Retuning of FMIT #3 from 70.0 MHZ to 78.0 MHZ Charley Schwartz 10/28/02 DRAFT

Transcription

1 Retuning of FMIT #3 from 70.0 MHZ to 78.0 MHZ Charley Schwartz 10/28/02 DRAFT This document serves two purposes, an entry into the engineering records of the process and data accumulated during the tuning process and as a basis for the formalization of a procedure to standardize the transmitter tuning process. There are two tuning procedures for the HF transmitters which will vary in sequence and detail depending on the historical data available. When historical data is available for a previous tune the tuning parameters are set to a previous tune and adjusted to provide the proper operation. This tuning sequence does not perform tuning at low power levels. Some initial checks are made at low power levels and then the drive power is increased to the levels expected in normal operation. This allows the adjustment of the tuning elements at the resonant plate resistanances normally experienced at the normal operating levels. This tune process should take several hours once the testing apparatus is in place. When historical stat is not available for a previous tune the transmitter will be tuned from the signal source through the FPA in steps appropriate to the operating conditions found in each step. This is a very time consuming process requiring several days. General Sequence Analyze the previous tune data available from the logbooks etc. Gather the necessary test equipment. Calibrate the test cables. Generate a spreadsheet of the expected power levels from the directional couplers and the internal transmitter instrumentation. Gather adequate support staff to perform the tuning and provide an overview of the activity. Start the actual tuning process. o Initiate short low level pulses to identify the existing tune. o Tune the IPA to the desired plate impedance and transmission line match minimums. o Tune the Driver to the desired plate impedance and transmission line match minimums. o Tune the FPA to the desired plate impedance and transmission line match minimums. o Retune the IPA, Driver and FPA incrementally if needed. o Record the final tune digital panel meter displays and physical measurements of the tuning elements. o Test the transmitter with longer pulses and all the fast protection and monitoring circuitry operational. 1

2 Generate a tuning summary of the exercise and not any problems encountered. Initial Preparation Prior to Power Room Access Test Equipment Required Pulse Generator Digital Storage Oscilloscope (Fast) Digital Storage Oscilloscope Directional Coupler Type-N Connector Cables (2) Kalmus Output Directional Coupler <> Oscilloscope Cables (2) IPA Output Directional Coupler <> Oscilloscope Cables (2) Driver Output Directional Coupler <> Oscilloscope Cables (2) FPA Output Directional Coupler <> Oscilloscope Cables (4) BNC-BNC RG-174 or equivalent (Internal monitoring of transmitter metering) Cable (1) Gate generator to low level electronics rack Cables (3) BNC-BNC RG-58 or equivalent Pulse generator to Oscilloscope triggers The tuning procedure notes from a visit by Chris B. from the Princeton laboratory were reviewed and adapted to the current situation. The historical information available for tuning parameters was reviewed from Excel spreadsheets and logbook tuning notes. Actual tuning information for operation at 78 MHZ was not well defined. Several Excel spreadsheets were found which listed tuning information but some additional notes with the data indicated the information may have been the initial tuning information and not the final tuning information which was used during the experimental operations. Unique considerations for FMIT #3 & #4 Low Level Drive Controls The FMIT #3 and FMIT #4 systems operate at a common frequency, unlike FMIT #1 and FMIT #2 which can operate at different frequencies. The FMIT #3 and FMIT #4 system normally have their drive frequency controlled by a pair of Hewlett Packard frequency sources. The actual operating frequency is derived from the heterodyned difference in frequency from each source, e.g. Operation at 78.0 MHZ would be derived from the difference of one signal source at 30.0 MHZ and one signal source at MHZ... This process was disabled during the tuning process. One signal source was set at the desired operating frequency of 78.0 MHZ and gated to the low level amplifier chain. Tuning Procedure The tuning procedure is a continuing development of notes taken by David Terry during a visit by Chris Brunkhorst 04/23-04/ Comments have been added by C. Schwartz regarding concerns, problem identifications and enhancement needs. 2

3 General Considerations The tuning of the amplifier stages is an interactive process. The interconnections between each stage are through sections of 50 Ohm transmission line with a directional coupler in each line section (the directional coupler between the Klamus and the IPA is temporarily installed for tuning). In an ideal situation the output of the interstage transmission lines would be terminated into a 50 Ohm non reactive load. This would allow the output of an amplifier stage to be perfectly matched to the transmission line. The subsequent stage would then be tuned to the 50 Ohm transmission line resulting in near zero reflections on the transmission line. This is not practical in the daily operation so tuning will be accomplished to provide a reasonable match at power levels expected in normal operation by starting at a known reference position on the tuning elements and fine tuning the match at a given power level using short duration pulses. The plate (anode) load resistances of each amplifier stage will vary with power levels. The input impedances will remain more constant with varying drive levels. This will dictate that a perfect match is only available at one power level. Operation at different power levels will present different source impedances to the transmission lines resulting in a mismatch and reflections on the transmission lines. Warnings The test pulses must not repeat within thirty seconds to allow the operation of the RF analyzer in the event of a system fault. The anode and screen currents must be monitored to insure excess current conditions are avoided which could result component of subsystem damage. At high power settings and longer pulse width the capabilities of the dummy load must be considered. Initial Set-Up Hardware Required Filters Cables Calibration 3

4 Existing Measurements 70 MHZ The front panel digital panel meter and the actual physical measurements of the tuning parameters were measured prior to any tuning activity. Tuning Element Date: 2001/07/09-70 MHZ Previous Dim. Dig. Tun. Ind. Date: 2002/10/28-70 MHZ Previous Dim. Dig. Tun. Ind. in inches in counts in inches in counts IPA Input Tune 3 1/ / IPA Output Load 4 7/ / IPA Output Tune 2 2/ / Driver Input Load 15 1/ / Driver Input Tune 7 7/ / Driver Output Load 5 7/ Driver Output Tune 6 4/ / FPA Input Load FPA Input Tune 6 5/ / FPA Output Load FPA Output Tune 23 1/ / Chris B. Measurements C. Schwartz Measurements The above table represents the last FMIT #3 tune data that was available from the Excel spreadsheets from a tuning session on 2001/07/09 and the actual physical measurements performed on 2002/10/28 prior to the retuning to 78.0 MHZ 4

5 Initial Tune Mechanical 78.0 MHZ The tune information from a previous session on 2002/04/24 was used as a base po9int for the retuning to 78.0 MHZ using the digital panel meters. There were some discrepancies between the mechanical measurements when the tuning elements were set to the panel meter positions. (Note To reduce the mechanical errors from backlash in the tuning mechanisms all tuning was done by lowering the tuning element to a position with a lower digital panel meter display and then increasing to the desired position) Date: 2001/04/24-78 MHZ Tuning Element Previous Dim. Dig. Tun. Ind. Current Dim. Dig. Tun. Ind. Mechanical in inches in counts in inches in counts Discrepancy IPA Input Tune 2 3/ / /2 Inch IPA Output Load 3 7/ / /8 Inch IPA Output Tune 3 3/ / OK Driver Input Load 15 13/ / /16 Inch Driver Input Tune 11 15/ / /16 Inches Driver Output Load 4 9/ / OK Driver Output Tune 4 3/ / OK FPA Input Load 37 3/ / /8 Inches FPA Input Tune 5 3/ / /16 Inches FPA Output Load FPA Output Tune 24 1/ / OK The errors noted during the initial preset of the tuning elements are highly likely to be a mechanical issue with the drive hardware, the indicating elements and the electrical calibrations. The mechanical presets were adjusted to the previous dimensions and the digital panel meter displays recorded for the new positions. This will serve as the starting point for the retuning. 5

6 Mechanical Positions and Digital Panel Meter Displays Prior to Tuning to 78 MHZ The tuning elements were adjusted to the historical mechanical positions and recorded prior to applying power for tuning. The mechanical and digital panel meter displays will be recorded after the tuning is complete and adequate power is delivered to the antenna. Tuning Element Initial Dim. Dig. Tun. Ind. After Tuning Dig. Tun. Ind. in inches in counts in inches in counts IPA Input Tune 2 3/ IPA Output Load 3 7/ IPA Output Tune 3 3/ Driver Input Load 15 13/ Driver Input Tune 11 15/ Driver Output Load 4 9/ Driver Output Tune 4 3/ FPA Input Load 37 3/ FPA Input Tune 5 3/ FPA Output Load FPA Output Tune 24 1/ Initial conditions for 78 MHZ when adjusted to historical mechanical information Mechanical position after tuning at high power levels and successful tests. 6

7 Tuning Considerations - General Tuning Pulse Width The initial tuning pulse width should be adjusted to 20 microseconds. This will limit the activation of any of the internal overload relays while allowing for data collection of the interstage forward and reflected powers. Tuning Pulse Amplitude The initial tuning pulse amplitude should be zero. This should be increased until the IPA stage starts to display an increase in anode current. Tuning Changes No adjustments of the stage tuning should be exercised until some power is applied to the IPA and the Driver and FPA stages have been monitored to determine the proximity of the existing tuning positions to tuning positions which will deliver output power. Tuning Actions During this tuning exercise it was initially decided to make some measurements at low power levels rather than operate at higher power levels. This was done for two reasons, to reconcile the information regarding previous tuning exercised with respect to the effectiveness of the tuning elements and to insure adequate monitoring of system parameters were available to identify any problem areas. The tuning at low level was very limited in time. The matching between the low level RF (Klamus output) and the IPA appeared to be adequate to allow the IPA to draw anode current. The reflections on the lower level RF to the IPA were less than 10 percent. The decision to deliver drive power to the Driver was made. The initial tuning element to adjust was the Driver Input Load. Step Action Kalmus<>IPA IPA<>Driver Driver<>FPA (Driver Input Load) FWD REF FWD REF FWD REF NIL NIL Very poor match 2 Decrease to Improved 3 Increase to NIL NIL Very poor match 4 Decrease to NIL NIL Very poor match 5 Increase to Acceptable? 7

8 Once the IPA to Driver transmission line match started to look reasonable the next step was to match the IPA output to the transmission line using the IPA Output Tune control. Step Action IPA<>Driver Driver<>FPA FPA<>Load (IPA Output Tune) FWD REF FWD REF FWD REF N/A N/A Starting point 2 Increase to N/A N/A Decreased output 3 Decrease to N/A N/A Improved 4 Decrease to NIL NIL N/A N/A No Output 5 Increase to N/A N/A Better 6 Increase to N/A N/A Better 7 Increase to N/A N/A Decreased output 8 Decrease to N/A N/A Better 9 Decrease to N/A N/A Acceptable? This tune position appeared to be the best match at this power level. The low level drive was increased which resulted in a FPA Screen Overcurrent trip. The primary fuses of the power supply were opened. Problems with the crowbar chassis grounding lever prevented immediate repairs to the system. Once the FPA screen grid problem was solved the drive pulse width was reduced to a level shorter than the FPA screen grid protection relay s actuation time (< 10 microseconds). Subsequent tuning was accomplished at higher drive levels with shorter pulse durations. During the tuning the screen grid currents were monitored for changes. This was done by placing an oscilloscope in parallel with the meter movement. Calibration information for these measurements was not available so the numerical data is not presented here. The IPA to Driver tuning was thought to be the adjustment which required immediate attention. It was found that the IPA <> Driver tuning which resulted in the best match was: IPA Output Tune 3 3/ Driver Input Load 15 13/ The match between the Driver and the FPA appeared to be fine. The output power of the FPA appeared to be normal with the drive power applied so no further adjustments to the FPA were made. The system was tested into the dummy load and then into the vacuum. 8

9 Tuning Element Parameters after Tuning to 78.0 MHZ The digital panel meter readouts were recorded after the tuning session. The mechanical measurements wee not taken due to accessibility and the run schedule. Tuning Element Initial Dim. Dig. Tun. Ind. After Tuning Dig. Tun. Ind. in inches in counts in inches in counts IPA Input Tune 2 3/ IPA Output Load 3 7/ IPA Output Tune 3 3/ Driver Input Load 15 13/ Driver Input Tune 11 15/ Driver Output Load 4 9/ Driver Output Tune 4 3/ FPA Input Load 37 3/ FPA Input Tune 5 3/ FPA Output Load FPA Output Tune 24 1/ Initial conditions for 78 MHZ when adjusted to historical mechanical information Mechanical position after tuning at high power levels. --- END --- 9