FE-.5680A trimming resolution
Chris,
When you're down at the ns level, every ns counts even more.
There actually a "huge" difference between a UT and VP and
M12 and ...
Then again, it's not always about nanoseconds. There are also
issues of power and size, support, supply, price, the future.
Perhaps also RF sensitivity, feature set, upgrade path for the
likes of GLONASS or Galileo, acquisition time. Even RoHS.
Perhaps this doesn't matter for a one-off hobbyist, but if you're
making kits or products it can become an important factor.
If you are inclined to experiment, just for the sake of exploring
as many of us on the list are, then certainly you'd want to get
a u-blox at some point. It doesn't have to be right away, but it
is a pretty nice, very modern, ultra compact, timing receiver.
If low cost is the object it's hard to beat that MG1613S board.
/tvb
What is it these u-blox device can do that a cheaper Motorola Oncore
can't? Depending on the version the Oncore has for 50 to 5 nS
one-sigma error on the timing pulses and can do either 1PPS or 100PPS.
Single unit prices are from $18 to $60 very good documentation is
available.
If the u-blox was somehow much better than a Trimble thunderbolt or
Motorola Oncore MT12T I'd buy one even at the above price. But
really these older GPSs are already at the single digtit of
nanoseconds level and I don't see room for improvement except....
If the L2 band is also used. This is the way to get order of
magnitude improments
Chris Albertson
Redondo Beach, California
I think, a specialized GPS SDR can be build for less than 500 USD
in low (a dozen at max) volumes.
The USRP works for GPS L1 (though P/Y is a little undersampled at 8
Ms/s complex), but I didn't find a way to acquire both L1 and L2
simultaneously at useful sample rates (maybe current USRP hardware is
better). Also 16 or 8 bits is too much precision---2 bits is more
appropriate and for some reason wasn't a standard option. It was fun
to acquire and track L1 and L2C separately, but what I really want is
a no-holds-barred geodetic reference receiver.
A dedicated tri-band GPS front end could be built for less than $500,
I agree. Software can handle acquisition, tracking, and conversion to
RINEX. The hardware just needs to translate RF to bits on the wire
(gigabit Ethernet say) and be phase-stable over temperature.
One possible inexpensive design:
- RF input passively split three ways, with LC filters for the three
channels: L5/E5, L2, and L1/E1/Glonass - For each channel, a downconverter (Maxim MAX2121) feeding a ~65 Ms/s
ADC (e.g. MAX19505) - A low-cost FPGA (e.g. Spartan-6) that quantizes the channels to 2
bits, does AGC, assembles Ethernet packets - Ethernet PHY, power (PoE?), etc.
For a timing receiver, one could inexpensively add one more ADC that
samples a 10 MHz input signal and a 1PPS input signal. 1PPS packets
would be emitted only when transitions are detected, and the 10 MHz
signal could be downconverted to a low-bandwidth signal to be sent
over Ethernet with the others. This way one has reference signals
coherently sampled with the GPS signals.
I think LC filters would provide enough protection against strong
out-of-band interferers; semicustom ceramic-resonator filters or,
worse, full-custom SAW filters are not hobbyist-friendly and may not
be as stable over temp. Also I think the phase noise of the MAX2121
is acceptable. Possibly the FPGA should be doing the pulse blanking
for L5 since the FPGA still has the 8-bit signal available.
Is there a publically-available antenna design that's easy to
fabricate, has a stable phase center, covers 1100--1600 MHz, and has a
good pattern over this band with low cross-polarization? Even a large
choke-ring design would be okay if it's fully specified.
Cheers,
Peter
On Wed, 1 Feb 2012 11:03:19 -0800
Chris Albertson albertson.chris@gmail.com wrote:
On Wed, Feb 1, 2012 at 10:18 AM, Attila Kinali attila@kinali.ch wrote:
That said. I've contacted u-blox, but got a number that is way out of
what i've expected (approx 120CHF). I'm currently trying to get a lower
price.
What is it these u-blox device can do that a cheaper Motorola Oncore
can't? Depending on the version the Oncore has for 50 to 5 nS
one-sigma error on the timing pulses and can do either 1PPS or 100PPS.
Single unit prices are from $18 to $60 very good documentation is
available.
The Timing Appnote [1] says, that the 1PPS error's sigma is 6.7ns, before
sawtooth correction and 3.0ns after.
If the u-blox was somehow much better than a Trimble thunderbolt or
I dont have the numbers, but I doubt that a LEA-6T can beat a thunderbolt.
Motorola Oncore MT12T I'd buy one even at the above price. But
really these older GPSs are already at the single digtit of
nanoseconds level and I don't see room for improvement except....
If the L2 band is also used. This is the way to get order of
magnitude improments
They are not dual band receivers. Yes, that would be really an improvement..
But i'm not convincing you to buy a LEA-6T. Nor do i think that it's
a must have for anyone. Heck, you can get a complete thunderbolt with
supply and antenna for 200USD on ebay. No LEA-6T based system will ever
get that cheap. But there is not much way to tinker with a thunderbolt.
You cannot play with its design. If you build a system from scratch,
you can.
Attila Kinali
[1] https://www.u-blox.com/images/downloads/Product_Docs/Timing_AppNote_%28GPS.G6-X-11007%29.pdf
Why does it take years to find the answers to
the questions one should have asked long ago?
On Wed, 1 Feb 2012 11:49:51 -0800
Peter Monta pmonta@gmail.com wrote:
One possible inexpensive design:
- RF input passively split three ways, with LC filters for the three
channels: L5/E5, L2, and L1/E1/Glonass - For each channel, a downconverter (Maxim MAX2121) feeding a ~65 Ms/s
ADC (e.g. MAX19505) - A low-cost FPGA (e.g. Spartan-6) that quantizes the channels to 2
bits, does AGC, assembles Ethernet packets - Ethernet PHY, power (PoE?), etc.
Heh..That's pretty much the design i thought of, though using a higher
sampling frequency (100 to 200Msps) which would allow to coherently
decode the E5a and E5b signals together. There is an ADC from National
that can do 200Msps for 20 bucks, with FPGA friendly parallel output
(ADC08200).
Is there a publically-available antenna design that's easy to
fabricate, has a stable phase center, covers 1100--1600 MHz, and has a
good pattern over this band with low cross-polarization? Even a large
choke-ring design would be okay if it's fully specified.
That's a good question. I don't know. I think a dual band patch antenna
design would work (two stacked patches). This would be easy to fabricate
with very good horizontal tolerances (just use PCBs). But i have neither
designed such an antenna, simulated or even build and tested...
But the other designs i've seen are much more difficult to build with
home tools or need tuning which is not easily done if you don't have
access to good equipment.
Attila Kinali
--
Why does it take years to find the answers to
the questions one should have asked long ago?
Can I ask where does the Trimble Resolution-T fit between this other receivers ? I've used it and I do like it. I thought it was relatively modern and capable compared to the Oncore.
Isn't it comparable to the uBlox for example?
Regards,
Roberto EB4EQA
From: albertson.chris@gmail.com
Date: Wed, 1 Feb 2012 10:38:25 -0800
To: time-nuts@febo.com
Subject: Re: [time-nuts] GPS SDR (was: FE-.5680A trimming resolution)
On Wed, Feb 1, 2012 at 9:43 AM, Bob Camp lists@rtty.us wrote:
Hi
My guess is that the reality of parts sourcing will quickly get us right
back to the "group buy of LEA-6T" topic.
For timing I don't see why an LEA-6T is better then a Oncore or
t-bolt. You can buy an Oncore UT for about $18 on ebay and new (with
factory warranty) MT types for about $60. I just got a t-bolt from a
seller in California for $110.
For car navagation the LEA-6 looks much better because t has inputs
for odometer pulses and a turn rate gyro and the LEA-6 can use this
data for position and rate determination in tunnels and urban canyons.
Chris Albertson
Redondo Beach, California
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On 01/02/12 10:25, Attila Kinali wrote:
On Wed, 01 Feb 2012 01:52:16 +0100
Magnus Danielsonmagnus@rubidium.dyndns.org wrote:
Exactly that is the appeal of the Tbolts.
Yes, but can this be replicated with a standard GPS module?
Yes. If you look up old papers, they already did this with Oncores,
Cesium clocks and synthesis.
I have not seen any such papers yet. Do you have any pointers or hints
what to search for?
Let me see... yes, here it is:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA497270
Was not too hard to find.
HP5061B 10 MHz divided by 188 and then the internal TCXO divided by 359
and then phase-comparision lock of the TCXO. Need to have a matching
VCTCXO for that frequency, but other than that. Looks up a Oncore VP
receiver.
Notice the comment on the crystals performance compared to the need for
the experiment.
They don't really spent much time theorizing about it and waving hand,
as it is fairly assumed as common knowledge and just hand-waving over
the numbers is sufficient to convince that audience. It should also come
as no surprise.
Cheers,
Magnus
On 01/02/12 15:07, John Ackermann N8UR wrote:
There've been numerous threads on the Gnuradio mailing list about code
to receive GPS using the Ettus Research USRP hardware. I don't know
whether anyone has actually made it work, but it appears that it's been
the subject of quite a few academic projects.
I bought one of those GNSS samplers, pulled data from the air and
actually managed to have my GPS SDR written in C scan the doppler bins
and code-phases with FFT correlation techniques (with a little help of
FFTW), find a bird and lock the PLL up to it... locked onto the symbols
and started spitting out the sub-code info. I wrote all the code myself
except for the USB application. I also spent some time to start
calculating position and such.
I will have to get one of those Ettus Research devices to be able to do
anything useful thought.
So, it is certainly possible. Takes a few of the software GPS books
alongside the ICD-GPS-200 and traditional GPS books to piece together
the full suite of references, but it didn't take THAT long to get where
I where.
Cheers,
Magnus
In my opinion the work done locking the VCTCXO of the Oncore is different
from the TBolt OCXO management: the TBolt steers the OCXO based on the
received signal instead they locked the Oncore oscillator to a Cs
reference. Yes, if all the world is the same then there is no difference:
the Cs locks the VCTCXO frequency where it should be as if steered from
satellites but how can you drive an OCXO from an Oncore even with hardware
modifications?. There is an oscillator offset in the @@Ha status message...
maybe by using it?
On Thu, Feb 2, 2012 at 12:59 AM, Magnus Danielson <
magnus@rubidium.dyndns.org> wrote:
On 01/02/12 15:07, John Ackermann N8UR wrote:
There've been numerous threads on the Gnuradio mailing list about code
to receive GPS using the Ettus Research USRP hardware. I don't know
whether anyone has actually made it work, but it appears that it's been
the subject of quite a few academic projects.
I bought one of those GNSS samplers, pulled data from the air and actually
managed to have my GPS SDR written in C scan the doppler bins and
code-phases with FFT correlation techniques (with a little help of FFTW),
find a bird and lock the PLL up to it... locked onto the symbols and
started spitting out the sub-code info. I wrote all the code myself except
for the USB application. I also spent some time to start calculating
position and such.
I will have to get one of those Ettus Research devices to be able to do
anything useful thought.
So, it is certainly possible. Takes a few of the software GPS books
alongside the ICD-GPS-200 and traditional GPS books to piece together the
full suite of references, but it didn't take THAT long to get where I where.
Cheers,
Magnus
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On 01/02/12 19:12, Attila Kinali wrote:
On Wed, 1 Feb 2012 09:27:30 -0800
Chris Albertsonalbertson.chris@gmail.com wrote:
I thought it might be interresting but then found out you need to buy
$2,000+ worth of hardware for even start experimenting. Open Source
SDR needs to run on a common affordable platform or it will never gain
the critical mass of users that it take to make the project live
longer then a few months.
That's because the URSP is a general purpose system. It is designed
to do many things. That makes it expensive. And being expensive,
it has a low production volume, which makes it even more expensive.
I think, a specialized GPS SDR can be build for less than 500 USD
in low (a dozen at max) volumes.
I guestimate, that the RF/ADC part would cost somewhere between
100 to 200 USD in parts. The big uncertainty here is the FPGA.
I have no clue how much logic space for a GPS SDR would be needed
at minimum and how much would be desirable. Hence i have no guess
what the FPGA would cost (could be anything from a cheap 20USD
FPGA to a 300 USD one).
That's why you start of with using an Ettus box as a boiler plate. Once
you have working code, you can re-target it for a smaller device and
situation. You can do dry synthesis towards the new platform without
having it as a physical device. The basic design can still be running on
that Ettus platform. Come to think of it, I did get a few university
point on a 2-week coarse teaching exactly this point, spin on big-ass
FPGA machines and then go to target. :) That's... 18 years ago. Time flies.
I think the way to go is to find a commercial GPS chip that has a low
level interface and then build the uP controller using a common
development system. Both the chip and the uP board need to be,
common, well documented and cheap.
There are no common, well documented and cheap GPS frontend chips
out there. All chips that are still in production are for high volume
stuff. Without knowing someone inside those companies, you will not
be able to get them at single pieces. I searched quite a while some
time ago, and couldn't find anything that is not EOL. Finally i came
to the conclusion that it is easier to build a custom frontend from
scratch, from the available HF parts.
Front-end chips is still there. That's how they build these:
http://ccar.colorado.edu/gnss/
http://www.sparkfun.com/products/8238
http://www.sparkfun.com/products/10981
That will suffice to get you started in the SDR field.
Cheers,
Magnus
Hi All,
I've been lurking here for a while, learning lots - but I think I may be
able to contribute something to this discussion.
I have been looking at SDR GPS reception for a while, and have a number of
ideas as to how to go about this process. My first point of call is the
layout of a board using a MAX2769B (1) receiver chip attached directly to a
Spartan 3E- 500 FPGA and then to a USB interface. I have decided to do
this using an add-on board to the Papillio FPGA boards (2) that have been
linked here before, incorporating the receiver chip, level shifters, power
supplies, as well as a somewhat buffered SMA input for monitoring an
external signal. The original board design requires an external reference
oscillator for the MAX2769B, I am intending to operate it with a 10MHz
signal to begin with.
In addition to the above, the board also features an FTDI FT2232H (3)
device for USB data streaming, in addition to the 2232D chip on the
Papillio board used for programming. This should be able to handle
streaming the data from the 2769B chip, at least for some initial testing
and proof of concept.
As for progress, there is a small amount of routing to be finished and I am
intending to submit the board for manufacture in the next few days. It is
mainly a learning experience so far, but the general aim is to have
something similar to the GNSS Sampler boards for a smaller price (and
hopefully not a fraction of the performance!).
Anyway, I will be happy to share progress as it occurs if anyone is
interested?
Cheers for Oz,
Tristan
(1) - www.maxim-ic.com/datasheet/index.mvp/id/7267
(2) - http://papilio.gadgetfactory.net/
(3) - www.ftdichip.com/Products/ICs/FT2232H.htm
On 2 February 2012 11:22, Magnus Danielson magnus@rubidium.dyndns.orgwrote:
On 01/02/12 19:12, Attila Kinali wrote:
On Wed, 1 Feb 2012 09:27:30 -0800
Chris Albertson<albertson.chris@**gmail.com albertson.chris@gmail.com>
wrote:
I thought it might be interresting but then found out you need to buy
$2,000+ worth of hardware for even start experimenting. Open Source
SDR needs to run on a common affordable platform or it will never gain
the critical mass of users that it take to make the project live
longer then a few months.
That's because the URSP is a general purpose system. It is designed
to do many things. That makes it expensive. And being expensive,
it has a low production volume, which makes it even more expensive.
I think, a specialized GPS SDR can be build for less than 500 USD
in low (a dozen at max) volumes.
I guestimate, that the RF/ADC part would cost somewhere between
100 to 200 USD in parts. The big uncertainty here is the FPGA.
I have no clue how much logic space for a GPS SDR would be needed
at minimum and how much would be desirable. Hence i have no guess
what the FPGA would cost (could be anything from a cheap 20USD
FPGA to a 300 USD one).
That's why you start of with using an Ettus box as a boiler plate. Once
you have working code, you can re-target it for a smaller device and
situation. You can do dry synthesis towards the new platform without having
it as a physical device. The basic design can still be running on that
Ettus platform. Come to think of it, I did get a few university point on a
2-week coarse teaching exactly this point, spin on big-ass FPGA machines
and then go to target. :) That's... 18 years ago. Time flies.
I think the way to go is to find a commercial GPS chip that has a low
level interface and then build the uP controller using a common
development system. Both the chip and the uP board need to be,
common, well documented and cheap.
There are no common, well documented and cheap GPS frontend chips
out there. All chips that are still in production are for high volume
stuff. Without knowing someone inside those companies, you will not
be able to get them at single pieces. I searched quite a while some
time ago, and couldn't find anything that is not EOL. Finally i came
to the conclusion that it is easier to build a custom frontend from
scratch, from the available HF parts.
Front-end chips is still there. That's how they build these:
http://ccar.colorado.edu/gnss/
http://www.sparkfun.com/**products/8238http://www.sparkfun.com/products/8238
http://www.sparkfun.com/**products/10981http://www.sparkfun.com/products/10981
That will suffice to get you started in the SDR field.
Cheers,
Magnus
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