Re: [time-nuts] WWVB BPSK Receiver Project? (fwd)
Yes, in order to equalize group delay, you need to know what to
equalize. But with an educated guess as to the system response, he could
get close.
All this said, in 2012, I would rather the amplifier be simple gain, the
inductor not loaded with capacitance and the filtering done past the
amplifier. We aren't living in the era of 3 transistor circuits.
When delta-sigma converters came on the scene. I wisely found new design
skills. [They replaced much analog filtering.] So better just to do the
filtering in DSP IF there is no critical power budget.
On 3/17/2012 6:25 AM, Attila Kinali wrote:
On Sat, 17 Mar 2012 06:13:28 -0700
garylists@lazygranch.com wrote:
On 3/17/2012 5:44 AM, Attila Kinali wrote:
On Sat, 17 Mar 2012 10:15:17 +0000
"Poul-Henning Kamp"phk@phk.freebsd.dk wrote:
Either you need to characterize the exact behaviour of your filter
and build the necessary compensation for its phase/frequency behaviour
into your receiver, or you need a very flat filter (both freq+phase)
in order to reliably recognize the proper zero-crossing to track.
BTW: how do you compensate for the filter characteristics of your
magnetic loop antenna?
Any filter's group delay can be equalized by all pass filters.
Delay builds up at the filter corner. Since everything in the real world
is causal, you add delay outside that corner frequency but in the
passband to equalize it. This is to say, you can't remove delay, but
just add it to flatten out the group delay.
Sorry, i asked in a misleading way. I didnt mean to ask what technique
to use to flaten the phase delay, but rather how does phk know how the
compensating filter should look like? For this, one needs to exactly
characterize the antenna-amplifier chain...AFAIK
Attila Kinali
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.
Yes, exactly. Excuse my loose speech before not explicitly mentioning LC
tuned circuit.
Marek
Hi
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.
Bob
On Mar 17, 2012, at 9:40 AM, gary lists@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:
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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Yes, in order to equalize group delay, you need to know what to equalize. But
with an educated guess as to the system response, he could get close.
All this said, in 2012, I would rather the amplifier be simple gain, the
inductor not loaded with capacitance and the filtering done past the
amplifier. We aren't living in the era of 3 transistor circuits.
When delta-sigma converters came on the scene. I wisely found new design
skills. [They replaced much analog filtering.] So better just to do the
filtering in DSP IF there is no critical power budget.
This may not be true, if you have some strong interference at the ferrite
rod input. Of course, if it would be strong and near the signal frequency,
it will not be attenuated much even by the 2nd order LC circuit, indeed.
However, for f0=77.5kHz and B=1kHz, the LC circuit with Q=40 gives phase
error over specified bandwidth about +-0.5deg p-p. Does such a phase
non-linearity bother you?
Marek
I think the tempco of the ferrite is more significant than drift in the
analog filter. Of course this again implies the better design is to not
load the inductor with a cap, i.e. stay broadband, and then just filter
post the preamp.
The open circuit voltage will be lower without the resonant circuit.
This is a case where the BF862 might do some good since the goal is to
not load the inductor, either resistively or with capacitance. When you
are dealing with components that are around 1nV/sqrt(hz), you can afford
to throw some circuitry at the problem especially since the atmospheric
noise will dominate.
On 3/17/2012 6:38 AM, Marek Peca wrote:
Any filter's group delay can be equalized by all pass filters.
Delay builds up at the filter corner. Since everything in the real
world is causal, you add delay outside that corner frequency but in
the passband to equalize it. This is to say, you can't remove delay,
but just add it to flatten out the group delay.
Yes, the compensation can be made and it has been also pointed out in
the first comment by Poul-Henning.
The only remaining question is, how stable are the analogue filter
parameters over time, to be compensated by fixed digital filter. It
seems to me, that some very small phase errors produced by such a
filter-filter mismatch may be acceptable.
At least for low-cost device which I would like to rebuild and offer for
WWVB audience (which is not present in our land).
Best regards,
Marek
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I think the tempco of the ferrite is more significant than drift in the
analog filter.
Perhaps I was unclear in this as well. I do not use nor plan to use any
other filter than the (ferrite-L)-C resonant circuit itself. So, yes, the
tempco of the ferrite makes its coefficients variation.
The question is, whether phase errors <1deg p-p over 1kHz band are
significant.
Marek
That would be 36ns group delay variation if I did the math correctly.
However, what material are you using for the ferrite? The material can
have a significant tempco.
On 3/17/2012 7:17 AM, Marek Peca wrote:
However, for f0=77.5kHz and B=1kHz, the LC circuit with Q=40 gives phase
error over specified bandwidth about +-0.5deg p-p. Does such a phase
non-linearity bother you?
Marek
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That would be 36ns group delay variation if I did the math correctly.
OK
And in article P. Hetzel: Time dissemination via the LF transmitter DCF77
using a pseudo-random phase-shift keying of the carrier, 2nd EFTF
Neuchatel, 1988., they conclude with timing results of about 2..10e-6 s
RMS over ~1000km distance.
However, I do not know what is the reality and whether such a performance
is limited by atmosphere/ground conditions, or whether it could be better
within LF band.
However, what material are you using for the ferrite? The material can have a
significant tempco.
In my project, I have used noname rod taken from within DCF77 alarm clock.
If I will recreate it, I will look for something defined at the store.
Regards,
Marek
OK, assuming type 61, it is 0.1%/deg C. Let's go with +/- 5 degrees,
which would be for indoor use. I don't have the equation handy for a
damped LC. Certainly undamped would be worst case. f=1/(2pisqrt(LC)).
When the dust settles, the frequency shift is the square root of the
temperature shift, so half a percent due to temperature ends up being a
quarter percent frequency shift.
Hi Marek -
I don't know where you are in CZ. I'm on the boarder in DE near PL and
CZ. The distance to DCF77 is about 450km and if I check the amplitude
across 24h I see considerable very deep fading effects! I think it is
useless as a phase-coupled time receiver. At least in specific
positions. It will loose phase at least for twice the day for approx. 2h
! That was the report for a ferrite rod.
The other way would be a high-impedance FET-preamp vertical-wire
antenna. I think this will resist much more fading effects. But it is
unchecked at the moment. You're welcome to do it.
The benefit of a resonated ferrite rod is the good bandpass filtering
for local interferers like TV. The FET vertical wire will need heavily
filtering thereafter. All in the whole dynamice range, of course.
Ferrites can be temperature controlled. They have big spreads in
parameters anyway! The production procedure is explained in the
classical book about Ferrites: Snelling "Soft Ferrites".
- Henry
Marek Peca schrieb:
That would be 36ns group delay variation if I did the math correctly.
OK
And in article P. Hetzel: Time dissemination via the LF transmitter
DCF77 using a pseudo-random phase-shift keying of the carrier, 2nd EFTF
Neuchatel, 1988., they conclude with timing results of about 2..10e-6 s
RMS over ~1000km distance.
However, I do not know what is the reality and whether such a
performance is limited by atmosphere/ground conditions, or whether it
could be better within LF band.
However, what material are you using for the ferrite? The material can
have a significant tempco.
In my project, I have used noname rod taken from within DCF77 alarm clock.
If I will recreate it, I will look for something defined at the store.
--
ehydra.dyndns.info