Coherent (VQ) Detector Error Analysis Software

Software for analysis and alignment of I/Q Detectors is described using the results developed by Kyle in " Precision Alignment of I/Q Detectors Using Fourier Transforms". The relationship of the continuous and discrete Fourier transforms is described. The measurement technique uses two locked synthesizers to provide sinusoidal responses at the outputs, I and Q, of the detector. Measurement data acquisition uses sampling techniques to obtain time sequences of the detector I and Q outputs. The sampling process is performed by a Tektronix 390AD giving 2048 values (10 bits each) for each input. The 390AD data is processed to give floating point time samples of the I and Q response. Availability of the time response sequences permits use of digital signal processing techniques for detector characteristics analysis. Discrete Fourier Transforms are introduced and compared with the continuous tranforms. The FFT of the time sequences i (t), q (t) are performed resulting in frequency domain (spectrum) sequences I (n) and Q (n). The spectral coefficients are identified for the input signal and analyzed to determine ka, the detector amplitude balance, and ϕ, the detector phase error. The power spectrum is developed and Image Rejection is evaluated. The power spectrum is plotted with the amplitude balance, phase error, and image rejection shown. Finally window functions are shown to provide equivalent results for the non-ideal i (t) and q (t) sequences. The technique is shown to provide fast-accurate results of the determination of I/Q detector characteristics.