Setup of Gain Control System (MGC/AGC)

Transcription

1 Setup of Gain Control System (MGC/AGC) Comark Optimum and Ultimate ATSC Transmitters This service bulletin provides the procedure to properly align the manual gain control (MGC) and automatic gain control (AGC) functions in Comark Optimum and Ultimate solid-state ATSC Transmitters. Normally, it is not necessary to modify the setup of the transmitter gain control system once the installation is complete. However, should major exciter subassemblies be replaced or the correct set up of the gain control system be in doubt, it may become necessary to perform a complete realignment of this system on-site. CAUTION: The procedure contained below is for advanced users only and should not be attempted without a full understanding of the theory of operation of the transmitter gain control system. Incorrect set-up of the gain control system could create the potential for serious damage to the transmitter amplifiers resulting from RF overdrive conditions. MGC/AGC Simplified Theory of Operation The transmitter gain control system uses the principal of negative feedback to maintain a steady operating power despite changes in amplifier gain due to temperature shifts or other component drift. (see Figure 1) Each power amplifier (PA) module has a built-in RF detector that provides a DC voltage output proportional to the module RF output power level. These AGC voltages pass through the modular power supplies and are collected at the multiplex card(s). The AGC voltages from the individual PA modules may be read on the local control panel and typically average about 2V. A diode-or circuit formed by the parallel connection of the emitters of the final buffer transistor in the AGC output line of each PA module ensures that only the highest module AGC voltage is passed to the next processing stage. Note that the diode-or action is shown as actual diodes in Figure 1 only in the interest of simplicity. Depending on the transmitter vintage, the highest AGC voltage from the multiplex card(s) will either pass directly to the AGC input (J17) on the ADAPT exciter or be compared to other AGC samples in the A300 System Metering Assembly (P/N ). When present, the A300 System Metering Assembly compares the highest module AGC voltage with additional AGC voltages derived from RF samples taken at the exciter output and the final system output. The system metering assembly selects the highest AGC voltage according to a diode-or selection rule. This voltage is then passed to the AGC input (J17) of the ADAPT exciter. The diode-or selection is such that the transmitter gain control system will only track the highest AGC voltage coming in from the PA cabinet. For transmitters without the system metering chassis, this always corresponds to the PA module with the highest AGC voltage level (i.e. highest gain and power level, assuming AGC potentiometer set correctly). This is appropriate, as the PA module with the highest gain is the most susceptible to overdrive conditions. If this PA module should be removed from service, the diode-or function automatically switches to the next highest AGC voltage, and the module with the next highest gain will seize control of the AGC system. Because the AGC system maintains the gain and output level of the individual PA modules approximately constant, the power will drop each time a PA module is removed. The amount of power drop is given by the following equation. Power = (PA modules in service / total PA modules) ^2 x 100%

2 For example, an eight PA module transmitter with one PA module removed will fall to approximately Power = ( 7 / 8 ) ^2 =.77 or 77% When present, the A300 System Metering Assembly seeks to prevent this drop (within reason), by tracking a AGC voltage derived from an RF sample at the total system output. (see attached drawings and ) Under normal operation, this AGC voltage from the total system output sample (J7) is adjusted via potentiometer R9 to be slightly higher than the PA module AGC voltage coming in from the multiplex card (J13). The diode-or function (emitter of Q1) ensures that the gain control system will track the total system output power instead of the individual PA module output powers. As PA modules are removed from service, the total system output AGC voltage will drop and the AGC system will increase exciter output power to compensate. The power output of the remaining PA modules will increase as each additional module is removed. Eventually, the power output of the individual PA modules reaches a point where the PA module AGC voltages exceed the total system output AGC voltage and the diode-or switches over to tracking the PA module AGC voltages (Q1 turns off). At this point, the mode of AGC operation switches to that described previously for the systems not equipped with the system metering assembly, and the power will drop as more PA modules are removed. The degree to which the total system output AGC voltage is adjusted to exceed the highest PA module AGC voltage (under normal operation) determines how much AGC boost the system will apply before switching to the module-limited mode of operation. CAUTION: The total system output AGC described above (with the A300 System Metering Assembly) may only be employed on those transmitters operating below the nominal transmitter rating (e.g. 5K00 transmitter operating at 2.5 kw). In is not advisable to increase the drive to PA modules in transmitters already operating at full nominal power in an attempt to compensate for removed modules. Incorrect set-up of the gain control system could create the potential for serious damage to the transmitter amplifiers resulting from RF overdrive conditions. An additional level of protection is provided by the AGC voltage derived from the on-air exciter output sample (DHY1) (J9). The exciter output AGC voltage is typically adjusted via R29 to be slightly below the PA module AGC voltage (Q2 off). This gives the exciters freedom to increase their output without being restrained by the AGC system, but will prevent excessive overdrive should an intermediate stage fail. If an exciter output level were to rise unexpectedly, the corresponding exciter AGC level would seize control of the AGC system (Q2 turns on) and prevent further increases in power. NOTE: The procedure contained in this bulletin will not make use of the exciter output AGC described in the preceding paragraph. Rather, damage due to exciter overdrive will be prevented by other means. The AGC voltage from the multiplex board or system metering assembly (where applicable) passes through connector J17 to the CUDC module in the ADAPT exciter. This voltage is scaled in the AGC potentiometer and then inverted, such that an increase in AGC voltage input causes a decrease in the RF level command to a variable attenuator in the RF signal chain. The exciter control system has the ability to modify this RF level command voltage according to actions taken through either the ADAPT Control software or the transmitter CPU control system (power level adjustment/reduction commands). When the AUTO gain mode is selected via the transmitter control panel, the RF level command voltage is passed directly on to the variable attenuator. When the MAN gain mode is selected, an alternative RF level command voltage is obtained from either the exciter control system or the local MGC potentiometer. While in MAN gain mode, the exciter still maintains an overdrive protection via a clamping diode connected to the AGC system. If the RF level command coming from the MGC system becomes too high

3 relative to the AGC voltage (approx 2.5dB greater), the clamping diode conducts, thereby restraining the MGC voltage. For this reason, the AGC potentiometer will still act to limit the maximum power when the exciter is in the MAN gain mode. Figure 1. Simplified block diagram of transmitter gain control system. Figure 2. Photograph of A300 System Metering Assembly P/N

4 Procedure : MGC / AGC Setup Procedure ATSC Solid State Applicability Prerequisites Equipment Required Comments Optimum ATSC, Ultimate ATSC All transmitter subassemblies functioning correctly. ADAPT Control software installed and connected to exciter. Spectrum analyzer 50 ohm RF cable PC with ADAPT Control software RS-232 null modem cable Average power meter Generally required only during initial turn-on, for major changes in TPO, or replacement of major subassemblies affecting AGC. Procedure is for the complete gain control system setup and is closely based on the procedures employed in the Comark factory for initial transmitter turn-on and test. 1. Press transmitter OFF button on transmitter local control panel. 2. Remove RF drive to amplifiers by removing output drive cable from J23 at rear panel of ADAPT exciter(s). Remove all attenuators from J Locate AGC and MGC potentiometers on exciter CUDC module. Set exciter output power to maximum by turning both potentiometers fully clockwise. 4. Press transmitter ON button on transmitter local control panel. Verify presence of output signal from CUDC by checking RF monitoring port on front of the ADAPT preamplifier module with spectrum analyzer. 5. Using ADAPT Control software program, send command Compute > Max Cudc. Verify that reply message states that AGC range is OK. The digital power control system within exciter is now properly calibrated to true power output capabilities of the unit. Issue Copy > Current_cs Restart_cs command to save the computation. 6. Repeat steps 1-5 for second ADAPT exciter, if installed. 7. Switch transmitter to manual gain mode by issuing following commands via transmitter local control panel: CONTROL > CONTROL NEXT > CONTROL NEXT > GAIN = MANUAL GAIN (colorized screen) or CONTROL > CONTROL LEVEL 2 > CONTROL LEVEL 3 > GAIN MODE = MAN GAIN. (monochromatic screen) 8. Ensure operator power level control is set to 100% by issuing following commands via transmitter local control panel: CONTROL > CONTROL NEXT > RF LEVEL > REQUIRED POWER > cur value = 100% (colorized screen) or CONTROL > CONTROL LEVEL 2 > REDUCT = 0dB (monochromatic screen). 9. Set exciter output power to minimum by turning AGC and MGC potentiometers fully counterclockwise. Press transmitter OFF button on transmitter local control panel. 10. Re-install attenuator pads at ADAPT exciter output(s) at J23. Attenuator pads installed at installation should be approximately those listed in Table 1. Reconnect appropriate exciter output cable to J23 output(s).

5 Table 1 ADAPT output attenuation Transmitter power 2KW or less ADAPT attenuation at J23 10dB > 2KW to < 3KW 7dB 3KW to < 5KW 5KW to < 12KW 6dB 4dB > 12KW 0dB 11. For systems with an A300 System Metering Assembly: Disconnect and tie back RF cable to A300-J9, and adjust potentiometers R29, R41, and R9 fully counter-clockwise. This will ensure that total system output AGC and exciter output AGC voltages remain inactive during this stage of the setup procedure. The cover of system metering assembly must be removed to have access to these adjustments. The A300 chassis is typically located in lower, left rear of PA cabinet in liquid-cooled transmitters and behind a rack filler panel below the transmitter local control panel in air-cooled transmitters 12. Press transmitter ON button on transmitter local control panel. Slowly adjust AGC potentiometer clockwise, verifying that no power output is detected, until potentiometer is at maximum clockwise position. CAUTION: Although mechanically similar, the CUDC modules used in Comark DCX, Advantage, and Affinity transmitters are modified such that the local MGC potentiometer is disconnected (set to maximum). Do not attempt to use a CUDC module from these transmitters in an Optimum or Ultimate transmitter. Cease this procedure and contact Comark if you observe output power at this stage (i.e. with MGC potentiometer still fully CCW) and do not have local MGC adjustment capability. 13. Slowly raise output power using MGC potentiometer until power is ¼ ( 6dB) of rated system output. Check all amplifier status screens, and verify that readings are at their nominal values. Also check amplifier AGC readings to see if they are all close to each other (at least within one volt). Let the system run at this power level for approximately five minutes. NOTE: Full power in this procedure is the nominal nameplate rating of the transmitter, not the desired station TPO, for those cases where reduced power operation is anticipated. 14. Increase system output power to approximately 50% of rated output. Re-check amplifier and AGC readings. If OK, increase power to rated output level. Again, re-check amplifier and AGC levels. Let the system run for 15 minutes. 15. This would be a good opportunity to check power meter calibrations with either calorimetric methods or a true average power meter.

6 16. Slowly rotate AGC potentiometer counterclockwise until power level begins to drop. Reduce power by 2dB using AGC potentiometer. This indicates that overdrive clamp is restraining MGC system and ensures that AGC will not overdrive transmitter when AGC mode is enabled (next step). 17. Switch transmitter to automatic gain mode by issuing following commands via transmitter local control panel: CONTROL > CONTROL NEXT > CONTROL NEXT > GAIN = AUTO GAIN (colorized screen) or CONTROL > CONTROL LEVEL 2 > CONTROL LEVEL 3 > GAIN MODE = AUTO GAIN. (monochromatic screen) 18. Slowly increase system output power back to full using AGC potentiometer. 19. Repeat previous steps for second ADAPT exciter, if installed. It is not necessary to let system run an extended length of time at the lower power levels with second exciter. 20. Adjust all amplifier AGC levels to be within.05v of each other. The AGC absolute level is dependent on amplifier type, cabinet power level, and channel frequency. 21. Using spectrum analyzer connected to RF monitoring port on ADAPT exciter preamplifier, establish reference for nominal exciter output power during 100% operation (1dB/division scale), using averaging and/or narrow video bandwidth to obtain a stable reading, as necessary. Save this trace on the spectrum analyzer screen, if possible. Figure 3. Exciter nominal output level on spectrum analyzer (1dB/div. scale) 22. Press transmitter OFF button on transmitter local control panel. CAUTION: Never attempt the following step with the transmitter in the ON state. This could create the potential for serious damage to the transmitter amplifiers resulting from RF overdrive conditions. 23. Disconnect RF drive to PA modules by disconnecting RF input to directional coupler DHY1. This is the directional coupler immediately after the exciter switching coaxial relay in dual drive transmitters. It may be found between the exciters by opening the rear doors on air-cooled transmitters. It may be found behind an access panel just below the local control panel on liquid-cooled transmitters. Be sure to disconnect the input side to DHY1, the side with the RF cable coming from the coaxial relay (or exciter in single drive transmitters).

7 NOTE: If transmitter does not have DHY1, disconnect RF output from both exciters (where applicable) by disconnecting J23 from the exciter backplane(s). 24. Press transmitter ON button on transmitter local control panel. With no drive applied to PA modules, no AGC voltage will return to exciter. Exciter will respond by increasing its output power to maximum possible in an attempt to compensate for missing gain. Exciter output sample may jump anywhere from 5 to 20 db on spectrum analyzer. 25. Add attenuator pads to J20 at rear panel of exciter to decrease input level to ADAPT preamplifier module. Add attenuation until exciter output level on spectrum analyzer is only 1 db above reference level recorded earlier (nominal output at 100% power). This ensures that exciter can never overdrive transmitter by more than 1dB over nominal, should the AGC sample cable ever be disconnected or the AGC system otherwise fail. This will protect PA modules and RF system components from catastrophic failures due to overdrive. Figure 4. Exciter maximum output level compared to (stored) normal output level on spectrum analyzer (1dB/div. scale) 26. Add same amount of attenuation to second exciter, where applicable. Repeat steps 21 through 25 to ensure that overdrive capability on second exciter is properly limited to 1 db. 27. Press transmitter OFF button on transmitter local control panel, reconnect RF input to DHY1, press transmitter ON button, and return to 100% power operation. 28. Procedure is complete for those transmitters that will operate at full rated power. 29. For those transmitters that will operate at a reduced TPO from the maximum rated power and do not have an A300 System Metering Assembly, reduce transmitter power to desired TPO in MAN and AUTO gain modes by adjusting MGC and AGC potentiometers, respectively. Recalibrate power level display to reflect reduced TPO, as necessary. Procedure is complete. 30. For those transmitters that will operate at a reduced TPO from the maximum rated power and do have an A300 System Metering Assembly, invoke total system output AGC by adjusting potentiometer R9 in A300 assembly clockwise until power drops to desired TPO. Verify that total system output AGC is working by removing a PA module. Exciter output level should increase in an attempt to compensate for missing PA module. Amount of compensation possible will depend on how much power was scaled back via R9 to achieve reduced TPO. (more back off = more boost possible). Re-calibrate power level display to reflect reduced TPO, as necessary. Procedure is complete.

8 CAUTION: If the total system output AGC is being employed, never use the AGC potentiometer on the exciter CUDC module to make power adjustments. The adjustment of power via the AGC potentiometer could create the potential for serious damage to the transmitter amplifiers resulting from RF overdrive conditions. The digital power level set function, accessed via the local control panel, should be used for all power level adjustments. For large, permanent adjustments to transmitter power level (e.g. in response to a licensed TPO change), potentiometer R9 of the A300 System Metering Assembly should be readjusted. Please contact the Comark Customer Service Department (800) for any additional information you may require on this subject. Comark Communications LLC 104 Feeding Hills Road Southwick, MA U.S.A. (800) Attn: Customer Service Attached drawings: Americanized System Metering Assembly (Americanization) Americanized System Metering Assembly (Americanization) System Interconnect (Water 1200 Americanization Kit)

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