Short Circuit Protection for signal generators

[technoqwerty123]

Hello,
How is output short circuit protection scheme implemented in a standard signal generator with frequency range upto few MHz?

Regards

 

[FvM]

Signal generators with MHz frequency range mostly implement 50 ohm impedance matching and are typically short circuit resistant. Additional protection, e.g. aigainst reverse power, may be provided by output relays.

 

[technoqwerty123]

Hello Sir,

...are typically short circuit resistant.

How is this accomplished from a circuit point of view? I have seen lab signal generators fail after an accidental output short and wish to implement an add on short circuit protection feature to prevent future failures.

Regards

 

[FvM]

How is this accomplished from a circuit point of view?

By designing sufficient power rating for output transistors and 50 ohm series resistor.

If you don't need to grant load impedances below 50 ohm, you can implement current limiting and optionally thermal overload protection for the output transistors, e.g. by monitoring the heatsink tempearture.

 

[dick_freebird]

If the output is source (series) terminated then it's a simple matter of sizing
for the power level. If it's shunt terminated then the output driver is exposed.

A simple thing would be to put a "pad" (attenuator) on the output, one whose
series R suffices to protect. But this will cost you on the upper end of output
amplitude.

 

[RCinFLA]

Unless it is a high power output generator, generator with output in the +20 dbm max. or less need no short circuit protection.

As previously mentioned, if generator is considered a test set for a transceiver, like a cellphone, there may be protection from accidentally keying the transmitter with a couple of watts of RF power back into the generator which can damage in output step attenuator (which typically can tolerate a maximum of half a watt of reverse power injection).