Almost all receivers have semiconductor noise diodes for calibration of the measurements. The noise power is fed into the receiver input via a directional coupler. It can be switched synchronously with the measurement cycle and can usually be set to one of a few power levels.
The size of the calibration signal is determined normally at the beginning of an observing period of a receiver using a hot/cold body measurement and/or by comparison with a radio source. The value is available in Effelsberg. It is not however monitored as routine. It is the task of the observer to be sure about the correct calibration of the data. The size of the calibration signal is unlikely to be constant over the input range of the receiver because of the frequency characteristic of components in the calibration circuit hardware. The value quoted by the Systems Group is then an average over the bandwidth used for the determination.
Some receivers, used for spectroscopy measurements with the ON/OFF technique have one or two noise diodes for noise balancing. The noise power can be switched and is fed to the receiver input. The level of the signal can be set in steps from the control room. In ON/OFF observation it provides a compensation in the OFF position for the continuum radiation of the source by making the input noise level the same for both observing positions. This is done in order to improve the base line of the receiver system when measuring spectral lines.
When using this technique, however, the observer must realize that additional measurement errors (baseline ripple) can be introduced in broad band observations due to frequency variation of the injected noise signal. Usually test measurements must be made in any given case to make a final decision whether the noise balance technique can be used to improve the observation.