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

[Marek Peca]

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