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Converter Rack

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       The Converter Racks A and B are the first stage of processing beyond the antenna where the optical fibers carrying the IF signals from the receiver room on the GBT terminate at the Optical Receivers in the Equipment Room in the Jansky Laboratory Building over 6 km from the GBT.
       The signals are demodulated from optical to RF in the Optical Receivers then sent to the input switches on the 1-8 GHz Converters by coaxial cables manually patched on the front panels of the Converter Racks, the specific patching is shown in the IF Manager screen.  In the Converters the signal is shifted by mixing the 1-8 GHz IF with a local oscillator signal designated LO2 and provided to pairs of Converters by one of eight Gigatronics 605 synthesizers, then filtered to a 1.85 GHz bandwidth centered at 9.43 GHz.  LO2 is variable over a 10.5 to 18 GHz range providing tuning capability over the entire 1-8 GHz range of the first IF, and components in the Optical Receivers and 1-8 GHz converters are specified to operate from 100 MHz to 8 GHz should the need arise to carry IF signals below 1 GHz.  After filtering at 9.43 GHz the signal is mixed with a 10.50 GHz signal designated LO3 and converted down to a filtered 150-2000 MHz IF and switched to any of four outputs:  1. The 1.6 GHz samplers,  2. A 500 to 1000 MHz band pass filter in turn feeding the Very Long Baseline Array Data Acquisition Recorder,  3. A spare output covering the full 150-2000 MHz bandwidth or  4. To a 550 MHz low pass filter feeding a four way power divider in turn feeding:  1. The Spectral Processor,  2. The 100 MHz Converter/Filters, and  3,4. Two spare outputs.  LO2 and LO3 are derived from a 10 MHz reference signal which in turn is derived from the master hydrogen maser in the GB Site Timing Center. The power level of the 150-2000 MHz IF is determined by coupling the signal to a square law detector.  That detected voltage is indicated on the Converter CLEO screens under 'CM Power (V)' and on a meter on the front panel of the 1-8 GHz Converter.  Each detector is measured to determine its individual voltage versus power characteristics and the data from those measurements is available here.
 

Initial CLEO Converter Rack Screen:


 

The top row of the initial CLEO Converter Rack screen has the Menu Bar common to most CLEO screens.  The next row has five tabs to open a screen for Converter Rack A containing Converters 1-8 and LO2 Synthesizers 1-4, Rack B containing Converters 9-16 and LO2s 5-8, a screen of parameters for all sixteen Converter Modules, a screen of parameters for all eight LO2 synthesizers and a tab showing voltages for the rack power supplies.  At the bottom of the screen is the State/Status frame common to CLEO device and manager screens.

The third row of the screen has four tabs, each allowing access to information and parameters for the Converters and their dedicated LO synthesizer.  Adjustable (after unLocking the module) parameters include Input buttons for selecting the IF signal from one of two optical receivers, parameters in the path to enable the user to set attenuation levels from 1/8 to 31.875 dB in 1/8 dB steps, buttons for which of the four Outputs to be selected, and buttons to place the Converters in or out of control of the Converter Rack Manager labeled 'CMx Select' in a frame with an indicator for the Converter control status.  Indicators for the IF3 RF power level after the Attenuator show a voltage output from a detector for each Converter.  This voltage is exponentially proportional to the RF power level, a voltage of 0.1 volts is approximately -40 dBm, 1 volt is approximately -30 dBm and 10 volts approximately -20 dBm, better accuracy can be obtained from the individual module detector's calibration data.  Between the two Converter signal paths and indicators is a box with parameters for placing synthesizer 'Gx' in or out of rack control, an indicator for the state of the rack control and entry parameters for the output frequency and power level fed to the IF2 mixer in the Converter Modules.  Note that the negative RF power reading in the CM5 module is outlined in red, indicating that this reading is outside of the normal operating range.  In the case of the RF power reading, the negative reading is caused by a small offset and indicates there is no RF power in the channel.
 
 

Blue Buttons:
On each of the screens for specific modules there are blue buttons next to function selections that will open the IF Manager screen information box on that particular function.  On screens controlling all the particular modules the blue button opens a sub menu from which a specific input, output or function information screen can be chosen.  Information such as where the signal on that particular input or output is coming from or going to, frequency, bandwidth, polarization of that signal and the manager controlling that function is available on a screen like this one:
 


 
 
 

All Modules Tab:
 


 

Clicking the All Modules tab allows the same access and control to all Converter Modules that the four Converter pair tabs allow to the pairs of Converters but without access to the information and control of the synthesizer parameters.
 
 
 

All Synthesizers Tab:
 

Clicking the All Synthesizers tab allows the user to control the frequency and output power levels of all eight synthesizers similarly to the control over individual dedicated synthesizers in the synthesizer box on the Converter pair screens.  Frequency resolution is 1 kHz, level resolution is 0.1 dB, but it is recommended that the RF level be set for +10 dBm for best operation.
 

Power Supplies Tab:
 

The power supplies tab gives the user the power supply voltages and indications if LO3 is faulty.
 

 Online Technical Documentation Link

 

 


Copyright 2000 Associated Universities, Inc. Washington D.C., USA
Modified: 23 October, 2001 by Ronald J. Maddalena