Application of time-domain techniques for measurement and equalization of a millimeter-waveguide line in the 40- to 110-GHz band

The use of time-domain techniques for measurement of the transfer characteristics of circular millimeter-waveguide lines in the 40- to 110-GHz band is described. By phase modulating a millimeter-wave carrier, energy is concentrated within the 500-MHz-wide channel of interest. Fast Fourier transform processing of the quadrature components of the coherently detected waveform with transmitter and receiver back-to-back and with transmission over the channel yields the desired transfer characteristic. The transfer function information for the 500-MHz-wide waveguide channel is then used to synthesize an equalizer to compensate for deviations from the desired channel characteristic. The technique described in this paper ensures that adequate signal-to-noise ratio is obtained to yield excellent accuracy (0.1 dB rms in amplitude and 0.1-ns rms in delay) within the 20-MHz resolution bandwidth of the measurement. The frequency resolution obtained is approximately ten times the resolution of time-domain techniques using fast subnanosecond baseband pulses to cover band-widths of 10 GHz or greater. An error analysis is presented which treats the problem of thermal noise entering at intermediate frequency as well as the errors caused by phase jitter of the coherent reference. Actual measurements of a millimeter-waveguide line are included to demonstrate the use of this technique.