Studying Behavior of an EEG Device
A device to apply deterministic voltages is fairly easy to achieve. 

As an EEG needs to be very sensitive, you really need battery powered, which will isolate you from ground loops.

I suggest a 3v source, say 2xAA batteries or a CR2032 if you want it smaller, to drive an HC14 relaxation oscillator. This form of oscillator has very low power consumption, at least at low frequencies.

A hex chip contains 6 inverters, from which you can make 6 independent oscillators. Set them by choice of R and C to 100Hz, 7Hz or whatever you need. 

Each will give you a 3v square wave at the inverter output, or a roughly 1v triangle wave at the capacitor. The inverter output voltage will faithfully follow the battery voltage, which you can measure with a DVM if you need to know the amplitude of the signal very accurately. The triangle wave will not follow the battery voltage, so is less useful for accuracy.

Now divide down these high voltages with resistive dividers, until you get the mV, uV or nV that you need for the EEG input. To get 3uV from 3V, a 10^6:1 divider is needed. You could use 10meg series with 10 ohm. Any noise generated by the 10meg resistor must be compared against the 3v signal that is dropped across it, a negligible effect. If you want to use smaller resistor values, then you could cascade several (say) 10k:10ohm pairs, each getting 1000:1 ratio. The tolerances of these resistors are largely irrelevant. If each was 1%, that gives you 2% on the ratio, so outputting 3uV +/- 2%. 

If you use an EEG receiver to instrument a real human head, then there are a number of noise sources with which you must up put. EM fields from mains wiring and radio signals. DC contact potentials caused by differences in body chemistry and contact material. Johnson noise from resistors. All of these will be present as well when using a battery powered signal generator as I describe, no worse, in fact the body contact one should be better. 

If your EEG recevier has a low enough noise level to pull a 100nV signal off a human head, it will do the same with a 100nV signal from a calibration signal generator. If it hasn't, it won't.