GPS first LO need to be locked?
HI
On Apr 9, 2017, at 4:28 PM, Attila Kinali attila@kinali.ch wrote:
On Tue, 4 Apr 2017 17:58:11 -0700
jimlux jimlux@earthlink.net wrote:
The advantage of such a system would be that there is only a single
path through the system for all signals, especially through the filters.
Thus the variability of the differential phase shift between the
frequency bands would be significantly reduced, which would result
in better stability.
Oh, I'm not sure about that. It would depend on the filter kind and
topology.
If it's a SAW or BAW filter, it's all one "brick", but I think you'd
still need to calibrate the differential phase shift vs temp. And it
might be very predictable in a "measure 10 of them, and now you know the
characteristics of the next 1000"
The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference.
Why would you substitute an expensive IF filter for a cheap front end filter?
Bob
And at the same time, because the IF frequency
being low, you don't need any specialized filter components that
might not be available in a couple of months.
Of course, this doesn't really work that way when significantly
wider signals (E5) have to be caught together with "narrow band"
signals (L1 C/A or L2C).
Of course, that's the theory. Whether things work
out this way in reality is a different question. What can be said for
sure is, because of the high IF frequency of >200MHz, the standard tuner
chips cannot be used anymore and the RX chain has to be build from
"discrete" components,
There's a ton of integrated demodulator/ADC parts out there these days
that go up to 6GHz.
AD9361 for example
it will do 56 MHz BW through the IF, with 12 bit ADC feeding a 128 tap
FIR filter, etc.
Unfortunately, the AD9361 does not offer the IF bandwith necessary.
Even though it has a high sample rate and can offer high bandwidth
capture of signals, the zero-IF nature of its design doesn't work
for this design approach. The IF of the AD9361 has a low pass filter
of at most 56MHz, ie it offers to capture a bandwith of 56MHz of
frequency space (using both I and Q channels). But the above approach
would need an IF of >200MHz, but it would be enough to only have a
single channel.
I looked up the prices for the components and figured that the prices for
mixer and IF amplifiers are actually quite low (a 2-4 USD per IC) so it
isn't that much more expensive to build such a system than using a 3 tuner
approach (eg using MAX2120 as Peter Monta did with the GNSS Firehose).
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
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On Apr 9, 2017, at 4:29 PM, Attila Kinali attila@kinali.ch wrote:
On Wed, 5 Apr 2017 08:27:58 -0400
Bob kb8tq kb8tq@n1k.org wrote:
Galileo E5 is a bit of a strange case. It’s really E5a and E5b.
You can either grab it all as one giant signal or as two separate signals.
You may (or may not) care about the data on E5a or b depending on what you
are trying to do. Getting the entire very wide signal likely has some
interesting benefits when it comes to working out very small differences
in location or … errr… time.
I wouldn't call it strange, but rather neat :-)
The E5 signal is created as a single, 8-PSK signal(see [1]), which is
modulated such, that the positive and negative frequency parts get
a specific signal structure. This is done in order to allow an extremely
wide band signal to be demodulated in parts. I guess they feared that a
receiver for a 50MHz wide signal would be too expensive for the
commercial market and made it possible to process the signal as two
20MHz wide pieces. There is a slight loss in correlation energy in this
case, but for most applications it should not matter. The bigger issue
is that the path delays for the two receiver channels would need to be
calibrated and tracked during operation in order to make full use of
the E5 signal.
BTW: I have been told, that using the full E5 signal makes the use
of any other signal kind of unnecessary as its extremely wide bandwidth
allows a very fine tracking of the signal. Thus the use of any other signal
(e.g. E1 OS) would actually degrade the receivers timing performance than
improve it.
Without a “second frequency” you can’t do local ionosphere corrections. That’s
true regardless of the bandwidth of the signals …..
Bob
One way to do the E5 signal would be a dual (duplicate) IF ISB downconverter.
How practical that turns out to be is an open question. The more conventional
approach is to take a monstrous chunk of L band down to a high speed sampler.
As I have written above, to be able to do this is the reason for the E5's
signal structure. And apparently the designers thought that this would be
the way how most users would decode it. I am currently not aware of any
commercial E5 receiver that is already on the market, so it is kind of moot
to ask what the common way to decode E5 is.
BTW: Rodriguez' PhD thesis[2] (which is the basis of navipedia) gives a very
nice overview of the trade-off's that went into the Galileo signals and
gives a few hints where future GNSS signals could further improve things.
Attila Kinali
[1] Galileo OS SIS ICD Issue 1 Revision 2,
Section 2.3.1.3 "Equivalent Modulation Type"
[2] "On Generalized Signal Waveforms for Satellite Navigation",
by José Ángel Ávila Rodríguez, 2008
https://athene-forschung.unibw.de/node?id=86167
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
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Hi Attila,
On 04/09/2017 10:29 PM, Attila Kinali wrote:
On Wed, 5 Apr 2017 10:37:07 +0200
Magnus Danielson magnus@rubidium.dyndns.org wrote:
Also, you don't really need to keep the bands fully separate in their
mixed-down form, since they do not correlate except for the P(Y), but
keeping enough frequency difference, such that doppler shift does not
remove correlation margin, they remain uncorrelated. Some of the
literature pay much attention to the band not wrapping around the
band-edge, but I'm not convinced it is such a big issue.
If part of the signal wraps because you are at the bandedge,
then you lose this part of the signal and the part it wraps over.
This is due to the signal coherently overlapping in frequency space.
As far as I understood the math, there isn't a way to seperate them
again (at least there isn't any I am aware of). Thus this signal energy
is lost for the decoding process.
Your generated signal would have the same wrapping. A single bit sampler
would be feasible to loose in, but for multibit ADCs I'm more skeptic.
Cheers,
Magnus
God Morgon Attila,
On 04/09/2017 10:28 PM, Attila Kinali wrote:
On Tue, 4 Apr 2017 17:58:11 -0700
jimlux jimlux@earthlink.net wrote:
The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference. And at the same time, because the IF frequency
being low, you don't need any specialized filter components that
might not be available in a couple of months.
Of course, this doesn't really work that way when significantly
wider signals (E5) have to be caught together with "narrow band"
signals (L1 C/A or L2C).
You got it backwards.
You need to protect your LNA and mixer from other signals, not to be
blocked out by out of band signals which is strong. That's why you have
SAW filters to start with. This has become a larger issue these days.
So, considering that you already have them, then what good do they do
for the different scenarios.
Unfortunately, the AD9361 does not offer the IF bandwith necessary.
Even though it has a high sample rate and can offer high bandwidth
capture of signals, the zero-IF nature of its design doesn't work
for this design approach. The IF of the AD9361 has a low pass filter
of at most 56MHz, ie it offers to capture a bandwith of 56MHz of
frequency space (using both I and Q channels). But the above approach
would need an IF of >200MHz, but it would be enough to only have a
single channel.
Only if you need the Galileo E5.
I looked up the prices for the components and figured that the prices for
mixer and IF amplifiers are actually quite low (a 2-4 USD per IC) so it
isn't that much more expensive to build such a system than using a 3 tuner
approach (eg using MAX2120 as Peter Monta did with the GNSS Firehose).
Cheers,
Magnus
BTW: Rodriguez' PhD thesis[2] (which is the basis of navipedia) gives a very
nice overview of the trade-off's that went into the Galileo signals and
gives a few hints where future GNSS signals could further improve things.
Attila Kinali
[2] "On Generalized Signal Waveforms for Satellite Navigation",
by José Ángel Ávila Rodríguez, 2008
https://athene-forschung.unibw.de/node?id=86167
Thanks for that pointer! Most interesting. I wonder whether anyone has an
updated document detailing the different systems and their current state, as
that information is approaching 10 years old?
Cheers,
David
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv
Hi
On Apr 10, 2017, at 1:00 AM, Magnus Danielson magnus@rubidium.dyndns.org wrote:
God Morgon Attila,
On 04/09/2017 10:28 PM, Attila Kinali wrote:
On Tue, 4 Apr 2017 17:58:11 -0700
jimlux jimlux@earthlink.net wrote:
The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference. And at the same time, because the IF frequency
being low, you don't need any specialized filter components that
might not be available in a couple of months.
Of course, this doesn't really work that way when significantly
wider signals (E5) have to be caught together with "narrow band"
signals (L1 C/A or L2C).
You got it backwards.
You need to protect your LNA and mixer from other signals, not to be blocked out by out of band signals which is strong. That's why you have SAW filters to start with. This has become a larger issue these days.
So, considering that you already have them, then what good do they do for the different scenarios.
Unfortunately, the AD9361 does not offer the IF bandwith necessary.
Even though it has a high sample rate and can offer high bandwidth
capture of signals, the zero-IF nature of its design doesn't work
for this design approach. The IF of the AD9361 has a low pass filter
of at most 56MHz, ie it offers to capture a bandwith of 56MHz of
frequency space (using both I and Q channels). But the above approach
would need an IF of >200MHz, but it would be enough to only have a
single channel.
Only if you need the Galileo E5.
The other point with E5 is the nature of the data on the various sub signals. Galileo has three
classes of service and only one of them is free (open). As with traditional L1 / L2 survey receivers, you
don’t have to recover full data from a signal for it to be useful. That said, the free (open) service
is only on one of the two sub signals. If you are building a L1 / L2 / L5 GNSS receiver, you might
well opt to only grab the lower part of the E5 signal.
You might also decide on a setup that only used two of the three bands. That would give you all
the data and ionospheric correction. It is a bit unclear what the third band would add other than a “cool factor”
if traditional criteria are used for the receiver design. There are various arguments for L1 / L5 and L1 / L2. One
could even make a case for L2 / L5.
Even if ionospheric correction is not a bit issue in your design, jamming probably should be for a design
targeted to run for many years into the future.. A broadband jammer (intentional or accidental) can fairly
easily take out one of the bands. It’s quite a bit harder to take out all of them at once. A lot would depend
on just how nasty an environment you intend to operate in, and how sensitive you are to occasional issues.
Lots of choices ….
Bob
I looked up the prices for the components and figured that the prices for
mixer and IF amplifiers are actually quite low (a 2-4 USD per IC) so it
isn't that much more expensive to build such a system than using a 3 tuner
approach (eg using MAX2120 as Peter Monta did with the GNSS Firehose).
Cheers,
Magnus
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
Hi Bob,
It is a bit unclear what the third
band would add other than a "cool factor"
Even quicker RTK convergence.
http://www.navipedia.net/index.php/Carrier_phase_ambiguity_fixing_with_three_frequencies
--
Björn
Hi,
On 04/10/2017 03:00 PM, Bob kb8tq wrote:
Hi
On Apr 10, 2017, at 1:00 AM, Magnus Danielson magnus@rubidium.dyndns.org wrote:
God Morgon Attila,
On 04/09/2017 10:28 PM, Attila Kinali wrote:
On Tue, 4 Apr 2017 17:58:11 -0700
jimlux jimlux@earthlink.net wrote:
The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference. And at the same time, because the IF frequency
being low, you don't need any specialized filter components that
might not be available in a couple of months.
Of course, this doesn't really work that way when significantly
wider signals (E5) have to be caught together with "narrow band"
signals (L1 C/A or L2C).
You got it backwards.
You need to protect your LNA and mixer from other signals, not to be blocked out by out of band signals which is strong. That's why you have SAW filters to start with. This has become a larger issue these days.
So, considering that you already have them, then what good do they do for the different scenarios.
Unfortunately, the AD9361 does not offer the IF bandwith necessary.
Even though it has a high sample rate and can offer high bandwidth
capture of signals, the zero-IF nature of its design doesn't work
for this design approach. The IF of the AD9361 has a low pass filter
of at most 56MHz, ie it offers to capture a bandwith of 56MHz of
frequency space (using both I and Q channels). But the above approach
would need an IF of >200MHz, but it would be enough to only have a
single channel.
Only if you need the Galileo E5.
The other point with E5 is the nature of the data on the various sub signals. Galileo has three
classes of service and only one of them is free (open). As with traditional L1 / L2 survey receivers, you
don’t have to recover full data from a signal for it to be useful. That said, the free (open) service
is only on one of the two sub signals. If you are building a L1 / L2 / L5 GNSS receiver, you might
well opt to only grab the lower part of the E5 signal.
You might also decide on a setup that only used two of the three bands. That would give you all
the data and ionospheric correction. It is a bit unclear what the third band would add other than a “cool factor”
if traditional criteria are used for the receiver design. There are various arguments for L1 / L5 and L1 / L2. One
could even make a case for L2 / L5.
L2 and L5 is so close, that bringing them down together is relatively
simple.
If you make a receiver today, it should be able to use any set of bands,
including L2 only or L5 only. Even more refined than that, any set of
signals. One should attempt to get any of the transmitted signal, so
there might be L1 from one, L2 and L5 from one, L1 and L5 from one etc.
Each signal contributes. Signal pairs and tripples allow for ionospheric
estimation, with the added benefit.
Even if ionospheric correction is not a bit issue in your design, jamming probably should be for a design
targeted to run for many years into the future.. A broadband jammer (intentional or accidental) can fairly
easily take out one of the bands. It’s quite a bit harder to take out all of them at once. A lot would depend
on just how nasty an environment you intend to operate in, and how sensitive you are to occasional issues.
Lots of choices ….
Indeed. I have been advocating for use of multiple bands, besides L1,
and for multiple GNSS. This helps to build signal redundancy. Such
receivers should not be too expensive eventually.
Cheers,
Magnus
On Sun, 9 Apr 2017 18:13:48 -0400
Bob kb8tq kb8tq@n1k.org wrote:
The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference.
Why would you substitute an expensive IF filter for a cheap front end filter?
Availability: Although there are L1/L2 filters available, they are not
easy to get unless you buy them in bulk. The standard L1 filters you
can buy are rather narrow band (just 2-4MHz) and don't allow the
reception of the modern signals. L5 filters are very rare and E5 filters
simply do not exist yet.
And keep in mind that the IF filter does not need to be a special
ultra-steep filter. The high sampling rate of the ADC allows to place
the input signal such, that the stop band can be quite far from the
pass band. Also, the filter is only really necessary to filter out
narrow band interference, which is hopefully far from the signal anyways.
Having a bit of noise fold in is, as Jim noted earlier, not a problem at all.
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
On Mon, 10 Apr 2017 07:00:27 +0200
Magnus Danielson magnus@rubidium.dyndns.org wrote:
God Morgon Attila,
On 04/09/2017 10:28 PM, Attila Kinali wrote:
On Tue, 4 Apr 2017 17:58:11 -0700
jimlux jimlux@earthlink.net wrote:
The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference. And at the same time, because the IF frequency
being low, you don't need any specialized filter components that
might not be available in a couple of months.
Of course, this doesn't really work that way when significantly
wider signals (E5) have to be caught together with "narrow band"
signals (L1 C/A or L2C).
You got it backwards.
You need to protect your LNA and mixer from other signals, not to be
blocked out by out of band signals which is strong. That's why you have
SAW filters to start with. This has become a larger issue these days.
I don't think they are necessary anymore. Todays LNAs have a very high
IP3 (in the order of 10-30dBm) and even IP1dB is usually around 0-10dBm.
Ie unless there is a very strong, narrow band interference, the LNA will
not cause any problems. Same goes for modern mixer.
Or, to make it a bit more practical: if you take an RTL-SDR dongle,
then you have a cheap, zero-IF system that has no frontend filter
and relies solely on selectivity of the antenna (which is often
a cheap puck without any filter) and its IF low-pass filters.
I have used this a few times and have not seen it fail. I took my
bladerf a few times and looked at the spectrum around 1575GHz and
haven't seen any strong interferer yet.
Unfortunately, the AD9361 does not offer the IF bandwith necessary.
Even though it has a high sample rate and can offer high bandwidth
capture of signals, the zero-IF nature of its design doesn't work
for this design approach. The IF of the AD9361 has a low pass filter
of at most 56MHz, ie it offers to capture a bandwith of 56MHz of
frequency space (using both I and Q channels). But the above approach
would need an IF of >200MHz, but it would be enough to only have a
single channel.
Only if you need the Galileo E5.
With only L1 and L2 it is still ~180MHz. The two bands are ~350MHz appart.
You cannot get around that without introducing a second down-mix step.
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson