Loopbacks

What's the difference between a Satellite Loopback and a Terrestrial TX/RX loopback? I know on most FDMA modems there is an option for a Terrestrial TX/RX loop, and for a Sat Loop you change your demod freq to match the mod freq.

But both test your system/data integrity all the way out to the satellite and back right?

I'm terribly confused...

For example, a Comtech CDM570 modem has a number of test loopback modes (see the modem manual for detailed circuit block diagrams):

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. (never had to do this !)

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 requency, rather like an artificial satellite. e.g. 14-14.5 GHz in and 10.7-11.2 GHz out (or 950-1450 MHz out).  I've a couple of 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.

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.  

As Greg says, loop testing is valuable way of diagnosing problems and identifying where the problem is located.

Best regards, Eric.