in reply to: Re: First bit of preamble posted by Anirudh on 15 December 2009 at 07:20:36.
So it's carrier start up you are concerned with, rather than the start of the HART message itself.
I don't think it matters how you start the carrier, so long as its transient effect meets the carrier start/stop transient requirement of 10mV maximum across 250 ohms as the output of a 2-pole 25Hz filter. See spec.054r.8.1 para. 188.8.131.52.
There's a diagram somewhere which shows the carrier amplitude building up over a few cycles. Is that what most HART modems actually do, I wonder?