carrier level and ignores short-term variations. The receiver gain that results is much more appropriate.


Fig. 5   Sixty seconds of carrier from two shortwave broadcast stations

Fig. 5 shows sixty seconds of carrier from two shortwave broadcast signals. I recorded the upper trace in the 25-meter band in the daytime, while the lower trace is from the 31-meter band at night. I obtained these plots by tuning my Kenwood TS-930S to the carrier in CW mode with the 500-Hz IF and audio-peaking filters engaged to reject modulation sidebands. AVC was off. You can see that carrier loss usually is sharp and brief, often bounding right back to the prior level. An AVC with a time constant of several seconds would have no trouble equalizing the long-term carrier level of these signals while riding through the short-term variations.

Do Your Own Experment

It's easy to see how badly fast AVC behaves using your own receiver. Find an AM signal with pronounced selective fading. Listen to it for a while with fast AVC enabled to get a feeling for the character of the fades and the audio distortion. Then turn AVC off. Adjust the RF gain so that the audio level is about the same as before. Then listen to the audio. If you're like me, you may be surprised not to hear any distortion the first time you try this. I assumed that propagation conditions had changed and selective fading had gone away. Switching AVC back on made clear that it was still there. With AVC off, listen carefully when the audio level drops a bit. You should hear some distortion, though it won't sound nearly as raucous as with AVC enabled. What's unexpected is that if you listen casually, the residual distortion with AVC off may drop below your awareness threshold.

Using manual gain control is a very good way to reduce selective-fading distortion. If you're willing to track longer-term variations in signal strength by riding the RF-gain control, you'll be rewarded with much cleaner audio. But most of the time you won't want to pay such close attention while receiving. This is when the use of a very long AVC time constant makes sense.

Implementing Very Slow AVC

In my receivers I use a time constant of about 300 ms for fast AVC and about 5 seconds for slow. It's impractical to have only slow AVC available, at least AVC this slow. When you tune away from a strong signal, it may take several seconds for the receiver gain to rise enough to hear weak signals. I use fast AVC for tuning and slow AVC for listening.

Don't make the fast AVC too fast. If your receiver's audio system has good bass response, an AVC time constant of less than 200 ms may cause audible distortion on low-frequency tones. Residual audio on the AVC capacitor can change the receiver gain at an audio rate, distorting the signal envelope.

Even with your receiver parked on one frequency, there will be times when signal strength increases rapidly. This may happen when propagation suddenly changes or when a stronger signal appears. It's helpful if the slow AVC has fast attack. Otherwise you may hear a loud burst of audio as the AVC slowly comes to the right level. The automatic noise limiter described in the previous article provides fast-attack AVC as a bonus. This ANL uses the AVC capacitor to clamp audio impulses. When a strong signal appears, the clamp diode quickly charges the AVC capacitor and reduces receiver gain. The ANL and AVC systems work synergistically to limit audio impulses and bursts, and to properly set receiver gain.

I use a 4.7MΩ resistor feeding a 1-µF low-leakage capacitor for slow AVC. The charging current available from the ANL circuit depends on the IF output impedance, diode conductivity, and series resistance. Using a very large AVC capacitor may cause some audible distortion if the ANL can't charge it quickly when a large signal appears. I found it easy to arrive at suitable component values by experiment. Be sure to evaluate your circuit under a wide variety of signal and propagation conditions.

Some receivers connect the main AVC capacitor to the controlled grid circuits through RC filters for RF decoupling. The capacitance of these filters often makes only a minor contribution to the overall AVC time constant. It's best not to modify the AVC time constant by altering the value of a decoupling capacitor. These capacitors often exhibit a small reactance, and altering a value may degrade RF tracking or the IF response.

If no decoupling networks are used and the AVC capacitor functions as an RF or IF grid-circuit bypass, replace the original capacitor instead of paralleling another. The series-resonant frequencies of capacitors of unequal value differ. A parallel combination will exhibit a high impedance at some intermediate frequency where the capacitive reactance of one capacitor equals the inductive reactance of the other, effectively disabling the bypass. Replace the original capacitor using the same attachment points and lead lengths since RF tracking may depend on stray circuit inductances. It's a good idea to check RF tracking and IF alignment after capacitor replacement.


July 12, 201088–108 MHz

Originally published in Electric Radio, October 2003