Virtuoso Envlp Analysis Problem with Thermometer Waveform as Input

Hi All,

I am a beginner in TX IC design. Now I am using virtuoso IC6.1.5.72 and MMSIM101 to simulate a RF modulator. I need to test its linearity and EVM. So I want to perform the envlp analysis. However I met a problem:

First I use the ppwlf to source my saved matlab ASCII data file, and then feed to a verilogA block written by myself, which transforms the pwl waveform to the thermometer coded waveform. I need it as switching input signal to my modulator(kind of switching PA). For simplicity, I just use 1LSB to do single bit test. However, the envlp results show that the voltage spectrum after my verilogA decoder is all 0 level. The pwl input signal spectrum of the decoder is all right. I also did a comparison simulation, feed my modulator directly with ppwlf source, the signal will not be pulled all down to 0, and the specturm of the input is OK. But I think this PWL signal is not the signal I want. I simulated my circuit using transient analysis, it works. Does envlp only support PWL waveform as input? Can it be thermometer coded waveform? Does anybody know how to solve this problem?

Best Regards,

Richard