Turn linear phase into truly linear phase
There’s a simple brute-force approach. Since using one of these filters on your signal causes occasional unwanted phase flips by pi radians, using two in series must cause phase jumps of 2pi radians. That’s the same as no phase jump at all! And indeed, if in the Creator filter customizer we cascade two identical instances of any filter suffering from the pi phase jump issue, we get an overall phase plot that is free from the problem. Though sometimes the plot will still show a jump of 2pi – now only a cosmetic rather than processing issue, but still a little annoying (note to self – get this fixed!). The down-side is that we almost double the number of coefficients in our filter, which might not suit your hardware. We also significantly change the filter response, of course. There will be twice the dB level of droop or ripple in the passband (bad, probably) and twice the dB stopband attenuation (good, probably, but maybe now over-engineered).
Figure 1 shows an example filter. This time, I used a third-party filter design program, FIRBBC, to create the coefficients for a small eight tap equiripple filter.
FIRBBC has some really interesting polyphase capabilities, by the way, and is useful for simplifying the process used in “Sample multiple channels simultaneously with a single ADC”. If we cascade two of the Figure 1 filters, we get a 15-tap filter (Figure 2) that has twice the passband droop, twice the stopband attenuation – and no stopband phase jumps!
Cascading two of Figure 1’s filters gets rid of the stopband phase jumps
Another approach is to manipulate the FIR filter’s impulse response directly to eliminate the jumps. One never gets something for nothing, so let’s look at the tradeoff we might have to make. As we saw last time, all filters with this rogue phase issue have one thing in common: the Fourier transform of the impulse response has some regions in which its value is negative. In the case of filters with many taps, there can be many such regions. We can fix that quite easily, though. Consider the Fourier transform of a pulse that has a non-zero height only for a single sample. It has a constant magnitude for every frequency component, and this magnitude is real and positive if the pulse occurs at time t=0. So, what we can do is ‘tweak’ the impulse response of our ‘jumpy’ filter by adding a little bit of single-sample pulse to it, just enough so that the positive components cancel out the most negative parts of the Fourier transform. It should be fairly obvious that we should add this pulse right at the centre of symmetry of the impulse response, so this technique is much simpler to apply in the case of an odd number of taps.
How much of this sample do we have to add? Well, let’s look at another example. I used FIRBBC to design a 15-tap filter, shown in Figure 3. It has an equiripple stopband with plenty of stopband nulls and plenty of pi phase jumps. Figure 4 shows the amplitude of the Fourier transform of the coefficient series, correcting for the signal delay through the filter. Strictly, it’s an FFT of the 15 taps padded out to 1024 samples with zeroes. It shows how the response gets inverted when the Fourier transform goes negative.
Figure 3: A 15-tap equiripple lowpass filter
Figure 4:
Fourier transform of the coefficients used to produce the filter in Figure 3
Closer inspection of the stopband shows that it rises up to -30.34 dB, which corresponds to a linear value of 0.0304. The largest coefficient value of the filter is 0.1952 so we increase just this coefficient to 0.2256. This is imperceptible when you just look at the impulse or step response. When we now plot the overall Fourier transform we find that it doesn’t go negative, see Figure 5. And when we plot the response of the filter using PSoC Creator (Figure 6), the phase jaggies are gone! The group delay is unchanged.
15-tap filter, centre coefficient modified for non-negative Fourier
transform
The eagle-eyed will notice that the stopband rejection has deteriorated a little. That’s to be expected. We added enough signal at every frequency to cancel out the most-negative portions of the Fourier transform. In those stopband regions where the response is already non-inverting, but with the same magnitude as the most negative parts, we might therefore as much as double the signal level. So, when using this technique, over-design your stopband by 6 dB, to allow for this reduction.
The technique also ‘works’ for that simple averager case. By changing the centre coefficient value from 0.14286 to 0.37593, and adjusting the gain to get back 0 dB at DC, we get no ‘jaggies’(Figure 7) and therefore no inversion in the stopband (Figure 8).
Figure 7: The seven day averager tweaked to eliminate the stopband inversion
Figure 8: Time response of the tweaked averager to the six day fluctuation
Now, if you start with a filter that has a half-decent stopband – which should mean in the majority of cases, if you’re series about your filtering – this isn’t going to make that much difference to the signals passing through. But by eliminating this source of indeterminacy in the spectrum of your signal, you may find that analyses relying on Fourier transforms don’t fall over so much due to discontinuities in sign. Why don’t you try this out and let me know if you have an application where it makes a difference? Don’t let your filter give you that old excuse: “it’s just a phase I’m going through!” Best / Kendall
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P.S. Check out my Cypress blog at www.cypress.com/go/thefilterwizard – you can also email me from there.
About the author:
As well as being The Filter Wizard, Kendall’s a skilled circuit designer, product engineer and all-round analog expert. He’s a widely published communicator of theory and method in many electronics disciplines. His fascination with electronics and audio dates back to boyhood. He spent 21 years designing filters, precision instrumentation, signal processing equipment and music systems. Then 11 years selling and designing in analog semiconductors. He’s created advanced products to chase signals across many domains, extract the information from them and do something useful with it.
These days, Kendall is with Cypress, doing system architecture, product definition and strategic application analysis for their precision analog and mixed-signal devices. And also, of course, supporting customers with all sorts of analog and digital filter designs. He’s also educating and mentoring a new generation in the ways of analog and systems thinking. Striving to improve the world one dB at a time!
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