Updating the Automatic Noise Limiter

A noise limiter clips an audio signal to reduce the amplitude of impulse noise. It prevents the waveform, which includes the desired signal and noise pulses, from exceeding a certain level. This can be effective when the noise pulses are thin and tall, less so when they are wide and short. An aggressive noise limiter may target the latter by using a clipping level well within the normal amplitude range of the desired signal. This will generate audible distortion by limiting the signal itself.

If the clipping level is adjustable with a front panel control, you can trade noise reduction for audio distortion as conditions warrant. This is a nice arrangement, but it requires additional parts, wiring, front-panel space, and operator attention. Many communications receivers settle for an automatic noise limiter, where circuitry automatically sets the clipping level based on the carrier level.

ANL is a good idea, but the implementations I've encountered distort the audio so badly that I never use them. Apparently the designers were so keen to limit noise amplitude that they were willing to sacrifice considerable audio quality. Perhaps they reasoned that since the signal was already corrupted by noise, additional distortion to quiet things down was justified.

But audio clipping has benefits beyond limiting external noise. For example, many receivers generate impulses of their own when you change bands or invoke other functions. I once made the mistake of wearing headphones while changing bands on a Hallicrafters SX-24. The impulse the bandswitch generated was so intense that my ears rang for several minutes. I was afraid I might have damaged my hearing. Later I looked at the audio waveform on a scope. The normal audio level was about 1 V, but the band-change impulse was 100 V or more. I never imagined the receiver could generate so much short-term energy. Had the receiver's ANL been so transparent that I could have left it always engaged, this impulse never would have reached my ears.

In addition to limiting receiver switching impulses, ANL has another benefit for receivers with good bass response. It can eliminate the loud thumps that may occur when tuning rapidly across a band filled with strong signals.

Conventional ANL circuits generate distortion another way by measuring the carrier level with an RC filter on the detector output. As described in the modulation acceptance article, this network can seriously distort the detected waveform by back-biasing the detector diode. This can generate audio distortion even when ANL is not engaged.

Improved Circuit

Fig. 1   Improved ANL circuit

Fig. 1 show the ANL circuit I prefer to conventional circuits. It eliminates the ANL's RC network and uses the AVC capacitor to measure carrier level. R1 + R2 is the detector load, while R2 ⁄ (R1 + R2) determines when clamp diode D2 conducts. D2 limits detected audio to the voltage on AVC capacitor C3. (C2 removes any residual IF signal and isn't essential to ANL operation.)

The sum of R1 and R2 affects modulation acceptance, while their ratio determines when ANL clips. I pick the sum as described in the modulation acceptance article, and the ratio so that D2 conducts only when modulation exceeds 125%. This is the maximum upward modulation permitted for AM broadcast stations. You may find one or two local signals that exceed this limit some of the time on some program material. These signals are useful for testing. To pick R1 and R2, I tune to a local signal that hits modulation peaks of 150%. I adjust R1 and R2 until D2 begins to conduct on signal peaks. Then I check that D2 never conducts on signals with 125% modulation. With the ANL threshold set this way, I never hear ANL artifacts or distortion, and I can leave it on all the time. I used 60kΩ for R1 and 46kΩ for R2 for the ANL circuit in my National NC-57.

I use my spectrum analyzer in nonscan mode with its linear detector to find a broadcast signal with the modulation peaks I want. If I didn't have a spectrum spectrum analyzer, I would find the value of R1 and R2 where most broadcast signals just begin to clip. Then I would reduce R2 until no sign of clipping remained.

When limiting occurs, D2 injects some charge into C3 so that repetitive noise pulses tend to increase the AVC voltage and reduce the audio level somewhat. The effect isn't great unless D2 conducts for a sustained period. If you encounter a strong signal while tuning, for example, D2 will quickly charge C3. Once C3 charges to about 40% of the carrier level, D2 stops conducting and R3 charges C3 the rest of the way. This provides a very useful fast-attack AVC characteristic for strong signals. It permits you to use a long AVC time constant without suffering sustained audio blasts. The next article will explain how a very long AVC time constant can minimize selective-fading distortion.

If your receiver shorts C3 to disable AVC, D2 will clamp the audio to ground. C3 must remain operative when AVC is disabled unless you disable ANL at the same time. A SPDT contact that switches the AVC line between ground and C3 will solve the problem, but sometimes you'll have to improvise. I've installed a SPDT relay in receivers that had only SPST switch contacts.

Although there is no steady carrier in CW mode, the detected BFO signal will keep C3 charged to a level where CW tones do not clip.

With R1 and R2 selected as described above, noise limiting is not as great as when the clipping threshold is within the normal modulation range. When I pick these component values, I trade some noise reduction for clean audio. I don't use ANL to suppress external noise because I keep my neighborhood free of impulse noise sources. Instead, I use ANL to suppress receiver switching impulses and tuning thumps, and to permit a very long AVC time constant. You can adjust R1 and R2 to put the clipping threshold wherever you want it.

Performance on Impulse Trains

Fig. 2   Noise pulses 17 ms apart, ANL disabled

Fig. 3   Noise pulses 17 ms apart, ANL enabled

To demonstrate the ANL circuit, I generated a 70%-modulated AM signal near 10 MHz with one signal generator and combined it with the output of a second generator sweeping 1 to 20 MHz every 17 ms. As it passed the receiver's frequency, the swept signal demodulated to a pulse 300 µs wide. I varied the relative levels of the two generators to obtain the impulse level I wanted. Fig. 2 shows the voltage across R2 in my NC-57 with the ANL circuit disabled. Fig. 3 shows it with ANL enabled. While ANL has greatly reduced the impulse amplitudes, it also has dropped the signal about 3 dB. Pulses this rapid and strong raise the AVC level somewhat. This prevents the clipping threshold from being very close to the demodulated signal level.

Fig. 4   Noise pulses 100 ms apart, ANL disabled

Fig. 5   Noise pulses 100 ms apart, ANL enabled

Figs. 4 and 5 show ANL action for pulses 100 ms apart. Here I used a 100%-modulated signal. Because the pulse density is lower, AVC is less affected and the pulses are clamped closer to the desired signal.

The circuit of fig. 1 permanently engages ANL. If you want to be able to turn it off, add a switch in series with D2. Since ANL is permanently on in my receivers, I use the ANL panel switch to select AVC time constants. The only time I find ANL undesirable is when sweeping the IF during receiver alignment. An internal switch (or unsoldering a lead) solves the problem.

With ANL always engaged, you begin to trust your receiver not to startle you with an unexpected outburst. I find that having it on all the time makes tuning and listening more pleasant.

January 10, 201388–108 MHz

Originally published in Electric Radio, September 2003