[time-nuts] WWVB BPSK Receiver Project? (fwd)

[Attila Kinali]

On Fri, 16 Mar 2012 20:02:27 -0700
gary <lists at lazygranch.com> wrote:

> I lost track of who wrote this, but why is it assume a ferrite rod has 
> non-linear phase. [Group delay error I presume). Now I assume this 
> presumes the rod is used in a LC circuit, but if the Q is not high, the 
> phase linearity won't necessarily be bad.
> 
> Basically I'd like to hear more from whomever wrote this.
> 
> 
> "The useful bandwidth of LF to HF radio is about 9kHz, DCF77-like 
> standards with PRBS is about 1.5kHz. Of course the ferrite rod as an 
> input filter *will* have a non-linear phase, but it still seems to me it 
> is the simplest and most common receiptor for LF time signals."

I'm not the one who wrote this, but it is true :-)
Any filter has its phase dependend on the frequency. As a rule
of thumb: the higher order the filter, the faster the phase changes.

As long as you just do straight filtering, with no feed back, you
care seldom about the phase. It's the amplitude of the signal you
are interested in. But if you now go time nuttery, phase change means
delay. And you dont know how large it exactly is, because you dont
know where exactly the resonance frequency of the filter is. And more
importantly, you cannot say how it changes over time (tempeture dependece,
aging, etc).

That said, i think this can be ignored for all practical purposes
in an VLF receiver, as the enviromental changes in the atmospheric
signal path are much larger than the small error you get from the
filter. But then again, time nuts are time nuts ;)

			Attila Kinali

-- 
Why does it take years to find the answers to
the questions one should have asked long ago?