Edit: I can force the input voltage to be 9V if I set the supply to 11Vish...Is that what I need to do? Is that normal?
Thank you,
Tyler
No, that is not normal. Shunt regulators like zener diodes regulate by maintaining the total current through both the zener and the load as a constant. That means that if the zener current is to be 20 mA when the load draws 30 mA then the dropping resistor, R2, must be calculated to drop the required voltage at that sum current. Then, should the load draw less current for some reason the zener will immediately take up that slack and draw up to the full 50 mA to maintain its regulation voltage. Should the load take more, but not more than 50 mA, the zener will take less current, down to zero. So R2 needs to be (9V - 5.6V)/50 mA value.
Shunt regulators are one of the most wasteful types of regulator as the current drawn from the supply is ALWAYS maximum regardless what the load is doing.
EDIT: Sigh, I just re-read what you are seeing on the bench and it looks like your signal generator has non-zero output impedance. In fact we can calculate just what output impedance it has since it drops from 9V with no load to 7.7V
with load of (7.7 - 5.6)/68 = 30.9 mA so is (9 - 7.7)/0.0309 = 42 Ohms.
One way to fix this is to get a better source. Or another way is to incorporate the generator source impedance into the total value of R2. That is, subtract the generators source impedance value from the 68 Ohms you calculated and put in a resistor for R2 at that resulting value. Then the sum of R2 and the Gens source impedance will be the required 68 Ohms and the circuit should operate as expected.