In simplest terms, here's what the system does: As the collected signal passes through the mixer its combined (or mixed) with a synthesized signal generated by the LO. The result of this mixing process is the sum and the difference of the collected signal and the synthesized signal. Only one of those resulting signals continues on, the other is filtered out. The signal that continues is refered to as the IF (intermediate frequency). In most cases the IF signal is lower in frequency than the original signal, but has all of the original signal's characteristics. The signal is mixed a total of three times at different places in the data stream before it is useable by GBT back-end equipment.
In each of the installed front-ends a frequency conversion is performed to produce the first IF signal. The mixer for the first conversion is located inside the prime focus front-end box or on a plate attached to the Gregorian front-ends' dewar, and the first mixer local oscillator signal is provided by the system LO1 synthesizers via the LO Router. The Test Tone Router outputs connect to the front-ends for testing purposes.
Coaxial cables connect the Noise Source module and the first IF signals to the IF Router. The IF Router allows selection of several of the IF signals for connection to Optical Fiber Driver modules. Eight optical fibers driven by the Optical Fiber Driver modules go from the Receiver Room through the antenna elevation and azimuth cable wraps to the Equipment Room.
At the Equipment Room, the optical fibers connect to Optical Receiver modules. The Optical Receiver module outputs connect to the 1-8 GHz Converter Modules via coaxial cable on the front of the Converter Rack. The 1-8 GHz Converter Modules provide switch selectable outputs for various supported backends. LO2 and X21 Multiplier/LO3 module outputs are connect by coaxial cable to the converter modules. Mixing is done within the 1-8 GHz Converter module.
The 1-8 GHz Converter module output ports are connected by coaxial cables to the input ports of the 1.6 GHz Sampler Filter modules, the 100 MHz Converter/Filter modules, the VLBI rack and the Spectral Processor at varying frequencies.
1.6 GHz Sampler Filter -------Spectrometer/Sampler Rack for sampling
DCR via fiber optics
100 MHz Converter/Filter---Spectrometer/Sampler Rack for sampling.
DCR via fiber optics.
There is one variation to the above IF/LO signal flow through the system. The prime focus has 4 optical outputs for direct connection to the DCR, bypassing the IF/LO system completely.
The GBT operator is responsible for assisting astronomers in carrying out observing. Refer to the GBT Operational Manual, procedures and checklists for specific guidance on setting up and using this system.