Hilbert Transform in FPGA
Introduction
Most of you familiar with signal processing would’ve heard the term Hilbert Transform. We’ll call it HT from here on. There are other transform in DSP like Laplace, Fourier, Z etc which are more popular & used across multiple domains. Basically transform, as the word suggests is a mathematical process which converts one form of signal to another. Some are used only for purely analytic purposes (Fourier) while some are used for real-time signal processing like Hilbert.
HT ‘transforms’ the phase of the signal. It performs a phase shift of -90°. Yes you read it right, it is negative 90° phase shift. Generally, it is mentioned as a 90° phase shift without explicitly mentioning positive or negative shift. The reason is due to the concept of positive & negative frequencies. HT would phase shift positive frequency component by -90° while a negative frequency would be shifted by +90°.
Where could we use HT?
I’m sure there are multiple uses but the one I’m aware of is Quadrature processing. The term quadrature processing refers to dealing with In-phase, Quadrature-phase signals, i.e I,Q signals. I,Q signals are used for modulation & demodulation techniques. Quadrature modulation/demodulation offers many advantages over conventional approaches which can be discussed in another post.
For example, SSB demodulation by phasing method uses Hilbert transform to phase shift the incoming modulated signal to -90°.
FPGA Implementation (Xilinx)
Hilbert transformers can be easily implemented in Xilinx FPGA’s using FIR compiler IP. HT coefficients can be generated using Matlab FDA Tool. It is important to note that the Hilbert coefficient structure should contain alternate zeroes for FIR compiler to infer the filter as a Hilbert Structure.
Snapshot shows Hilbert Transformer using FDA Tool.
Generated Coefficients for Hilbert transformer
As you can see, every alternate coefficient is zero valued, which the FIR compiler infers as a Hilbert Filter structure.
FIR IP Core generates I, Q output (dout_i, dout_q in IP symbol) if the Hilbert structure is inferred properly. The output is -90° phase shifted. HT of cosine is sine and HT of sine is -cosine
HT is predominantly used in quadrature Signal Processing.