Independent sideband modulation/demodulation using digitally implemented Hilbert transforms

This paper describes a modulation/demodulation technique which uses digitally implemented Hilbert transforms for achieving independent sideband (ISB) modulation/demodulation of baseband signals, each up to 3 kHz wide, on a single RF or IF carrier. The Hilbert transforms are used to provide a wideband 90 deg phase shift in one of the baseband channels of a modulator/demodulator. The phasing method of generation/recovery of ISB modulation can thus be realized with up to 60 dB of undesired sideband rejection. Digitally implemented filtering is also used to provide the required out-of-band attenuation of the signal spectrum. The trade-offs between the number of Hilbert transform sections and the undesired sideband rejection are discussed. The results obtained with an experimental digitally implemented two-channel ISB modulator/demodulator are described.