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. 7.3.2.1.
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?