Hello, gary,
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.
It was me, a time-nuts newbie. My previous related posts were:
http://www.febo.com/pipermail/time-nuts/2012-March/065049.html
http://www.febo.com/pipermail/time-nuts/2012-March/065003.html
http://www.febo.com/pipermail/time-nuts/2012-March/065009.html
etc.
and
http://www.febo.com/pipermail/time-nuts/2012-March/065135.html
"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."
Let me clarify the unclear statement. I was reacting to Poul-Henning
Kamp's (true) statement, that: "The reason I use 1MSPS is that it allows
me to use a very sloppy low-pass filter filter which just cuts off
somewhere around 150-200 kHz, and do everything else in software. This
means that I have no phase/group-delay distortion in the analog part that
I need to compensate in software."
In my design, I have used a ferrite rod LC circuit as and antenna and also
the only element of selectivity in front of sampling. So, there was a 2nd
order only filter.
The useful signal of DCF77 (afaik yout WWVB is very similar now with BPSK)
spans over ~1kHz. In my design, in contrast to P.-H. K.'s approach, I use
only ~40ksps, so the 2nd order ferrite rod circuit should pass 1kHz, but
it should attenuate somewhere around +-10..20kHz.
I.e., the result will be always a compromise. Unfortunately, I don't have
a measurement of my worked circuit's Q, but let us assume Q=20..100 can be
realistic value for ferrite rods. Then, the filter's BW will be somewhere
0.8..4kHz, what means, that its phase over the interesting 1kHz band will
_not_ be straight line, but somewhat curved.
This is the only thing about ferrite rod and phase I meant.
To conclude, I would like to repeat, that in my oppinion the ferrite rod
is easy and common antenna for LF signals, so that in such a case the
phase will be curved anyway. Of course you can feed the P.-H. K.'s 1Msps
input by more wide-band antenna, not the ferrite rod, to get more linear
phase without SW compensation.
Greeting from Marek
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