Converting to digital II
Dec 7, 2008 12:15 PM, By Russell Brown
One problem with single IOT transmitters is that some are not configured to be able to direct the output into a station load before the mask filter — without this, some tuning problems can be difficult to diagnose because the mask filter will “mask” the true output of the IOT. It’s always better to be able to look at the output of an IOT directly into a station load to examine the true output of the tube.
Figure 2
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Magic-T combiners are always built for a certain channel and are not capable of being used on a different frequency — not even a single channel change. The paddles within the magic-T, as well as the cavities, are shaped and sized to shift the phase of the signal passing through it. If the frequency is different than what it was designed for, the phase shift will be incorrect and the combiner will not work.
Other frequency-dependent components found in the transmitter’s high-power amplifiers (HPAs) are driver amplifiers, circulators and even some directional couplers. These must all be examined to ensure they will work at the new frequency. The sampling probes for forward and reverse power use low-pass filters (because they are inserted at the tube’s output before any other filtering) and should not be an issue with a single channel change.
Of course, the analog exciter will be removed, the 8-VSB exciter will need to have its output split and one of the outputs will need a variable attenuator placed between it and one of the amplifier cabinets. This will allow for balancing of the two amplifiers, so they will put out the same amount of power. If the two were being combined, a tuner (trombone) would also be added to match the phase of the two amplifiers and gain maximum power output by reducing the reject power on the magic-T combiner.
Control
When combining any high-power systems to work together, controlling them is of prime importance. By combining the control systems, a single button can take care of an emergency changeover, making it much simpler for operators as well as sleepy engineers in the middle of the night. Sometimes this is not possible, such as when the two systems are not from the same manufacturer or one system is much older than the other, but even this can be overcome to some degree with certain transmitter remote control systems that allow for complex operations to be carried out.
Figure 3
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KMTP’s DTV transmitter uses a central GUI controller that’s capable of controlling several HPA cabinets, but the analog TX is manually controlled. (See Figure 2, above.)
The analog amplifier cabinet will need to be upgraded to enable it to work with the DTV GUI controller; this mainly involves adding an I/O interface module and connecting it to the amplifier’s control and status lines. The I/O interface module is then connected to the GUI controller’s communications network. The transmitter manufacturer will then need to reprogram the GUI controller to work with the added cabinet. The DTV GUI controller already controls the common cooling system for both cabinets as well as the RF switching for digital (antenna or dummy load). There are several advantages to creating a unified system, but the station budget may not allow it, in which case the added cabinet will be treated as a separate unit. (See Figure 3.)
Acknowledgments
Todd Loney of Richland Tower, Michael Boyle at L-3 Communications and Fred Stefanik and Ted Karam of Thomson contributed to this tutorial.
Next time
The next “Transition to Digital” will continue with digital conversion regarding the RF system.
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