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

lists at lazygranch.com lists at lazygranch.com
Sat Mar 17 10:27:03 UTC 2012


I've designed filters for datacom chips. I know filtering. My point is the original author is making some assumptions in the design which are not stated.  

What I don't have a lot of hands on experience is with open circuit magnetics. (I do with closed circuit magnetics.) But I claim if the ferrite rod antenna is not capacitively loaded to resonate at the comm frequency, then there isn't significant group delay error. 

The antenna will have a natural resonant frequency comprised of the inductance and parasitic capacitance. But this represents an upper frequency limit. So simply operate below resonance and the group delay error is minimized. Filtering can be done following the preamp that connects to the antenna, and thus will not interact with it.

 

-----Original Message-----
From: Attila Kinali <attila at kinali.ch>
Sender: time-nuts-bounces at febo.com
Date: Sat, 17 Mar 2012 10:47:23 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] WWVB BPSK Receiver Project? (fwd)

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?

_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.


More information about the time-nuts mailing list