Demonstrated direct sampling & demodulation of UHF & S-band signals

A set of time-skewed analog-to-digital converters (ADCs) in combination with a complex finite-impulse response (FIR) filter has been utilized to directly sample and demodulate RF signals. This architecture is referred to as a complex analog-to-digital converter (CADC) B.W. Tietjen. The clock signals driving the ADCs are staggered in time such that the system achieves an effective sampling rate commensurate with the carrier frequency of interest, while each ADC need only run at a rate greater than the immediate bandwidth of interest. The output of each ADC is fed into the complex bandpass filter, which suppresses the input signal's image frequency to produce I and Q data. The data rate out of the filter is equal to the ADC clock rate; thus the RF band of interest is aliased down to baseband or a low IF band. Hardware implementations of the CADC architecture have been achieved for both UHF and S-Band. The measured values of signal-to-noise ratio (SNR) and image rejection for both prototypes closely match expected results. Furthermore, by using the CADC to down-convert the RF band of interest to a low IF band, a second digital filter can be used to easily suppress any harmonics introduced by the ADC hardware. This provides superior spurious-free dynamic range (SFDR) and increased SNR performance. This technique is demonstrated using the UHF prototype. An automatic filter-tuning algorithm is also implemented to adaptively balance the amplitude and phase of the ADC channels composing the CADC. This technique allows the CADC filter to operate as expected despite hardware imperfections.