A comparison of wide bandwidth quadrature demodulators using computer modelling

In radar systems that need to measure target range and velocity accurately the transmitter and receiver chains use the same local oscillators. This ensures that the phase of the target return only contains the targets range information. The radar receiver demodulates the signal into inphase (I) and quadrature (Q) signals, using a quadrature demodulator. A conventional analogue quadrature demodulator is shown. The nature of demodulating one signal into two signal paths means that any imbalance in the characteristics of the paths causes an error (called the image) in the IQ samples. The image can appear as a target which does not exist and is therefore undesirable. Traditionally the demodulation has been performed using analogue components, then converting to digital once the signals are at baseband. With advances in ADC and DSP technology, digital methods are becoming possible at radar bandwidths. These techniques have been widely documented over the last few years. What has not been documented to our knowledge is a comparison of wide bandwidth quadrature demodulator techniques. That was the objective of a study by Racal Radar Defence Systems.