Notch Filter in Ham Radio Receivers: Manual vs Auto Notch
A notch filter is the tool you reach for when a single steady tone ruins an otherwise good contact: the whistle from a carrier that sits a few hundred hertz from the station you want, the beat note between two overlapping broadcasts or the whine of a digital device in the next room. Almost every modern transceiver has one, and many have two: a manual notch that you tune with a knob, and an automatic notch that finds the tone by itself. Used well, a notch removes a narrow slice of the audio and leaves the voice almost untouched. Used badly, it hollows out speech or does nothing at all. This guide explains how a notch works, how the manual and automatic versions differ, how to set them and what they cannot fix.
What a Notch Filter Does
In filter terms, a notch is a band-stop filter with a very narrow stopband. The Wikipedia article on band-stop filters defines a notch filter as one with a narrow stopband and a high Q factor, which means that it removes a small range of frequencies and passes everything on both sides. A band-pass filter does the opposite: it lets one range through and blocks the rest. In a receiver, the passband filter chooses the width of the signal you hear, for example 2.4 kHz for SSB or 500 Hz for CW, and the notch cuts a hole inside that passband to remove one unwanted component.
The unwanted component is usually a steady carrier. A single-frequency tone has almost no bandwidth, so a notch only a few tens of hertz wide can take it away. Voice, on the other hand, is spread across hundreds of hertz and is made up of many components that change all the time. A narrow notch removes only a small part of the speech, which is why the voice stays natural. This works because of the difference between a steady tone and a changing signal, and it fails in cases where the interference is not steady.
Where the Whistle Comes From
The beat note you hear is a result of the mixing of two signals. When two carriers a few hundred hertz apart arrive at the detector, they produce a tone at the difference frequency, which is the idea behind the heterodyne. On the shortwave bands this happens on the crowded broadcast frequencies and on busy SSB segments, where a station tuning up or an AM carrier sits next to your channel. A carrier 1 kHz away gives a 1 kHz whistle in your speaker, and the tone follows the carrier if it drifts. Local sources behave in the same way. A switching power supply, a charger or a computer clock may place a narrow line on a particular frequency of the band.
That gives a way to think about the problem before you touch any control. Is the interference a narrow, steady tone? Then a notch is the right tool. Is it a broad hiss, a crackle or a series of pulses? Then the right tools are the noise reduction and the noise blanker, described in our guide to the noise blanker and noise reduction, and not a notch.
Manual and Automatic Notch Compared
| Feature | Manual notch | Auto notch |
|---|---|---|
| How it is set | You turn a knob or touch a control until the tone disappears | The DSP searches for tones and places the notch itself |
| Number of tones removed | Usually one, sometimes two | Often several at the same time |
| Speed of reaction | Instant once set, but follows a drifting tone only if you re-tune it | Takes a moment to find a new tone and may follow a slow drift |
| Effect on voice | Small, because you place a narrow notch exactly where it is needed | Can affect vowels and make the audio sound processed, depending on the algorithm |
| Best use | A steady carrier close to the wanted signal | Several or wandering tones, hands-free operation, digital mode audio |
| Weakness | Needs your attention | May treat a CW signal or a data tone as a carrier and remove it |
In practice the manual notch is the one worth learning first, because it is predictable. The auto notch is convenient and works well in many situations, but the way it behaves depends on the algorithm and on the radio’s software, so test it on your own receiver and learn its habits.
Setting a Manual Notch Step by Step
- Tune the signal you want and note the whistle in the background. If the whistle is strong, reduce the AF gain a little so that the radio is not stressed.
- Switch the notch on and, if there is a choice, choose the narrow setting. On some radios the notch width is a menu item and a wide notch removes more of the voice.
- Sweep the notch frequency slowly through the pitch of the whistle. The tone will become quieter and then vanish, and a little further it will start to return. Stop in the middle of the quiet region.
- Fine tune with the audio or with the waterfall display if the radio has one. A spectrum scope shows the carrier as a spike, and many radios show the notch position as a marker so that you can place it exactly on the spike.
- Check the voice. If the speech sounds boomy or thin, narrow the notch or move the passband slightly so that the carrier is near its edge.
On many radios the notch operates in the IF or in the DSP section after the IF filtering. A DSP notch acts on the audio after the main selectivity, so a strong carrier can still trigger the AGC. If you hear the whole receiver pumping up and down each time the carrier is present, the notch is working at the wrong point in the chain for that problem; a narrower IF filter or a different offset will solve it better. The guide to AGC attack and hang time explains why a strong unwanted signal makes the whole band fall silent, and the discussion of receive filter bandwidth and roofing filters shows what happens earlier in the chain.
Using the Auto Notch Well
The auto notch is a convenient choice when the interference changes. It is especially useful on a crowded band when several carriers drift in and out, since a manual notch can handle only one at a time. A few habits make it more useful:
- Turn it on only when you need it. An auto notch that is active all the time may remove the carrier of a CW signal you are trying to read, or treat a tuning signal as an interferer.
- Do not use it on digital modes unless you have tested it. A steady tone is exactly what many digital signals look like, and an auto notch can remove part of an FT8 or RTTY signal. In most cases the decoder works better with the notch off.
- Listen for the artefacts. A watery or warbling sound suggests that the filter is hunting. In that case, switch to the manual notch or reduce the passband.
- Combine it with a narrow passband. On SSB, moving the filter edge away from the carrier may cut the whistle at its root, without an extra process in the audio path.
The Notch in CW and Digital Modes
On CW the useful signal is itself a steady tone, so the notch must be used with care. Keep the notch away from the pitch of the signal you are copying. In a pile-up of CW stations, a narrow IF or DSP filter around the tone you want is a better tool, and a notch is for the carrier that sits inside that narrow band. A typical case is an unmodulated carrier from a transmitter tuning up near your pitch: the notch cuts it, and the CW signal stays.
For digital modes the software decoder has its own signal processing, so interference from a carrier within a few hundred hertz is usually dealt with by the decoder or by choosing a different frequency. A notch in the radio may help if a strong carrier overloads the audio level, but check the effect on the decode rate before you rely on it. If you use a software defined radio, the notch is often part of the receiver program rather than the radio itself, and you can place it on the waterfall with the mouse.
When a Notch Is the Wrong Tool
A notch cannot repair a signal that is already damaged by the interference. If a strong carrier overloads the first stages of the receiver, the distortion appears before any audio filter, and the notch cannot see the real cause. An attenuator or a change of antenna is needed in that case, as discussed in the article on receiver dynamic range and blocking. A notch also does nothing for broadband noise, since it removes only a narrow slice, and it can do harm if you place it inside the signal you want. Finally, the notch removes the unwanted tone at the output of the radio but does not reduce the carrier’s effect on the AGC, which can still hold the gain down. If you have the choice, a narrower IF filter that excludes the carrier is the cleaner cure.
Some local noise sources create a set of evenly spaced lines. These are often better dealt with at the source: unplug devices in turn and listen for the change, or use the methods in our guide to finding and killing RF noise. A notch is a bandage, and the cure is to remove the source of the tone.
Notch, IF Shift and Passband Tuning Together
A notch is only one of three controls that shape the passband, and they work best together. IF shift or passband tuning moves the whole filter window up or down without changing the pitch of the received voice very much, while the notch removes a single tone from inside the window. When a carrier lies on one side of the wanted signal, the first move should be to shift the window so that the carrier falls outside it. Only if it falls inside the passband in spite of the shift, because it is too close to the signal, do you add the notch. This order keeps the audio as natural as possible, since a shifted window costs nothing in quality and the notch removes some of the voice around its frequency.
A quick test helps to decide which control to use. Switch the notch on and off while you listen. If the tone drops by a large amount, the notch is effective, and you should leave it on. If you cannot hear a difference, the tone is either outside the notch or it is not a simple carrier. In the second case, switch to the other tools, or check whether the sound comes from your own computer or from the receiver chain. Doing this once on a quiet band teaches you more about how your radio responds than reading the manual twice.
The Bottom Line
A notch filter removes a narrow slice of the received audio, which makes it ideal for a steady carrier or heterodyne whistle. A manual notch is precise and predictable, so learn it first: switch it on, sweep it through the whistle and stop in the middle of the quiet region. An auto notch is convenient for several or wandering tones, but it may affect CW and digital signals, so use it only when you need it. If the interference is broad, impulsive or overloads the front end, a notch is the wrong tool, and a narrower filter, noise reduction, an attenuator or a cure at the source works better.
Frequently Asked Questions
What is a notch filter on a ham radio?
It is a very narrow band-stop filter that removes a single tone, such as a carrier or heterodyne whistle, from the received audio while leaving the rest of the signal almost unchanged.
What is the difference between a manual notch and an auto notch?
With a manual notch you tune the frequency yourself and place it on the tone. An auto notch uses DSP to find and remove tones without your input, often several at once.
Why does my voice reception sound strange with the notch on?
The notch may be too wide or placed inside the voice range. Narrow it or move the passband so that the carrier sits near its edge.
Can I use a notch filter on CW?
Yes, but only to remove an unwanted carrier near the pitch of the signal. Keep the notch away from the tone you are copying, because it would remove your own signal.
Should I use the auto notch on FT8 or other digital modes?
Usually not. A digital signal can look like a steady tone to the auto notch, and the decoder may lose part of the signal. Test it on your radio and turn it off if the number of decodes falls.
Does a notch filter reduce broadband noise?
No. A notch removes a narrow slice of the spectrum. For broad hiss or impulse noise use noise reduction, a noise blanker or a cure at the source.