Voltage and power sources in a circuit simulator

V_IN is really the voltage at the input of the circuit the generator is hooked to, and is NOT the generator voltage V_G . V_IN differs from V_G by Z_GI_IN which accounts for voltage drop across the generator internal resistance Z_G . Z_G can be complex although typically real in common simulations. In ADS power sources for single tone or sweeping simulations in the frequency domain are found under "Sources-Freq Domain" and either the P1_tone or the P_AC palettes. By default both power sources include a Z_G=50 Ω internal resistor that is schematically represented in the symbol:

Fig. 1

When the circuit input impedance (i.e. the load seen by the power source) is matched to the generator impedance Z_IN=Z_G (which is real or in case of a conjugate matching condition), then the power delivered to the matched load is maximum and equals P (or Pac) in the ADS symbol. Since:

(1)

then V_G and V_IN can be directly extracted from P. Also, because the power delivered to the input of the controlled circuit is:

(2)

then

(3)

For the typical case of internal impedance Z_G equal to system reference impedance Z₀= 50 Ω and under a 1W signal, V_IN and V_G are 10V and 20 V, respectively. It should be noticed that V_G depends on the source internal impedance but NOT on the matching condition, i.e. even away from the conjugate match the source is still set to supply the voltage V_G in (3). V_IN depends on the matching condition as well and away from the conjugate match it is not necessarily provided by (3) since (1) no longer applies.