How can I use the solver FixedStepDiscrete when a Ramp Block and a Discrete Time VCO Block are connected?

Hi,

Actually I'm trying to simulate a GPS's signal and I have a problem with the Ramp Block. I have to connect a Ramp Block to a Discrete Time VCO Block, because the Ramp Block will simulate the change in frequency as a function Doppler Rate. When I start the simulation, I have this: Error reported by S-function 'sdspsigattrib' in 'GPS_Signal_Simulator/Discrete-Time VCO1/Check Signal Attributes/Check Signal Attributes': Continuous sample times not allowed. I don't know how can I resolve it. Someone can help me please.

Thanks