GE BRIVO MR355, Service Training, Page: 149 / 227

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1. The Mega Switch provides the RF routing circuit which routes the four RF inputs to one RF
outputs. The main functions of the RF router block are as following.
• Provide the receive Interface of RF signal
• Route the RF receive signals to proper down conversion channels.
• Provide protection circuitry at each RF signal input to protect the components from being
damaged by the transmit RF signal during scan, ESD(Electronic Static Discharge) in
installation and other occasions.
2. Loopback interface is used to distribute loopback signal from Exciter to all receive chain
for R1 calibration and diagnostics.
•
R1 calibration is very important in SV system. It is performed when resetting TPS and
rebooting the system every time. Loopback signal is split into eight separate but equal
signals and are sent to each receive channel via SPDT(Single-Pole Double-Throw)
switches in the RF router, then to Mixer Module for amplifying and mixing, then output to
RRX.
• Meanwhile, system will set the R1 value from 13 to 1, and send them to RRX via DVMR
link. Then RRX sends the R1 data to Mixer Module in Mega Switch. Eight Mixer Modules
amplify or attenuate the loopback signal according to different R1 value.
•
In VRE, system will check the signal power of every channel when R1 value is from 1 to
13, if the signal level is in the specification, R1 calibration is successful and system can go
to next step. But if it fails, system will report R1 calibration failure and stop.
• If RF disable switch is located in disable mode in Exciter, system will not get loopback
signal during TPS resetting rebooting, this will cause system fail.