[Radio Wrangler]

Laboratory type signal generators usually have a feedback amplifier with very low output impedance followed by a series resistor to create an accurately controlled output impedance of whatever is needed, 50/75/600 Ohms. This is then followed by switched attenuators usually in 10dB steps to allow coverage of low output levels. The drive level to the output amp is variable to allow interpolation of the attenuator steps, and the application of AM if needed.

Makers varied in how they specified the output levels. As a power level, dBm (decibels with respect to one milliWatt) the displayed figure only makes sense into a matched load. power into anything else can be calculated and included as a mismatch loss.

As a voltage, then some makers specified the EMF, the driving voltage at the input to an ideal impedance-setting series resistor. It's usually thought of as the off-load terminal voltage, but with such a mismatch, you may have to consider the effect of the mismatched transmission line length from the impedance setter to the connector if you're working at high frequencies. Some makers specified the PD, Potential Difference voltage, which would apply into an ideal impedance load.

This all created quite a bit of confusion to put it mildly. Modern gear with microprocessors usually let you switch between the different output level formats. EMF to matched PD is a simple 2:1 ratio so you can easily do it in your head. 0dBm is close to 224mV RMS into 50 Ohms, or close to 775mV into 600 Ohms but these test your mental arithmetic a bit more.

David