Re: [time-nuts] WWVB BPSK Receiver Project? (fwd)
On Sat, 17 Mar 2012 10:15:17 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
In message 20120317111119.9536107ebf82050fe14ee85c@kinali.ch, Attila Kinali w
rites:
On Sat, 17 Mar 2012 10:01:13 +0000
Could you explain why? Yes, you need a higher BW for Loran-C,
but the phase(f) function will give you only a distortion of
the signal and a constant time delay in your signal recovery.
But that shouldnt degrade the usefullness of the system.
What am i missing here?
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.
The more you disturb a Loran-C pulse, the more it just looks like
a bit of a sine-function, and the harder it is to lock on the right
zero-crossing.
Ah.. so it is because Loran-C uses the third zero crossing as specified
measurement point, which you thus have to capture with the greates
possible resolution.
Am i right that for DCF77, WWVB and the like, where there is no such
requirement on the zero crossing of a pulse, one can just lock to the
carrier and the distortions from filters are not so relevant?
Attila Kinali
--
Why does it take years to find the answers to
the questions one should have asked long ago?
On Sat, 17 Mar 2012 10:27:03 +0000
lists@lazygranch.com wrote:
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.
Ah.. you have here a "wrong" assumption. Normal antennas are resonant
at the wanted frequency. This is in order to get maximum gain i the
first stage (not to mention that the antenna is the only "amplifier"
with no noise). Also all DCF77 antennas i have seen so far are ferrit
rods with an attached capacitor, to form a resonant antenna.
I think, it would be possible to use a non-resonant antenna. I don't
know what the total noise would then be. But it would definitly be
interesting to know whether a non-resonant antenna design would be
better or worse.
Attila Kinali
--
Why does it take years to find the answers to
the questions one should have asked long ago?
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?
Attila Kinali
--
Why does it take years to find the answers to
the questions one should have asked long ago?
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.
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?
Attila Kinali
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
On Sat, 17 Mar 2012 06:13:28 -0700
gary lists@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
--
Why does it take years to find the answers to
the questions one should have asked long ago?
Dear Poul-Henning,
My only argument against your versatile and well-performing solution is
that it is a little bit overkill.
As if running a handfull precision oscillators just for fun isn't
"overkill" also ? :-)
I don't know -- are there any limits for the fun in a time-nut sense? :-)
I hope not. The point is, with which kind of toy we would like to play.
In other words, it would be certainly better to buy USRP N210,
Actually that would be a very idea, because you cannot get rid of
the down-sampler in the USRP and that would make Loran-C reception
very tricky to implement.
Are you sure there are such a limitations? I must reveal, that I have not
even once played with USRP N210, but I hope it does not have any BW
limitations up to the Gig-Eth speed.
Anyway, it would be an expensive and heavy receiver for LF-only signals.
My point is to do something with relevant performance wrt. <10kHz wide
LF signals.
The crucial question is if your are doing timenuttery or radionuttery.
If you are doing timenuttery, you want you ADC synchronized to your
OCXO/Rb/Cs or whatever you have,
Yes, I would like to have an option of external frequency standard.
However, I would like to lock ordinary onboard quartz too, since many
people without Rb (though they are pretty cheap these days) may use it as
a disciplined frequency source, too. I mean no time-nuts, but ordinary
hobbyists, going to tune their filters etc.
Or people, wanting some time signal in place of poor GNSS reception
without good NTP access (I know such a set i almost empty :-)).
and you don't want to have to deal with getting your IF frequency locked
too.
Soundcards use inconvenient frequencies and are seldom built to take
an external clock signal.
So this is why I would like to supply a little bit tweaked "sound card",
tailored to receive LF-HF band signals up to say 10..20kHz of width.
The useful bandwidth of LF to HF radio is about 9kHz,
You need more than 25kHz for good Loran-C
OK, thank you for this notice. I have not yet thinked about Loran, so I
must look in more detail on it.
Best regards,
Marek
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.
Yes, my fault, I didn't write it properly, so by a "ferrite rod" in
context of DCF/WWVB reception, I meand a "ferrite antenna in an LC tuned
circuit".
Apologies for all who have been confused.
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.
Yes, see above. I meant an LC circuit, containing the ferrite rod antenna
as the L.
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.
Thank you for your understanding.
Marek
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
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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.