Ultra-Wideband Direct Conversion Receivers for Satellite On-Board Processing – System Analysis and Digital Error Compensation –

In this contribution, a direct conversion receiver (DCR), zero IF or homodyne receiver applying in-phase/quadrature (I/Q) sampling has been revisited in conjunction with digital error compensation. Contrary to common practice, our main interest is focused on the investigation of performance degradation in conjunction with the processing of ultra-wideband frequency division multiplex (FDM) signals. To this end, all potential error sources have been identified and introduced into a comprehensive error model. This error model comprises path individual DC-offsets, gain, phase and time delay mismatch, bidirectional crosstalk between the input ports of the down-converters, quantisation and overflow saturation (clipping) errors, as well as time jitter due to sample-and-hold circuits and clock generator. For all of these error sources an exact error analysis has been carried out. Furthermore, a typical error scenario taken from an ultra-wideband application in satellite communications has been used to demonstrate the overall performance degradation as a result of the combination of these error contributions. For this application, the major degradations are caused by DC-offsets that heavily impair the channel centred at zero frequency, branch time delay difference that are most pronounced in edge channels, and gain mismatch. Each of these three error sources introduces, on its own, a performance degradation such that error compensation measures are compulsive. In contrast, all other error contributions are of minor influence. To reduce degradation, blind digital error compensation methods have been investigated to estimate and compensate the most important errors. In particular, a novel approach to compensate for I/Q time delay mismatch has been developed. The combination of this novel delay mismatch compensation method with known DC-offset, gain, and phase compensation techniques results in highly improved signal to noise and distortion ratio (SNDR). This enhancement due to digital error compensation will be demonstrated by simulation by applying the above ultra-wideband scenario.