GPS active antenna delay ?
On 2/18/15 1:49 AM, Attila Kinali wrote:
You could try tinyVNA[1]. I have used it once, it has some quirks
(it's half hobby, half commercial project and that shows) but works
otherwise. I have no idea how accurate it is.
Attila Kinali
And tough to find any substantive information on the hardware design. I
poked around for a few minutes, but all I was able to find was software
user manuals, but no hardware description.
I assume that it's the usual 2 port, 1 direction thing.. A DDS for the
RF source, and some sort of dual channel detector with directional
couplers or a bridge on the output port.
What would be interesting is to know what detection method they're using..
HI Tom...
Good to see you getting into Time-Nuttery!
How accurately do you need to know the delay? For absolute time, it is important, but it doesn’t really help much with position since the calculation doesn't know which direction your receiver is located from the antenna. For a time-base it probably doesn’t matter much either; the PPS will simply be out of sync with 'real' seconds. The above comments will likely get me chewed up by the gnat's-eyelash crowd but that's how we mere mortals learn.
My first guess on the prop delay of the antenna would be the reciprocal of the bandwidth, but that is just a shot at getting into the ballpark. No doubt it varies over the BW and the amplifier adds a bit. Which leads back to how accurately do you need to know.
I have some ideas for helping you out with the measurements if you want to contact me off list. My TAPR email will work just fine.
Tom Holmes, N8ZM
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of Tom McDermott
Sent: Tuesday, February 17, 2015 9:33 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS active antenna delay ?
Hi Dave - agree that VNA is one good way to measure the delay. If required
accuracy is less than about
0.5 nsec, then Tx antenna to Rx antenna mutual impedance starts to become
an issue. Above about
1 nsec error probably most of these can be ignored. No access to a vector
VNA that works at 1.5 GHz.
unfortunately.
Normally I would use a difference measurement (substitute known reference
Rx antenna for Rx
under test, and difference the two), but am afraid that the (reference
Rx-to-Tx) and the (Rx under test-to-Tx)
might have different mutual Z.
Thanks for the pointer to the Keysight VNA discussion list.
-- Tom, N5EG
On Mon, Feb 16, 2015 at 6:12 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby@kirkbymicrowave.co.uk> wrote:
On 7 Feb 2015 19:18, "Tom McDermott" tom.n5eg@gmail.com wrote:
Has anyone on the list measured or otherwise estimated the active antenna
delay including the amp and filters?
-- Tom, N5EG
I have never done this, but suspect that using a VNA is the best way to
go. With a simple passive dipole on one port, and the active antenna and
cable on the second port, a transmission measurement (S21) is probably the
way to go. But I was unsure of exactly how to do it.
Your question prompted me to ask on the Keysight VNA forum. There's a reply
by Dr. Joel Dunsmore - one of the worlds leading authorities on VNAs.
http://www.keysight.com/owc_discussions/thread.jspa?threadID=39151&tstart=0
He asks what accuracy is needed. Being time-nuts, I think the answer is "as
high as possible". He writes
"With this method, 1 nsec is reasonable, but if you need 100 psec or 10
psec, then we will have to be much more careful."
You might want to contribute to that forum post.
Dave.
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On Wed, 18 Feb 2015 07:24:32 -0800
Jim Lux jimlux@earthlink.net wrote:
And tough to find any substantive information on the hardware design. I
poked around for a few minutes, but all I was able to find was software
user manuals, but no hardware description.
I assume that it's the usual 2 port, 1 direction thing.. A DDS for the
RF source, and some sort of dual channel detector with directional
couplers or a bridge on the output port
When i had one on my desk, i opened it up, but apparently i forgot to take
pictures. From what i remember, i think it works either similar to the
VNWA design done by DG8SAQ[1,2] or by N2PK[3]. The tinyVNA can only do
S11 and S12, lacking the additional source at the second port.
Attila Kinali
[1] http://sdr-kits.net/DG8SAQ/VNWA/baier_VNWA10_QEX.pdf
[2] http://sdr-kits.net/DG8SAQ/VNWA/QEX-Letters.pdf
[3] http://n2pk.com/VNA/n2pk_vna_pt_1_ver_c.pdf
--
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 Thu, 19 Feb 2015 10:51:04 +0100
Attila Kinali attila@kinali.ch wrote:
When i had one on my desk, i opened it up, but apparently i forgot to take
pictures. From what i remember, i think it works either similar to the
VNWA design done by DG8SAQ[1,2] or by N2PK[3]. The tinyVNA can only do
S11 and S12, lacking the additional source at the second port.
Correction, a little bit of googling brought up [1] (sorry, mostly german).
The discussion contains top[2], bottom[3] pictures and the block diagram[4]
Attila Kinali
[1] http://www.mikrocontroller.net/topic/351273
[2] http://www.mikrocontroller.net/attachment/238726/image.jpg
[3] http://www.mikrocontroller.net/attachment/238727/image.jpg
[4] http://www.mikrocontroller.net/attachment/243812/vna_scm_col_v3.jpg
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 2/19/15 1:59 AM, Attila Kinali wrote:
On Thu, 19 Feb 2015 10:51:04 +0100
Attila Kinali attila@kinali.ch wrote:
When i had one on my desk, i opened it up, but apparently i forgot to take
pictures. From what i remember, i think it works either similar to the
VNWA design done by DG8SAQ[1,2] or by N2PK[3]. The tinyVNA can only do
S11 and S12, lacking the additional source at the second port.
Correction, a little bit of googling brought up [1] (sorry, mostly german).
The discussion contains top[2], bottom[3] pictures and the block diagram[4]
Thanks..
I can read the German, so that's ok.
I just have to look up some of the part numbers.
On 18 Feb 2015 04:20, "Tom McDermott" tom.n5eg@gmail.com wrote:
Hi Dave - agree that VNA is one good way to measure the delay. If
required
accuracy is less than about
0.5 nsec, then Tx antenna to Rx antenna mutual impedance starts to become
an issue.
I don't see why. The two antennas can (and should) be a reasonable distance
apart. There will be no significant interaction between them.
The biggest problem might be reflections from the ground.
Above about
1 nsec error probably most of these can be ignored. No access to a vector
VNA that works at 1.5 GHz.
unfortunately.
If it was a 10-15 minute job I would not mind doing it for you as I have
several VNAs that cover 1.5 GHz.
But it would take quite a bit of time to build the appropriate antenna then
mount the two in such a way that reflections were not an issue. It means
errecting two masts on non conductive poles. It is basically a days work.
If you have a spectrum analyzer, you could probably see the bandwidth of
the filters from looking at the noise spectrum. From what someone wrote
earlier, with typical delays vs bandwidth, you could perhaps get a rough
estimate of the delay.
I just wonder if there's some other way, especially if one has multiple GPS
receivers, a fast scope.... but I can't think of anything.
A VNA is the obvious tool to me, but I would not be surprised if there was
some trick that could be used without a VNA.
Thanks for the pointer to the Keysight VNA discussion list.
No problem. For answers to VNA related questions, the Keysight forums are
the place to go. Neither Anrisu or R&S have anything like this.
-- Tom, N5EG
Dave.