I want to build a meter tester for testing voltmeters and frequency meters.
My first thought was to use a 120v single ph VFD but they are few and far between and expensive. I'm also not sure if the serial on them would cooperate with Arduino.
Does anyone have a better way to adjust frequency and voltage separate from one another?
What parameters are you testing for? Accuracy is a phantom that will be very expensive to chase. If you want to test for various frequencies, get an audio generator or a function generator. Even used, certified, test equipment is very expensive, then need to be certified periodically.
An Arduino can output square waves at various frequencies. You'll need a trusted frequency meter to control the real output frequency of the Arduino.
Output of a voltage requires a DAC, not normally included in an Arduino. You can use PWM and a low pass filter for creating somewhat stable DC voltages. Again a trusted volt meter is required to control the real output voltage of that circuit.