The Holography Receiver is located in a rack in the receiver room on the GBT. Refer to Figure 1 for a proposed depiction of this rack. The guts of this instrument are mounted in three drawers labeled: Holography Correlator, Holography Downconverter, and Downconverter Power Supply. The drawer labeled Holography Correlator house digital hardware. IF section components are located in the drawer labeled Holography Downconverter. Power supplies are located behind the panel labeled as such. Listed below is a breakdown of the instrument by its two main sections with a brief discussion of their primary components.
This part of the backend is designed to accept an IF signal near 50 MHz. The IF consists of two channels with one used for a test signal, the other as a reference. Refer to Figure 2 to see how these components fit into the signal path.
Simply takes an input signal and amplifies it before sending it on. The output is displayed on the front panel as a digital voltage readout.
Used to lock in a specified phase before signals are sent on to the SSB downconverters. The "locked" status is displayed via an LED light.
Two of these are used to convert received signals. How? By first rejecting a specified bandwidth, splitting the results so that a 180-degree phase shift is introduced, then recombining the results to cancel any undesired sidebands.
This is a lowpass filter used to reject a specified frequency and provide the results as either a 10 KHz or 100 KHz bandwidth output.
This is a circuit that uses complex mathematics (i.e. the Hilbert Transform) to create an imaginary component of a real signal. This is needed in the case of holography where the data must be re-phased or inverse-processed before processing the data.
These are considered the most critical parts of the entire backend. They are used to convert analog signals to a digital format before further processing.
Components making up the computer digital logic part of the receiver are housed here. These components work together to integrate and manipulate (e.g., process) the data signals.
Generates all the clock and timing signals used by the section.
Handles the multiplying of data integrations.
This card accumulates data integration outputs (to include those from the multiplier) prior to their transfer to the interface card and on.
Takes the digitized data and a clock signal from the A/D converters and coverts it to a useable output for the correlator components.
Simply functions as an interface between the A/D converters and the receiver's display panels (lights, digital readouts, etc.).
Provides a means of controlling attenuation i.e. a reduction in signal strength.
This card provides the necessary "handshake" between a standard interface and the correlator so they can communicate with each other correctly.