Hello, Mark ! You can design an LPF and then multiply it's taps by the complex sinusoid exp(j*w0*t). This will result in a two sets of coefficients (real and imaginary). Each set represents a BPF centered at w0, but the two filters will have 90 degree phase response difference. I use such technique in my HF DSP transceiver design. All the best ! Oleg UR3IQOLINRADDARNIL