CHAPTER 11: RF BYPASS OPERATION
MDU Solutions
®
– DVIS/DVISm - Digital Video Insertion System & Mini Digital Video Insertion System - Installation & Operation Manual
11-1
RF BYPASS OPERATION
11. RF Bypass Operation
An RF bypass feature is integral to the hardware and is designed to react to a power outage or to the unlikely occurrence
of the failure of the internal QAM modulator. See
“Simplified Block Diagrams” on page 2-4
Bypass switches. The operation of the bypass ensures that the MDU or other facility will continue to receive the cable system
QAM channel content despite hardware or power failure. RF switches are installed on either side of the channel deletion filter
which allow the filter to be switched from the active RF path if required. There is a switch on the output of the QAM modulator
as well which removes the QAM RF output from the combiner network. The RF switches are all fail-safe and have a power off
rest state, which is the bypass state. The QAM modulator switch terminates the modulator RF output in the bypass state. All
3 switches always act together.
11.1 RF Bypass Operation
An RF switch controller monitors the RF output level of the built-in QAM modulator and controls the state of the bypass
switches.
There are 3 conditions where RF bypass will occur:
•
In the event of an internal QAM modulator failure, where the RF output level drops below a pre-determined threshold.
•
In the event of the loss of 115/230 VAC power or a failure of the internal power supply module.
•
During unit power-up (for 90 Seconds).
The RF bypass will, during a power failure or detected QAM modulator output failure:
•
Bypass the channel dropping filter from the input RF path restoring the original QAM channel from the cable plant.
•
Terminate the built in QAM modulator within the DVISm unit.
11.2 Power-up/Power Restoration
When power is first applied or restored following an outage, the RF switches will be in the bypass state and the bypass
controller will not restore the normal state of the switches for 90 seconds while the unit boots up. When the boot up cycle is
complete, the controller will restore the QAM channel deletion filter to the active RF path along with the internally generated
QAM carrier.