[time-nuts] WWVB BPSK Receiver Project? (fwd)
lists at rtty.us
Sat Mar 17 14:14:26 UTC 2012
The problem with delay compensation in a Time Nut environment is that to do it you add delay. Your all pass network adds enough delay to the "fast" part of the passband to make it come out the same as the slow part. In real circuits you inevitably add some delay everywhere with the all pass, so the net is somewhat higher delay everywhere than the worst of the original filter.
So far no problem.
Change temperature or let things age and all those delays change. Since they are a sum of many things, they likely change in a complicated fashion. Change in delay is change in time. That is a Time Nut problem.
On Mar 17, 2012, at 9:40 AM, gary <lists at lazygranch.com> wrote:
> Which basically matched my assumption. If the inductor is loaded, you have a narrowband filter. So again, this does not imply that a ferrite rod antenna per se has phase distortion. It is the LC filter than effects the group delay.
> On 3/17/2012 6:19 AM, Marek Peca wrote:
>> 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:
>>> "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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