Over the past 30 years, HF radio noise in urban areas has steadily increased. S6-S9 noise levels are common, which makes it hard to listen to the signals we want to receive.
I’ve been wondering if we can attenuate this noise using knowledge of the properties of the noise, and some clever DSP. Even 6dB would be useful, that’s like the transmitting station increasing their power by a factor of 4. I’ve just spent 2 months working on a 4dB improvement in my FreeDV work. So this week I’ve been messing about with pen and paper and a few simulations, exploring the problem of man-made noise on HF radio.
One source of noise is switching power supplies, which have short, high current pulses flowing through them at a rate of a few hundred kHz. A series of short impulses in the time domain produces a series of spectral lines (i.e sinusoids or tones) in the frequency domain, so a 200kHz switcher produces tones at 200kHz, 400kHz, 600kHz etc. These tones are the “birdies” we hear as we tune our HF radios. The shorter the pulses are, the higher in frequency they will extend.
Short pulses lead to efficient switch mode power supplies, which is useful for energy efficiency, and especially desirable for high power devices like electric car chargers and solar panel inverters. So the trend is shorter switching times, higher currents and therefore more HF noise.
The power supplies adjust the PWM pulse-width back and forth as they adjust to varying conditions, which introduces a noise component. This is similar to phase noise in oscillators, and causes a continuous noise floor to appear in addition to the tones. The birdies we can tune around, but the noise floor sets a limit on urban HF operations.
The Octave script impulse_noise.m was used to generate the plots in this post. Here is a plot of some PWM impulse samples (top), and the HF spectrum.
I’ve injected a “wanted” signal at 1MHz for comparison. Given a switcher frequency of 255kHz, with 0.1V impulse amplitude, the noise floor is -90dBV down, or about 10uV. This is S5-S6 level noise, assuming 0.1V impulse amplitude induced onto our antenna by local switcher noise, e.g. nearby house wiring, or the neighbors TV. These numbers seem reasonable and match what we hear in our receivers.
Single, isolated pulses are an easier problem. Examples are lightning or man-made sources that produce pulses at a rate slower than the bandwidth of the signal we are interested in.
A single impulse produces a flat spectrum, so the noise at frequency f Hz is almost the same as the noise at frequency f+delta Hz, where delta is small. This means you can use the noise at frequencies next to the one you are interested in to estimate and remove the noise in your frequency of interest.
Here is an impulse that lasts two samples, the magnitude spectrum changes slowly, although the phase changes quickly due to the time offset of the impulse.
Turns out that if the impulse position is known, and most of the energy is confined to that impulse, we can make a reasonable estimate of the noise at one frequency, from the noise at adjacent frequencies. Below we estimate the phase and magnitude (green cross) of frequency bin H(k+1) (nearby blue cross) from bin H(k). I’ve actually plotted H(k-1), H(k), and H(k+1) for comparison. The error in the estimation is -44dB down, so that’s a lot of noise removed.
Unfortunately this gets harder when there are multiple impulses in the same time window, and I can’t work out how to remove noise is this case. However this idea might be useful for some classes of impulse noise.
Another idea I tried was “blanking” out the impulses, buy opening and closing a switch so that the impulses are not allowed into the receiver. This works OK when we have a wideband signal, but falls over when just a bandpass version is available. In the bandpass version the “pulse” is smeared over time and we are no longer able to gate it out.
There will also be problems dealing with multiple PWM signals, that have different timing and frequency.
I haven’t looked at samples of the RF received from any real world switcher signals yet. I anticipate the magnitude and phase of the switcher signal will be all over the place, due to some torturous transfer function between the switcher and the terminals of my receiver. Plus various other signals will be present. Possibly there is a wide spectrum (short noise pulses) that we can work with. However I’d much rather deal with narrow bandpass signals consisting of just our wanted signal plus the switcher noise floor.
I might get back to my FreeDV work now, and leave this work on the back burner. I do feel I’m getting my head around the problem, and developing a “bag of tricks” that will be useful when other pieces fall into place.
The urban noise appears to be localised, e.g. if you head out into the country the background noise level is much lower. This suggests it’s coupled into the HF antenna by some local effect like induction. So another approach is to estimate the noise using a separate receiver that just picks up the local noise, through a sense antenna that is inefficient for long distance HF signals.
The local noise sequence could then be subtracted from the HF signal. I am aware of analog boxes that do this, using a magnitude and phase network to match the differences in signals received by the sense and HF antennas.
However a DSP approach will allow a more complex relationship (like an impulse response that extends for several microseconds) between the two antenna signals, and allow automatic adjustment. The noise spectrum can change quickly, as PWM is modulated and multiple devices turn on and off in the neighborhood. However the relationship between the two antennas will change slowly if they are fixed in space. This problem reminds me of echo cancellation, something I have played with before. Given radio hardware is now very cheap ($20 SDR dongles), multiple receivers could also be used.
So my gut feel remains that HF urban noise can be reduced to some extent (e.g. 6 or 12dB suppression) using DSP. If those nasty PWM switchers are inducing RF voltages into our antennas, we can work out a way to subtract those voltages.