Included within the LO system is the Receiver Room LO Rack, a LO Reference Distribution system, and components located within the Equipment Room. The Receiver Room LO Rack components consist of two LO1 synthesizers, the LO Router and the Test Tone Router. The LO Reference Distribution System along with LO2 and X21Multiplier/LO3 are located in the GBT equipment room adjacent to the GBT operations room.
Located in the Receiver Room LO rack are two LO1 synthesizers. These units generate the synthesized signal used in producing the IF.
The LO Router is also located in the Receiver Room LO Rack. The LO Router routes the signals that are generated by the LO1 synthesizer to the proper front-end ports leading to the front end equipment.
The last of the LO equipment located in the Receiver Room LO Rack, the Test Tone Router allows the user to inject test signals into the front-ends for system checkout.
There are eight LO2 Synthesizers located in the GBT Equipment Room. These synthesizers generate the LO2 signal that is used to upconvert all or a portion of the receiver IF1 bandwidth to the frequency band 8.5-10.35 GHz. The synthesizers which generate the LO2 signals are tunable over a 10.5 to 18 GHz range with 1 kHz resolution.
A fixed LO3 10.50 GHz source downconverts IF2 to the 0.15-2 GHz band, producing the output IF3 signal. A switch allows selection of either filtered or unfiltered outputs to the various backends.
The receiver room IF equipment consists of an IF Router, Noise Source, and Optical Fiber Driver Modules. These components are incorporated in the Receiver Room IF Rack. The remaining IF equipment is located in the GBT Equipment Room and consist of the Optical Receiver Module and 1-8 GHz Converter Module (mounted in Converter Racks A and B), and the 1.6 GHz Sampler Filter and 100 MHz Converter Filter (mounted in the Analog Filter Rack).
A single module that generates an amplified noise spectrum is provided for IF system diagnosis/test purposes. The user may select either a broadband (0.1-8 GHz) or narrowband (2.75-3.25 GHz) outputs.
The IF Router allows the user to select several of the IF signals for connection to Optical Fiber Driver modules.
Eight Optical Fiber Driver modules using microwave optical laser transmitters enable the IF to be sent over a fiber optic line to the GBT equipment room.
These modules demodulate the optical signal, and amplify and split the IF1 signal for distribution to converter modules in the same racks. In current receiver plans, all front ends produce first IF signals within the 1.0-8.0 GHz range. Extra bandwidth below 1 GHz is provided in the Optical Receiver modules in case the need arises in the future to transmit IF signals below 1 GHz. There are eight modules capable of producing thirty-two IF output signals.
The sixteen Converter Modules, with associated local oscillators LO2 and X21Multiplier/LO3, are tunable over the 1-8 GHz IF1 frequency range. They provide selectable outputs of different bandwidths for various backends. The converter modules upconvert IF1 to about 9.5 GHz with tunable local oscillator LO3 and then downconvert and filter to a third IF with several outputs covering bandwidths of 400 to 1850 MHz for the various backends. Output levels can be adjusted over a 32 dB range with 0.125 dB resolution via a step attenuator and monitored via the M/C system or the front panel meter. Control of the module is done remotely via the M/C interface.
There are eight 1.6 GHz Sampler Filter modules in the system. These modules contains bandpass filters for the 800 MHz and 200 MHz bandwidth modes of the GBT spectrometer. A third choice of filter can be connected to rear panel parts and selected as the "External SG Filter".
There are sixteen 100 MHz Converter/Filter modules, one for each 1-8 GHz converter. These modules filter the signal to reject the image frequencies, then amplify , and mix it with an external 500 MHz reference to produce an output band of 15-350 MHz. The signal is then bandpass-filtered, with selectable bandwidths of 50, 12.5 MHz or the bandwidth of an external filter connected to the rear panel parts and selected as the "External CF Filter".
The LO Reference Distribution system supplies timing to IF/LO System. It accepts a 100 MHz and 10 MHz signal from the hydrogen maser standard located in the Timing Center. There it multiplies the 100 MHz to produce 500 MHz, and transmits 500 and 10 MHz signals using single-mode optical fiber over separate links to the GBT Equipment Room and to the GBT Receiver Room. The 500, 10, and 5 MHz generated by dividing the 10 MHz signal) are distributed there. In addition, a 500 MHz signal is returned over separate fibers to the Timing Center where Round-Trip Phase modules are used to measure the round-trip delay for the 500 MHz signal. The measured delay is available via the GBT M/C system.