IF loop: This command temporarily reconfigures the receive parameters to match the transmit parameters and internally connects IF out to IF in. Useful to verify that the modem is OK. You don’t need to touch the external connectors.
RF loop: This command is similar to IF loop but does not connect IF out to IF in. You need to make the loop physically yourself, e.g. at the far end of the IF cable network. This is a good test for the connectors on the IF cabling.
There are two internal loopback commands.
I/O loop: This loops the data interface in both directions. Data sent into the terrestrial interfaces comes back out. Data received from the satellite is send back to the satellite. Good for diagnosing if the problem in on the terrestrial side of the modem or satellite side related. If done at a remote site the hub can verify that everything is OK right up to the modem terrestiral interface. Note that the hub may need to reduce bit rate so that the return carrier may be accommodated in the satellite transponder.
Digital loop: This command loops back the modem after framing (and Reed Soloman coding) but before main FEC coding. This would be useful if you suspected an internal modem fault.
Additional loop tests not involving modem commands:
Loop test translator: If you have a “loop test translator” this enables you to downconvert a sample of the BUC output back to a receive frequency, rather like an artificial satellite. e.g. 14-14.5 GHz in and 10.7-11.2 GHz out (or 950-1450 MHz out). The 14GHz to L band versions and they are very useful for testing BUC/HPA outputs if you don’t have a 14GHz spectrum analyser. They function like an LNB with LO=13.05 GHz LO frequency.
Ref: Radyne LTT6400/A03 C-Band Loop Test Translator
Satellite loop back: If your uplink and downlink are in the same beam then you can monitor your transmit carrier with a receive modem. Many VSAT hubs will have a test remote VSAT co-located for such test purposes.
Ref: http://www.satsig.net/cgi-bin/yabb/YaBB.pl?board=any1;action=display;num=1271640292
Testing modem A with Loopback(Near) from Modem B
Testing modem A with the Satellite
Testing modem A without the Satellite
In operating condition, both Loopback are disable.
