A radio-astronomy group measures the output of a low-noise amplifier (LNA) that sits…
PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Circuit foundations and passive networks · Circuit foundations and passive networks
Electronics for Measurement, Ch The bench: probes, cables, grounds and what the instrument shows
Problem
A radio-astronomy group measures the output of a low-noise amplifier (LNA) that sits in a housing bolted to a metal aerial mast and earthed there. The measuring instrument, a mains-powered spectrum analyser at the foot of the mast, has its chassis bonded by a short strap of negligible resistance to the mast's earthing point. The LNA is joined to the analyser by 5m of coaxial cable whose screen, of resistance 60mΩ, carries the signal's return and is connected at both ends. On the same mast, a mains-powered rotator motor for steering the aerial returns 0.60A rms at 50Hz into the mast at the LNA's earth connection. From there it can reach the earthing point by two paths: the length of strap that runs down from the LNA's earth connection, whose resistance is 40mΩ, and the cable screen. The wanted signal at the LNA output, at the frequency of interest, is 0.30mV rms.
((a)) The cable screen and that length of strap join the same two points. Find the voltage the rotator's return current puts along the screen, and so in series with the signal, and express it as a multiple of the wanted signal.
((b)) State why simply disconnecting the LNA's earth would not be an acceptable way to remove this interference, and propose the wiring change that removes it without leaving the LNA unearthed against a fault.