MEMORANDUM

In attendance: Fisher, Ford, Lacasse
 

Minutes of Previous Meeting

    Approved.
 

Call Outs for Interference

      None.

Action Item Review

         1. Measurement of OVLBI transmission field strengths at the GBT.

A schedule was received from Minter.  Need to gather equipment and try a test measurement.

In Chuck's absence, Rick reported that Chuck was having problems with the resistive power combiner for the two transmitters.  Intermod products are produced when both transmitters are on.  Several options are being considered to remedy the situation.

Pole setting for the monitor station has been postponed twice.

The continuum detector system for the anechoic chamber was tested.  First a modulated carrier was run through the system to show that the detector system could properly detect and filter it.  Second, two copies of a wide white noise bandpass were generated.  One was attenuated by 10 db and the other by 9.  These two signals were run through an RF single-pole double-throw switch whose switching state was controlled by a TTL signal generated by the continuum detector PC.  The output of the switch was detected and filtered by the detector/filter unit and sampled by the acquisition board in the PC.  The PC was then able to display the acquired waveform and calculate the SIG/REF quotient.  The quotient was in error by about 12%.  This may be due to bad calibration of the pads (they were not checked) or to large switching transients.  A blanking option will be added to the software and the experiment repeated more carefully.

The factory claims to have never received the enclosure that was returned to them due to shoddy workmanship.  Ford will trace the package.

A second enclosure is required for the cafeteria.  Another vendor, AAMCO will be used as supplier for this one;  AAMCO has supplied shielded racks for several systems at NRAO and their quality is expected to be better for the same price.

Some 7 GHz absorber was ordered.  This will allow for the construction of a 10:1 scale model.  There was some discussion about how to handle the floor of the serpentine entrance.  As a first cut, a metal floor will be tested.  Alternatively, plywood over absorber can be tested.

Ford will work with the laser group to shield their control unit enclosure.  Most signals in and out of the control unit are low frequency so should be easy to shield.  The 1500 MHz reference also must come out of the unit.  It was recommended that this signal be well shielded, both inside and outside the enclosure, and bandpass filtered as it leaves the enclosure to keep internally generated noise from following this conductor out of the enclosure.

Eric Knapp is a 140-ft. operator who has recently transferred to the lab to prepare for his new position as GBT operator.  He has been designated the "RFI expert" operator, and part of his training will entail working with the IPG group.  One possible task for him is to work on a triangulation system to locate RFI with the monitor stations.  Other possibilities include setting up and making RFI measurements.

No progress.  The project is still looking for money and and a site.

Fisher explained that this plan is to spell out the details of the installation of the monitor station on the GBT.  This will be used by McKinnon for coordinating the commissioning effort.

        11. Lab RFI suppression efforts (Rick)

        12. Interference at 1612.17 MHz

        13. RFI monitor for Education Center

All of these are on hold.

For continuum measurements, it is desirable to switch the DUT on and off to look for turn-on transients.  After some discussion, it was decided to design a switch when we have a device in hand that needs to be switched on and off.  This switch will probably take the form of an optically-isolated, zero-crossing solid-state relay, with I/O well filtered.   Lacasse will check on the feasibility of using a power supply test load built by Schiebel as a test load for an AC power switch box.

On hold.

Next Meeting: July 6, 0800.

file: mt062299.htm