This section of the website describes the configuration of the telescope as a noise-adding radiometer and provides a place to present our radiometric measurements. Until development of this section is completed I will simply place here comments, graphics, and photographs that illustrate the configuration of the radio telescope for absolute power measurements at 1420.4 MHz and also measurement results as they are obtained. The technique sacrifices some telescope sensitivity in order to receive a calibrated noise level alternately with signals from the target in real time, thereby enabling high-stability power measurements to be made that are substantially insensitive to system gain changes due to temperature variations and other effects that result in drift of system gain. The detected levels of calibrated noise and antenna signal will each be equally affected by system gain changes and thus, as long as the gain changes are slow relative to the switching rate, the output which is a difference signal between the antenna signal and the calibrated noise signal will be relatively insensitive to system gain variations.

The detection technique utilizes synchronous analog signal detection of the difference signal between the power level of calibrated noises pulses and the power level received from the antenna. A difference signal is measured using a dual-phase lock-in amplifier. A dual-phase lock-in amplifier, in which the input signal is split into X and Y signals that are phase shifted 90-degrees from each other with the final output level equal to the square root of the squares of the X and Y signals, is used to ensure that the magnitude of the measured ac signal is accurate even for extremely weak signals that are too weak to manually adjust for an appropriate single phase shift relative to the the reference signal of the lock-in amplifier. Results will be posted as they are obtained. LabVIEW running on a PC will be used to control the data acquisition events from the lock-in amplifier via a GPIB port on the amplifier.

Fig. 1. Configuration diagram of the 1420MHz noise-adding radiometer using a Mini-Circuits MSW-2-20 reflective switch to alternately switch the feedhorn (antenna) and calibrated noise to the input of the LNA at a 100Hz rate.

Fig. 2. Photograph of the reflective switch, LNA, and directional coupler assembly mounted onto the 1420MHz feed horn. The switch is used to alternately connect the LNA to the antenna and a calibrated noise level from the unterminated directional coupler at a 100 Hz rate. Insertion loss of the switch is 0.5 dB at 1420MHz, which significantly reduces sensitivity of the radio telescope but is nevertheless tolerated for radiometer measurements because of the dramatic increase in effective gain stability of the system resulting from employing the synchronous switching/detection scheme. A tuned (optimized length) coaxial section is used between the input of the LNA and the reflective switch to maximize signal to noise.