Measure Tsys with a calibration diode. Diode power injected via a directional coupler before the first amplifier. ... Turning the diode on and off gives Tsys in Units of the diode temperature. ... All drifts in electronic gain can be removed with the cal diode (assuming the diode is stable). Gain drift and correction Examples: . No cal correction: . ... Dotted line is 5% change in Tsys. total power with cal correction (.pdf) . ... Measure diode temperature using hot and cold loads . ...