Phase calibration of the Very Long Baseline Array (VLBA)

Summary form only given. The delay through a microwave receiver can be calibrated by injecting short-duration pulses (<25-ps risetime) with constant repetition rate (typically 1 MHz) through a coupler into the input of the receiver. The phases of the tones generated by the pulse train are measured to determine the receiver phase and group delay. For a calibration tone of -100 dBm injected through a 40-dB coupler the pulse cal temperature is 0.7 K averaged over 1 MHz. This tone can be measured to within 0.3 degrees in 1 s when extracted from a 1 MHz bandwidth for a receiver with a 30-K system temperature. The group delay is determined from the phases of several tones over the observing band.<>