> Thanks!  Using the transformer sounds like a real workable solution.

Should work well in many cases - but see below.

The FET solution below allows you to handle very high DC currents at a 
modest cost. The transformer solution is arguably more elegant but 
transformer saturation with DC may be an issue. As noise effects may 
change with DC load, being able to simulate high load levels may be 
useful. So -

Another possible approach is a MOSFET in series with the supply. 
Modulating it's gate takes about zero power and a pot divider (or 
variable sig gen output) would allow any degree of modulation desired.

If you put the FET in the ground return you can probably drive it 
directly from the sig gen. If it's on the high side you need to float 
the sig gen or AC couple the signal. Capacitive coupling would work 
fine for this as the impedance is very high.

In either case you could provide a DC gate offset with a pot and a 
battery and then add the AC signal to that so that the FET is never 
fully cut off. A 1 meg pot across a 9 volt battery with the FET gate 
on the wiper and one end of pot to FET Drain plus AC capacitively 
coupled to the wiper should work well. Battery will last indefinitely. 
Opto-coupler coupling also an option.



        Russell


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