I need to drive a transmitter chain with stored data.  Spectre's PWLF sources would do the job nicely except that they connect the data points with straight lines. The straight line filter is a first order hold. The real source is a DAC, which is only a zero order hold. The extra filtering introduced by the PWLF source gives me an optimistic output spectrum.  Does anyone know how to change the PWLF to a zero order hold instead of a first order hold?  

I was hoping to avoid an extra VerilogA block. Also, it seems that for convergence I must add a small delay to the VerilogA ZOH. The small delay introduces a small error in the sample. I can make the delay small enough to make the error negligible. I just wanted to make sure I was not missing something in the pwlf source.

There are other applications for the ZOH pwlf source. Sometimes the data in the file is not uniformly sampled in time.  For example, if I could not afford AMS and my system had no digital feedback, I might use Verilog to generate a file containing a set of digital control signals and then try to import those control signals into my analog simulation using the pwlf sources. In that case, the data is not necessarily clocked uniformly. I am sure I code up some sort of trigger in VerilogA to sample the data from the pwlf source but it would be so much easier if the pwlf source had an option that simply held each sample until the next one came along.

Thanks for the suggestions guys.

The zvcvs is also uniformly sampled. I have used this to create a sample-and-hold before but I don't remember if I had to introduce the small delay to avoid convergence errors.  You raise a good point about SpectreRF. One important application of the zoh pwlf is to drive a device level transmitter model and simulate with envelope following to assess spectral mask. The z-domain approach would not work and the veriloga solution would have implement the sample and hold with switches and capacitors or a resettable integrator. I still think this is a less-than-optimal solution because it seems like such a simple thing to fix at the source.

I filed a service request with Cadence to see if they had any ideas. They suggested duplicating each data point with a slight delay to create the same effect as a ZOH. This approach shifts the work onto the program that generated the data file but it is compatible with SpectreRF. I still think it would be so much cleaner to enhance the pwlf but I fully understand the balance of priorities and resources.

I checked the spectre -h zvcvs response and saw that there is a "sxz" feature that automatically transforms s-domain filters into a equivalent z-domain filters.  This is cool and I don't remember seeing it before. Is it new?

An alternative would be to generate the entire waveform in Verilog-A. You could then use the transition function to get the behavior you need.

-Ken

Are you aware of the use of the (* instrument_module *) attribute? Placing it in front of a module defintion instructs SpectreRF to treat the module as a test bench module, which has loosen hidden state constraints. I have gotten envelope to work with some very sophisticated sources with all kinds of hidden state when using this attribute.

-Ken

Jess,

It's available from IC5141 onwards. You use it by putting

Code:

(* instrument_module *) module myModuleName... 

What it does is disable the checks for hidden states - which is fine for a baseband signal source in envelope analysis, and for measurement blocks in envelope analysis - but not if the model is (say) a sample and hold, or divider in a PSS analysis.

In other words, you don't want to disable the check for cases where the hidden state will affect the results. In a signal generator for envelope this wouldn't affect the results.

Similarly it is possible to mark particular variables as not being state variables by using the (*ignore_state*) attribute. For example:

Code:

(*ignore_state*) real myVar; 

This might be useful if the variable is just used for (say) informational messages, but does not affect the true circuit state.

Regards,

Andrew.