[time-nuts] GPS splitter

[Mark Sims]

GPS receiver  antenna alarms tend to be "informational" only.   They monitor 
the current that the antenna is drawing.   Better ones report open, short, or 
OK.  Others provide just an OK / fault.   A few have no antenna monitoring at 
all.   The RFTG-m reports the voltages at each end of a 100 ohm sreies resistor 
feeding the antenna and you can calculate the exact current that the antenna is 
drawing and also see what voltage it is feeding the antenna...  nice...

None of the thirty or so receiver types that Lady Heather supports allows you 
to turn off the antenna alarm.   Also, none of them stop working if they see an 
antenna alarm...  as long as they are getting a usable signal.   

---

> Sometimes a GPS receiver will raise an "Antenna Alarm" if it does not see a
DC load but you can turn those off with a serial command
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Arnold Tibus]

sorry about, but

who is 'lifespeed', a robot or a real person with a natural name?

many thanks,

73, Arnold, DK2WT


Am 17.06.2017 um 23:07 schrieb Lifespeed via time-nuts:

Yes, one has to lock them at a high reference frequency so as to avoid 
multiplied-up phase noise.  I can manage the tracking loop design.  Some 
applications aren't line-of-sight, so the radio link doesn't solve every 
situation.  Fiber optic backup plan, but everybody hates cords.

This is my application as well, phase measurement of the signals separated by 
some distance.  Not a billion km, but even a few km requires similar 
considerations.

Lifespeed

-Original Message-
From: jimlux [mailto:jim...@earthlink.net]

Well, at JPL we regularly lock two crystal oscillators together that are over a 
billion km apart with added Allan deviation of less than 1E-15 at
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the distance 
and velocity to spacecraft (a few cm in range and mm/s in
velocity) and from that figure out the gravitational fields (among other
things)

So it is *doable*

The performance depends ultimately on the noise within your tracking loop 
bandwidth.



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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Lifespeed via time-nuts]

Yes, one has to lock them at a high reference frequency so as to avoid 
multiplied-up phase noise.  I can manage the tracking loop design.  Some 
applications aren't line-of-sight, so the radio link doesn't solve every 
situation.  Fiber optic backup plan, but everybody hates cords.

This is my application as well, phase measurement of the signals separated by 
some distance.  Not a billion km, but even a few km requires similar 
considerations.

Lifespeed

-Original Message-
From: jimlux [mailto:jim...@earthlink.net] 

Well, at JPL we regularly lock two crystal oscillators together that are over a 
billion km apart with added Allan deviation of less than 1E-15 at
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the distance 
and velocity to spacecraft (a few cm in range and mm/s in
velocity) and from that figure out the gravitational fields (among other
things)

So it is *doable*

The performance depends ultimately on the noise within your tracking loop 
bandwidth.



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Re: [time-nuts] GPS splitter

[John Ponsonby]

Hi
There is a poorman's way of making a -3dB splitter using lengths of cable of 
characteristic impedance equal to the system impedance. That is to say using 
only 50Ω cable in a 50Ω system. It is a variant on the well known square 
four-port quadrature hybrid. In the standard hybrid with ports A,B,C & D, with 
a signal entering at A, half the power (-3dB) emerges at B and the other half 
at C and in principle nothing emerges at D. The outputs at B & C are in RF 
phase quadrature with C lagging. The four sides of the square are all of length 
λ/4. The sides BC & DA are of 50Ω cable whilst sides AB and CD are of cable 
with characteristic impedance 50/√2=35.35Ω. The poorman's version is to replace 
the low impedance sides (AB & CD) with λ/8 lengths of 50Ω cable and add λ/8 
open ended shunt stubs of 50Ω cable at each corner. As a splitter port D should 
be terminated with a 50Ω load. The OC ends of the stubs should be shielded with 
caps but in a way that doesn't significantly add end ca
 pacitance. This poorman's hybrid is rather narrow-band but adequate to cover 
the GPS L1 band (1575MHz). Three such hybrids may be joined to form a 4-way 
splitter which in principle has -6dB from input to each output. There is no DC 
path from inner to ground.

John Ponsonby
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Re: [time-nuts] GPS splitter

[Magnus Danielson]

Hi,

Yes, I knew that, but wanted to play safe.

Funny how a quick hobbyist quick-and-dirty build was needed to get an 
operator running. Ah well.


Cheers,
Magnus

On 06/17/2017 05:46 PM, Bob kb8tq wrote:

Hi

In some cases, you can get away with a 470 ohm resistor on the Tee and leave 
out the inductor. A lot depends on
the threshold of the detect circuit in the GPSDO. Since the signal is normally 
well amplified before it ever gets to the
splitter, adding a small amount of loss generally is not a big deal.  Depending 
on this and that, you may see more
loss from the Tee than from the 470 ohm resistor :)

Bob


On Jun 17, 2017, at 10:12 AM, Magnus Danielson  
wrote:

Hi,

At one time I had to design a DC-load since the GPSDO did not experience enough antenna 
current due to a different antenna being used. So, a BNC-T was quickly converted with a 
SMD inductor and resistor to add 150 Ohm of more load, and that helped the telecom 
operator to get their GPS out of "no GPS antenna" warning and actually accept 
the GPS satellites it was already detecting fine.

Cheers,
Magnus

On 06/17/2017 02:40 PM, Bob kb8tq wrote:

Hi

The DC block requirement depends a lot on the design of the GPSDO’s you
are using. With some GPSDO’s a 50 ohm load on the eighth port of a splitter
will do a pretty good job of “antenna detect” signaling. In the more general 
case
of “I didn’t design this beast” dc blocks and dc shunts to ground is the best 
approach.
This fairly quickly gets you headed in the direction of the HP / Symmetricom
splitters.

Bob




On Jun 16, 2017, at 11:54 PM, Clay Autery  wrote:

This brings up some interesting questions:

If sharing an active GPS antenna, do you have to DC block all but one
receiver port to prevent multiple receivers trying to supply current to
the antenna?

On say a 26dB antenna (ignoring line loss, power divider insertion loss,
et al), what is the effective gain to each receiver?  (Sorry, having a
senior moment)

Should ALL unused ports have 50 ohm +/- 0j terminators on them?  I
assume so...  Thus, it would be "better" to always use the divider with
the minimum required ports?

I am assuming since this is a receive only situation, it will follow
approximately the same rules of physics that dealing with satellite
antenna installations.

I would LIKE to share one PC-TEL 26dB GPS antenna mounted at the top of
my 38 foot horiz.loop mast right  at the shack entrance, using
LMR-400-DB from antenna to Narda 2-way and thence to my current hacked
Nortel GPSDO and my soon to be complete RPi 2/3 w/ Adafruit Ultimate GPS
Hat NTP Server.  On that mast, the antenna would have a near 360 degree
view of the sky completely unobstructed.  (Eventually, I expect both of
those units to be replaced with commercial units).

I'm assuming that I DC block whichever unit is capable of providing the
LEAST current at 5VDC...  I suspect the Nortel unit can supply more
current than the RPi, but that's not a guarantee...  And I guess I could
block/turn off DC delivery on BOTH units and add a voltage adjustable,
current limiting DC injection unit into the line.

Thanks.

73,

__
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389

On 6/16/2017 7:26 PM, Tim Lister wrote:

On Fri, Jun 16, 2017 at 4:40 PM, Gregory Beat  wrote:

I have reached the point that I need a 4-port splitter for my GPS antenna 
(outdoor 5 volt).  Any recommendations of models (HP/Symmertricom/Microsemi) to 
acquire OR to avoid??

As we recommended to me when I asked a similar question, the Narda
4372A-4 was a brand I had not heard of before and didn't come up in
'gps splitter' searches. I got one on ebay for $24 plus a bit extra
for DC blocks on the n-1 other ports and it seems to work well and it
was handy to have an SMA-based solution as most of the gps receivers
and the antenna pucks seem to use SMA. This meant I only needed 1 N to
SMA converter cable for an external antenna (which has yet to be
externalized...). I found it smaller in real life  than it looks in a
lot of the pictures, about the size of a modern smartphone but about
double or more the thickness (the connectors are on the ends).


greg
---

Cheers,
Tim
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Re: [time-nuts] Hints on PPS Buffer design...

[Chris Albertson]

Why 100R?   If using a 74xxx logic part as the driver is has a maximum
drive current.  100R limits current in case of a short to ground or 5V
supply  to only 50mA  It is about the lowest value resister i'd want to use

If you need 50 ohm output them go with an small RF amplifier or a
transistors driver.  These can drive a 50R load without burning up.

On Sat, Jun 17, 2017 at 1:15 AM, Clay Autery  wrote:

> Trying to pin down a reasonably optimal buffer design for bringing PPS
> out...  I've looked at all the references, like the i3detroit.org site
> et al.
>
> Of the few schematics and devices I see, most are using a hex inverter
> (1 into the other 5 paralleled with series resistors for "balance" and
> setting output impedance?
>
> Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out,
> not counting the gate impedance on the hex inverter...
>
> Q2:  Anyone have a reference to the math for choosing the resistors for
> setting a 50 Ohm nominal out INCLUDING determining and including the
> gate impedance of a particular part.
> (Right now, I am going to use the TI SN74AC04 Hex Inverter)  I saw a
> refernence in the archive referring to a 4 gate setup using a different
> part needing 187 Ohm resistors... thus I can only include that I need to
> use something slightly more than 250 Ohms on a 5 gate parallel setup)
>
> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of
> the chosen resistors?
>
> I'm sure there are others...
>
> Thanks!
>
> --
> __
> Clay Autery, KY5G
> MONTAC Enterprises
> (318) 518-1389
>
> ___
> time-nuts mailing list -- time-nuts@febo.com
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> mailman/listinfo/time-nuts
> and follow the instructions there.
>



-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] GPS splitter

[Chris Albertson]

On Fri, Jun 16, 2017 at 8:54 PM, Clay Autery  wrote:

> This brings up some interesting questions:
>
>
> I am assuming since this is a receive only situation, it will follow
> approximately the same rules of physics that dealing with satellite
> antenna installations.
>

And guess what?  Satellite TV splitters work.  They even have the answer to
"how much attenuation" printed right on the splitter and they come with F
type connecters you can use the recommended TV type antenna cable too.
You can buy them with DC blockers too.  All this stuff is low cost because
it is mass produced  by the billions

Yes, it's 75 ohm not 50 and the splitters a 2+ GHz but it works just fine
if your antenna has the right gain. The splitters don't high pass the
signal.Trimble actually recommends using the 75 Ohm TV cable and
supplies it with their kits.

Sometimes a GPS receiver will raise an "Antenna Alarm" if it does not see a
DC load but you can turn those off with a serial command
-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] GPS splitter

[Bob kb8tq]

Hi

In some cases, you can get away with a 470 ohm resistor on the Tee and leave 
out the inductor. A lot depends on 
the threshold of the detect circuit in the GPSDO. Since the signal is normally 
well amplified before it ever gets to the
splitter, adding a small amount of loss generally is not a big deal.  Depending 
on this and that, you may see more 
loss from the Tee than from the 470 ohm resistor :)

Bob

> On Jun 17, 2017, at 10:12 AM, Magnus Danielson  
> wrote:
> 
> Hi,
> 
> At one time I had to design a DC-load since the GPSDO did not experience 
> enough antenna current due to a different antenna being used. So, a BNC-T was 
> quickly converted with a SMD inductor and resistor to add 150 Ohm of more 
> load, and that helped the telecom operator to get their GPS out of "no GPS 
> antenna" warning and actually accept the GPS satellites it was already 
> detecting fine.
> 
> Cheers,
> Magnus
> 
> On 06/17/2017 02:40 PM, Bob kb8tq wrote:
>> Hi
>> 
>> The DC block requirement depends a lot on the design of the GPSDO’s you
>> are using. With some GPSDO’s a 50 ohm load on the eighth port of a splitter
>> will do a pretty good job of “antenna detect” signaling. In the more general 
>> case
>> of “I didn’t design this beast” dc blocks and dc shunts to ground is the 
>> best approach.
>> This fairly quickly gets you headed in the direction of the HP / Symmetricom
>> splitters.
>> 
>> Bob
>> 
>> 
>> 
>>> On Jun 16, 2017, at 11:54 PM, Clay Autery  wrote:
>>> 
>>> This brings up some interesting questions:
>>> 
>>> If sharing an active GPS antenna, do you have to DC block all but one
>>> receiver port to prevent multiple receivers trying to supply current to
>>> the antenna?
>>> 
>>> On say a 26dB antenna (ignoring line loss, power divider insertion loss,
>>> et al), what is the effective gain to each receiver?  (Sorry, having a
>>> senior moment)
>>> 
>>> Should ALL unused ports have 50 ohm +/- 0j terminators on them?  I
>>> assume so...  Thus, it would be "better" to always use the divider with
>>> the minimum required ports?
>>> 
>>> I am assuming since this is a receive only situation, it will follow
>>> approximately the same rules of physics that dealing with satellite
>>> antenna installations.
>>> 
>>> I would LIKE to share one PC-TEL 26dB GPS antenna mounted at the top of
>>> my 38 foot horiz.loop mast right  at the shack entrance, using
>>> LMR-400-DB from antenna to Narda 2-way and thence to my current hacked
>>> Nortel GPSDO and my soon to be complete RPi 2/3 w/ Adafruit Ultimate GPS
>>> Hat NTP Server.  On that mast, the antenna would have a near 360 degree
>>> view of the sky completely unobstructed.  (Eventually, I expect both of
>>> those units to be replaced with commercial units).
>>> 
>>> I'm assuming that I DC block whichever unit is capable of providing the
>>> LEAST current at 5VDC...  I suspect the Nortel unit can supply more
>>> current than the RPi, but that's not a guarantee...  And I guess I could
>>> block/turn off DC delivery on BOTH units and add a voltage adjustable,
>>> current limiting DC injection unit into the line.
>>> 
>>> Thanks.
>>> 
>>> 73,
>>> 
>>> __
>>> Clay Autery, KY5G
>>> MONTAC Enterprises
>>> (318) 518-1389
>>> 
>>> On 6/16/2017 7:26 PM, Tim Lister wrote:
 On Fri, Jun 16, 2017 at 4:40 PM, Gregory Beat  wrote:
> I have reached the point that I need a 4-port splitter for my GPS antenna 
> (outdoor 5 volt).  Any recommendations of models 
> (HP/Symmertricom/Microsemi) to acquire OR to avoid??
 As we recommended to me when I asked a similar question, the Narda
 4372A-4 was a brand I had not heard of before and didn't come up in
 'gps splitter' searches. I got one on ebay for $24 plus a bit extra
 for DC blocks on the n-1 other ports and it seems to work well and it
 was handy to have an SMA-based solution as most of the gps receivers
 and the antenna pucks seem to use SMA. This meant I only needed 1 N to
 SMA converter cable for an external antenna (which has yet to be
 externalized...). I found it smaller in real life  than it looks in a
 lot of the pictures, about the size of a modern smartphone but about
 double or more the thickness (the connectors are on the ends).
 
> greg
> ---
 Cheers,
 Tim
 ___
 time-nuts mailing list -- time-nuts@febo.com
 To unsubscribe, go to 
 https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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>>> 
>>> ___
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>>> and follow the instructions there.
>> 
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Re: [time-nuts] GPS splitter

[Ben Hall]

On 6/17/2017 7:40 AM, Bob kb8tq wrote:

will do a pretty good job of “antenna detect” signaling. In the more general 
case
of “I didn’t design this beast” dc blocks and dc shunts to ground is the best 
approach.
This fairly quickly gets you headed in the direction of the HP / Symmetricom
splitters.


Hi Bob and list,

This is very good timing for this discussion...as I'm in the same boat.

As others have pointed out, the issue with power splitters not designed 
for GPS use, is that they are passive, not DC-blocked, and don't have a 
way to load-down the DC-block ports to "fool" antenna sensing circuitry 
as other here have posted.


For the folks who like to "roll their own", there is this design for a 
DIY GPS antenna splitter:




It uses a MAR-6 MMIC, but since I had none of those in stock here, I 
modified it for a PGA-103 amplifier and built onto FR4 using the 
toner-transfer / ferric chloride etch method.  Seems to work okay, but 
doesn't have DC-load on the ports, just DC-block.  I have not taken it 
into work yet to put it on the VNA to see how it does.


I picked up a couple of Mini-Circuits ZC6PD-1900W 6-way splitters on the 
e-place very cheap.  And by "very cheap" I mean that even if I destroy 
all the electronics inside, the enclosures and SMA connectors are worth 
what I paid for them.


I've been toying with the idea of modifying one of the units to add 
internal DC-blocking and resistors to ground for DC-load.  For DC block, 
seems like I could very carefully slice the trace on the board by the 
SMA connector, bridging the gap with an SMA capacitor.  Problem is, when 
I add the capacitor, I'm changing the path length.  I believe I could 
minimize this by using a very small slice and a very small SMA 
capacitor...but I don't know how much this will impact performance. 
Perhaps I need to do some modeling of the design and see what happens. 
Right now I don't have anything very small, with 1206 being the smallest 
I've got.


I also picked up a 40dB PCTEL GPS timing antenna on the e-place, with 
the idea being that even if I do use a passive splitter, I'm starting 
off with 40 dB gain at the antenna versus the typical 26 dB.  If the 
splitters have ~10 dB of loss (what I measured on the VNA at work) I may 
need to add some attenuation to knock it back down to 26 dB-ish.  I 
don't know what sort of dynamic range these GPS receivers have, so that 
might not been needed.


thanks much and 73,
ben, kd5byb

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Re: [time-nuts] GPS splitter

[Magnus Danielson]

Hi,

At one time I had to design a DC-load since the GPSDO did not experience 
enough antenna current due to a different antenna being used. So, a 
BNC-T was quickly converted with a SMD inductor and resistor to add 150 
Ohm of more load, and that helped the telecom operator to get their GPS 
out of "no GPS antenna" warning and actually accept the GPS satellites 
it was already detecting fine.


Cheers,
Magnus

On 06/17/2017 02:40 PM, Bob kb8tq wrote:

Hi

The DC block requirement depends a lot on the design of the GPSDO’s you
are using. With some GPSDO’s a 50 ohm load on the eighth port of a splitter
will do a pretty good job of “antenna detect” signaling. In the more general 
case
of “I didn’t design this beast” dc blocks and dc shunts to ground is the best 
approach.
This fairly quickly gets you headed in the direction of the HP / Symmetricom
splitters.

Bob




On Jun 16, 2017, at 11:54 PM, Clay Autery  wrote:

This brings up some interesting questions:

If sharing an active GPS antenna, do you have to DC block all but one
receiver port to prevent multiple receivers trying to supply current to
the antenna?

On say a 26dB antenna (ignoring line loss, power divider insertion loss,
et al), what is the effective gain to each receiver?  (Sorry, having a
senior moment)

Should ALL unused ports have 50 ohm +/- 0j terminators on them?  I
assume so...  Thus, it would be "better" to always use the divider with
the minimum required ports?

I am assuming since this is a receive only situation, it will follow
approximately the same rules of physics that dealing with satellite
antenna installations.

I would LIKE to share one PC-TEL 26dB GPS antenna mounted at the top of
my 38 foot horiz.loop mast right  at the shack entrance, using
LMR-400-DB from antenna to Narda 2-way and thence to my current hacked
Nortel GPSDO and my soon to be complete RPi 2/3 w/ Adafruit Ultimate GPS
Hat NTP Server.  On that mast, the antenna would have a near 360 degree
view of the sky completely unobstructed.  (Eventually, I expect both of
those units to be replaced with commercial units).

I'm assuming that I DC block whichever unit is capable of providing the
LEAST current at 5VDC...  I suspect the Nortel unit can supply more
current than the RPi, but that's not a guarantee...  And I guess I could
block/turn off DC delivery on BOTH units and add a voltage adjustable,
current limiting DC injection unit into the line.

Thanks.

73,

__
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389

On 6/16/2017 7:26 PM, Tim Lister wrote:

On Fri, Jun 16, 2017 at 4:40 PM, Gregory Beat  wrote:

I have reached the point that I need a 4-port splitter for my GPS antenna 
(outdoor 5 volt).  Any recommendations of models (HP/Symmertricom/Microsemi) to 
acquire OR to avoid??

As we recommended to me when I asked a similar question, the Narda
4372A-4 was a brand I had not heard of before and didn't come up in
'gps splitter' searches. I got one on ebay for $24 plus a bit extra
for DC blocks on the n-1 other ports and it seems to work well and it
was handy to have an SMA-based solution as most of the gps receivers
and the antenna pucks seem to use SMA. This meant I only needed 1 N to
SMA converter cable for an external antenna (which has yet to be
externalized...). I found it smaller in real life  than it looks in a
lot of the pictures, about the size of a modern smartphone but about
double or more the thickness (the connectors are on the ends).


greg
---

Cheers,
Tim
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Re: [time-nuts] Hints on PPS Buffer design...

[Tim Shoppa]

Clay, as to "why 20 ohms out", there is a long-time-nominal 50 ohm PPS
convention that calls for 5V pulses to be delivered into a 50 ohm load.

If the driving voltage was 5V and source resistance was 50 ohms, then you'd
never get more than 2.5V into the load.

Different references across the net show 5V chips driving parallel 5x100
ohms, or 5x47 ohms. These are all attempts at getting more than 2.5V and
closer to 5V into the load. If you run the driver chip at 6V (which is
actually permissible for some HC-derived logic chips) then you can get even
closer to 5V into a 50 ohm load.

Most devices that consume this 5V-into-50-ohm signal are actually fine with
less than 2.5V but that's very vague.

There are real coaxial cable driver chips as well as arrangements of
bipolar transistors that do a very good driving long 50 ohm coax cables to
5V. These were very popular in the 1960's and 1970's as coax drivers for
mainframe/minicomputer computer peripherals, and are still available today
but not nearly so popular.

A much more popular cable driver standard from the past 20 years would be
LVPECL or LVDS but those are nominally differential and everyone seems to
insist on coax even today.

Tim N3QE

On Sat, Jun 17, 2017 at 4:15 AM, Clay Autery  wrote:

> Trying to pin down a reasonably optimal buffer design for bringing PPS
> out...  I've looked at all the references, like the i3detroit.org site
> et al.
>
> Of the few schematics and devices I see, most are using a hex inverter
> (1 into the other 5 paralleled with series resistors for "balance" and
> setting output impedance?
>
> Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out,
> not counting the gate impedance on the hex inverter...
>
> Q2:  Anyone have a reference to the math for choosing the resistors for
> setting a 50 Ohm nominal out INCLUDING determining and including the
> gate impedance of a particular part.
> (Right now, I am going to use the TI SN74AC04 Hex Inverter)  I saw a
> refernence in the archive referring to a 4 gate setup using a different
> part needing 187 Ohm resistors... thus I can only include that I need to
> use something slightly more than 250 Ohms on a 5 gate parallel setup)
>
> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of
> the chosen resistors?
>
> I'm sure there are others...
>
> Thanks!
>
> --
> __
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> MONTAC Enterprises
> (318) 518-1389
>
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Re: [time-nuts] Hints on PPS Buffer design...

[Bob kb8tq]

HI

> On Jun 17, 2017, at 4:15 AM, Clay Autery  wrote:
> 
> Trying to pin down a reasonably optimal buffer design for bringing PPS
> out...  I've looked at all the references, like the i3detroit.org site
> et al.
> 
> Of the few schematics and devices I see, most are using a hex inverter
> (1 into the other 5 paralleled with series resistors for "balance" and
> setting output impedance?

If the “output stages” are all going to switch at the same time, you want to 
hit their inputs
with a fast edge. As a practical point in a stand alone buffer, the invert and 
then invert
process keeps you out of a classic “oops” mistake. 

> 
> Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out,
> not counting the gate impedance on the hex inverter…

Not everybody does :)

It all depends on what you are trying to accomplish. If the spec from the OEM 
you are
building it for wants a 5V level into 50 ohms, you use a bit smaller resistor … 
You need
to check the spec from your customer. 

> 
> Q2:  Anyone have a reference to the math for choosing the resistors for
> setting a 50 Ohm nominal out INCLUDING determining and including the
> gate impedance of a particular part.

Gates are not spec’d for output impedance. The output stages are MOSFET’s and 
they
go into current limit during switching. In the full on or full off state, they 
are close to a 
short circuit. A good guess is that they are in the 20 to 50 ohm range. For 
more detail you
would have to grab a network analyzer and pick a frequency range. 


> (Right now, I am going to use the TI SN74AC04 Hex Inverter)  I saw a
> refernence in the archive referring to a 4 gate setup using a different
> part needing 187 Ohm resistors... thus I can only include that I need to
> use something slightly more than 250 Ohms on a 5 gate parallel setup)

The AC04 gates are not as high output drive as some of the others. There are 
gates 
designed as high(er) current buffers that likely will do better. Usually they 
will supply
1.5X or 2X the current of a normal gate. 

> 
> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of
> the chosen resistors?

Yes, but power is the primary issue. Will you be running into a short? (again, 
back 
to the spec requirement). Do you need to run into a short and supply 5V (or 4V 
or >2.5V) 
into a 50 ohm load? Current (and thus power) can get pretty big pretty quick.




The much more basic set of questions revolve around what I’m calling “the spec” 
above.

If you need to drive 5V CMOS logic on the other end, that gives you one 
requirement. If
you need to drive 5V TTL on the other end, the requirement is much different. 
The same is
true if you have 3.3V logic of different families on the other end. There is no 
“one size fits all”
here. 5V logic is not a favorite anymore. The easy answer on the Rx end is to 
run TTL level
5V stuff if you must have 5 volts. Then it can be driven with a 3.3V source. 
The only “works
with everything always” solution is to drive 0 to 5V into a 50 ohm load from a 
50 ohm source. 
Indeed, even that does not work with everything. The 10V p-p signal into an 
open circuit will
nuke a 5V gate. 

Do you need termination on *both* ends of the cable? We have gone around on 
this a *lot* of 
times. Terminating one end or the other is generally adequate. Terminating both 
ends does work
better. Terminating *neither* end is never recommended, but it can work out. If 
you need terminations,
do they have to be DC terminations? …hmmm….

What range of “PPS” signals will the buffer be handling? Is it’s entire life 
going to be spent running
a 10 us wide pulse? Might that pulse get inverted somehow? Will you ever be 
running a 50/50 duty
cycle signal (or 50/50 pulse per every other second …). All of this drives the 
numbers on the power
in your driver. 

Will you have multiple PPS outputs and are they all in sync? A great big high 
power driver is really
neat. It also is a great way to generate a massive spike on ground and supply. 
Isolating multiple 
drivers to reduce cross talk is a bit exciting when the signal goes down to DC 
and up to GHz. LVDS 
gets fast signals moved without a lot of power or a lot of swing. 

What will you be driving with the PPS signal? The answer of “I don’t know” is 
not at all uncommon. 
Stop and think about what actually uses a PPS around your lab. Counters can be 
adjusted to any
level and any termination. PPS inputs on digital gizmos are most likely TTL 
level. What else will you
be feeding? 

How far / where will your PPS signals be traveling? Driving a 5 or 10 foot 
cable is fine. Sending a signal to 
the next building takes a very different approach. Output isolation is 
generally ignored in a lab setup.
It may be a really big deal if high voltage or high power gear is involved. 

No, it’s not simple. No there is no single right way to do it. 

Bob


> 
> I'm sure there are others...
> 
> Thanks!
> 
> -- 
> __
> 

Re: [time-nuts] GPS discipline oscillator vs phase lock

[jimlux]

On 6/16/17 10:55 PM, Lifespeed via time-nuts wrote:

Not too surprising to read locking two crystal oscillators together
without using a physical cable is difficult to impossible.
Essentially what I am looking for is the phase alignment accuracy
(and phase noise) one would get PLL’ing one oscillator to the other
using a cable, but over a longer distance.  Some modest phase noise
degradation might be acceptable, but not an order of magnitude.
Clearly not a trivial problem. Yes, the jitter (phase noise)
typically accomplished from a PLL phase comparing at 100MHz is better
than what one could get “locking” to GPS.  It was just a thought,
apparently not a realistic one.  Thanks for disabusing me of that
notion.



Well, at JPL we regularly lock two crystal oscillators together that are 
over a billion km apart with added Allan deviation of less than 1E-15 at 
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the 
distance and velocity to spacecraft (a few cm in range and mm/s in 
velocity) and from that figure out the gravitational fields (among other 
things)


So it is *doable*

The performance depends ultimately on the noise within your tracking 
loop bandwidth.


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Re: [time-nuts] Hints on PPS Buffer design...

[Clay Autery]

On 6/17/2017 4:01 AM, Hal Murray wrote:
>> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of the
>> chosen resistors? 
> It's a 1 Hz repetition rate, but the bandwidth depends upon the rise time.  
> If the rise time is ballpark of 1 ns, the bandwidth will be ballpark of 1 
> GHz.  So, yes, you want low inductance.  That includes the power to the chip 
> as well as the resistors.  Surface mount is your friend.  So are ground/power 
> planes.
>
> How good is your scope?
Not nearly good enough, as I don't have one.  Have to borrow.  Thank you
for pointing me in the right direction on bandwidth...


>> Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out, not
>> counting the gate impedance on the hex inverter... 
> The FIVE is 6-1.  The one is for isolation.  The 5 is the rest of the 
> package. You might as well use them all as drivers.  You don't want to use 
> them for another signal (even if it is supposed to be identical) or you will 
> get minor crosstalk when you do things like plug or unplug a cable.
Yes, I got this part... At least I generally get something.  ;-)
> I don't know why you are saying "gate impedance".  That's over on the input 
> side.  I would have said "output impedance" or "driver impedance" of the chip 
> or section.
Apologize, was parroting the term used in the archived Time Nuts post I
found referencing the need to address the "driver impedance" in
selecting series resistors when trying to set the output impedance.

> I'm not sure why they picked 100 ohms.  Assume the net source impedance is 25 
> ohms.  Suppose the far end is terminated with 50 ohms.  There won't be any 
> reflections so the source value doesn't matter.  25 ohms will provide a 
> higher voltage at the far end than 50.  If you have a CMOS driver and a CMOS 
> receiver, 1/2 the voltage at the receiver is nasty.  It might be OK if you 
> have HT type receivers.
OK... I'll have to try and study this some more, because for a simpleton
like me, and impedance mismatch between the PPS out port on the Nortel
and the device to which it is connected of 25:50 ohms, reduces to 1:2,
flipped around is a 2:1 VSWR, which means a fairly significant power
loss...  Maybe it doesn't matter much with a short cable.. but the cable
is also tiny/lossy.

> I would suggest a bit of lab work.  What are you going to use on the far end?
> Lots of gear has 1000 ohms rather than 50 so a 50 ohm source impedance takes 
> care of the reflections and leaves (almost) the full voltage at the receiver.
Yes, I intend to do extensive testing.  I was just trying to trim away
some of the waste up front... time, materials, et al.  :-)
>> thus I can only include that I need to use something slightly more than 250
>> Ohms on a 5 gate parallel setup)
> More than 250 divided by 5 and rounded up a bit for the output impedance of 
> the chip will be more than 50 ohms.  I'd do some experiments.

Yep... gonna need some more instruments asap...  I may just have to wing
it and accept "good enough" until I can round up the rest of the
required instrumentation.

Thanks for your assistance and not brutalizing me for my lack of
knowledge.  I'm trying to learn.

 - C. Autery
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Re: [time-nuts] The clocks at Windsor Castle, UK

[iovane--- via time-nuts]

THAT was the time is the best comment


>Messaggio originale
>Da: "Will Kimber" 
>Data: 17/06/2017 4.22
>A: 
>Ogg: Re: [time-nuts] The clocks at Windsor Castle, UK
>
>And when those clocks were made there was no thought that in few
>centuries time a system that decrees that the time be put forward and
>back would be invented. They ran continuously and THAT was the time!
>
>Will
>
>On 06/17/2017 01:05 PM, William H. Fite wrote:
>> The clocks at Windsor range from C14 wooden-geared pieces to French
>> masterpieces of haute horlogerie with multiple complications including
>> perpetual calendars, sidereal time, equation of time, true local solar
>> time, date of Easter, various star charts and astronomical data, orreries,
>> animated figures and other automata. The more exotic and delicate of these
>> are not run continually but are started up periodically to test their
>> operation and when the Royals wish to demonstrate them to guests.
>>

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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Bob kb8tq]

Hi

If you also need the phase noise of the OCXO’s to be quite good when operating, 
the PLL approach has some issues. If you 
are after -100 doc / Hz sort of numbers at 1 Hz offset at 100 MHz, then a PLL 
to GPS is not your friend. At GPS will degrade
that by many 10’s of db. If the phase similarity requirement of the two OCXO’s 
extends out into the 100’s of Hz (you need them
phase coherent to 10 MHz maybe..), you will not be able to do that with any PLL 
approach. Again, the “record a set of modulated
signals” approach is more likely to do what you need to get done. In some ways 
the modulation *helps* in this case. It lets you better
estimate cycle slips and other odd occurrences. 

Bob


> On Jun 17, 2017, at 1:55 AM, Lifespeed  wrote:
> 
> Not too surprising to read locking two crystal oscillators together without 
> using a physical cable is difficult to impossible.  Essentially what I am 
> looking for is the phase alignment accuracy (and phase noise) one would get 
> PLL’ing one oscillator to the other using a cable, but over a longer 
> distance.  Some modest phase noise degradation might be acceptable, but not 
> an order of magnitude.  Clearly not a trivial problem. Yes, the jitter (phase 
> noise) typically accomplished from a PLL phase comparing at 100MHz is better 
> than what one could get “locking” to GPS.  It was just a thought, apparently 
> not a realistic one.  Thanks for disabusing me of that notion.
>  
> Sorry I can’t go into a lot of detail about the overall system block diagram, 
> but this one aspect of the design does just reduce to phase-locking two 
> oscillators over a distance.
>  
> Bob, I think I understand your post processing method refers to the reality 
> that all broadcast signals from which phase information could be extracted 
> are modulated, introducing complications that would not be present with a 
> simple carrier.
>  
> Lifespeed
>  
> Hi
>  
>  
>> On Jun 16, 2017, at 7:24 PM, life speed > > wrote:
>>  
>> That sounds like phase-locking the oscillators to a local radio transmitter. 
>>  Not sure there is any difference post-processing vs. real time.
>  
> The advantage is that you capture a much wider bandwidth signal than you can 
> lock to. That lets you extract better “instantaneous phase” information. With 
> the narrow band loop normally used for locking, loop dynamics get into the 
> picture. That on top of the RF propagation issues is a bit of a mess. It also 
> is quite possible to capture multiple radio (or TV or …) transmissions and 
> post process against all of them.
>  
> The bottom line is still that “many degrees” at 100 MHz is far more practical 
> than “tenth of a degree”. There are very few options if your application 
> really does need roughly a tenth of a degree. 
>  
> Bob
>  
> 
> 
>  
> - Lifespeed
>  
> 
> Hi
>  
> A far more common approach is to let the two oscillators free run and to 
> record something like a local broadcast station. 
> You then post process all of the data to give you the phase accuracy. One of 
> several gotcha’s is the stability of any 
> radio link at the level you are looking for.
>  
> Bob
>  

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Re: [time-nuts] Hints on PPS Buffer design...

[Graham / KE9H]

Read the spec sheet on the part you are using for a driver.
The size of the resistor on the paralleled driver side will be set by the
maximum current of the devices.

Decide how conservative a design you want.
Do you want it to drive into a shorted load and survive?
[If each output is rated at 20 mA, in a 5 Volt system, then R=E/I, = 250
Ohms]

Or do you only want it to work into a 50 Ohm load (to ground)?

Or perhaps only a Thevenin load? 50 Ohm load, but 100 Ohms up to +V, and
100 Ohms to ground.

Remember that 50 Ohms in a 5 Volt system will draw 100 mA on the 'High'
which can be a lot of current
for a modern IC.

--- Graham

==

On Sat, Jun 17, 2017 at 4:01 AM, Hal Murray  wrote:

>
> caut...@montac.com said:
> > Q3: It's only a 1Hz frequency, but is low inductance a desired trait of
> the
> > chosen resistors?
>
> It's a 1 Hz repetition rate, but the bandwidth depends upon the rise time.
> If the rise time is ballpark of 1 ns, the bandwidth will be ballpark of 1
> GHz.  So, yes, you want low inductance.  That includes the power to the
> chip
> as well as the resistors.  Surface mount is your friend.  So are
> ground/power
> planes.
>
> How good is your scope?
>
>
> > Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out,
> not
> > counting the gate impedance on the hex inverter...
>
> The FIVE is 6-1.  The one is for isolation.  The 5 is the rest of the
> package.  You might as well use them all as drivers.  You don't want to use
> them for another signal (even if it is supposed to be identical) or you
> will
> get minor crosstalk when you do things like plug or unplug a cable.
>
> I don't know why you are saying "gate impedance".  That's over on the input
> side.  I would have said "output impedance" or "driver impedance" of the
> chip
> or section.
>
> I'm not sure why they picked 100 ohms.  Assume the net source impedance is
> 25
> ohms.  Suppose the far end is terminated with 50 ohms.  There won't be any
> reflections so the source value doesn't matter.  25 ohms will provide a
> higher voltage at the far end than 50.  If you have a CMOS driver and a
> CMOS
> receiver, 1/2 the voltage at the receiver is nasty.  It might be OK if you
> have HT type receivers.
>
> I would suggest a bit of lab work.  What are you going to use on the far
> end?
>
> Lots of gear has 1000 ohms rather than 50 so a 50 ohm source impedance
> takes
> care of the reflections and leaves (almost) the full voltage at the
> receiver.
>
>
> caut...@montac.com said:
> > thus I can only include that I need to use something slightly more than
> 250
> > Ohms on a 5 gate parallel setup)
>
> More than 250 divided by 5 and rounded up a bit for the output impedance of
> the chip will be more than 50 ohms.  I'd do some experiments.
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
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Re: [time-nuts] GPS splitter

[Bob kb8tq]

Hi

The DC block requirement depends a lot on the design of the GPSDO’s you 
are using. With some GPSDO’s a 50 ohm load on the eighth port of a splitter 
will do a pretty good job of “antenna detect” signaling. In the more general 
case
of “I didn’t design this beast” dc blocks and dc shunts to ground is the best 
approach. 
This fairly quickly gets you headed in the direction of the HP / Symmetricom 
splitters.

Bob



> On Jun 16, 2017, at 11:54 PM, Clay Autery  wrote:
> 
> This brings up some interesting questions:
> 
> If sharing an active GPS antenna, do you have to DC block all but one
> receiver port to prevent multiple receivers trying to supply current to
> the antenna?
> 
> On say a 26dB antenna (ignoring line loss, power divider insertion loss,
> et al), what is the effective gain to each receiver?  (Sorry, having a
> senior moment)
> 
> Should ALL unused ports have 50 ohm +/- 0j terminators on them?  I
> assume so...  Thus, it would be "better" to always use the divider with
> the minimum required ports?
> 
> I am assuming since this is a receive only situation, it will follow
> approximately the same rules of physics that dealing with satellite
> antenna installations.
> 
> I would LIKE to share one PC-TEL 26dB GPS antenna mounted at the top of
> my 38 foot horiz.loop mast right  at the shack entrance, using
> LMR-400-DB from antenna to Narda 2-way and thence to my current hacked
> Nortel GPSDO and my soon to be complete RPi 2/3 w/ Adafruit Ultimate GPS
> Hat NTP Server.  On that mast, the antenna would have a near 360 degree
> view of the sky completely unobstructed.  (Eventually, I expect both of
> those units to be replaced with commercial units).
> 
> I'm assuming that I DC block whichever unit is capable of providing the
> LEAST current at 5VDC...  I suspect the Nortel unit can supply more
> current than the RPi, but that's not a guarantee...  And I guess I could
> block/turn off DC delivery on BOTH units and add a voltage adjustable,
> current limiting DC injection unit into the line.
> 
> Thanks.
> 
> 73,
> 
> __
> Clay Autery, KY5G
> MONTAC Enterprises
> (318) 518-1389
> 
> On 6/16/2017 7:26 PM, Tim Lister wrote:
>> On Fri, Jun 16, 2017 at 4:40 PM, Gregory Beat  wrote:
>>> I have reached the point that I need a 4-port splitter for my GPS antenna 
>>> (outdoor 5 volt).  Any recommendations of models 
>>> (HP/Symmertricom/Microsemi) to acquire OR to avoid??
>> As we recommended to me when I asked a similar question, the Narda
>> 4372A-4 was a brand I had not heard of before and didn't come up in
>> 'gps splitter' searches. I got one on ebay for $24 plus a bit extra
>> for DC blocks on the n-1 other ports and it seems to work well and it
>> was handy to have an SMA-based solution as most of the gps receivers
>> and the antenna pucks seem to use SMA. This meant I only needed 1 N to
>> SMA converter cable for an external antenna (which has yet to be
>> externalized...). I found it smaller in real life  than it looks in a
>> lot of the pictures, about the size of a modern smartphone but about
>> double or more the thickness (the connectors are on the ends).
>> 
>>> greg
>>> ---
>> Cheers,
>> Tim
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[time-nuts] GPS splitter

[Mark Sims]

I use an HP / Symmetricom 58517A 8 port splitter (the 58536A splitter is a 4 
port version and the 58535A is a two port version).  They have an amplifier 
built in.  It is powered by any/all of the the connected GPS devices and feeds 
power to the antenna.  It work very well... I have it driving 7 receivers at 
the moment.  No need for DC blocks or external power supplies (but you can 
waste a port and use it to power the thing (or use a "T" and DC block to supply 
external power and not waste the port)).
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Re: [time-nuts] Hints on PPS Buffer design...

[Hal Murray]

caut...@montac.com said:
> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of the
> chosen resistors? 

It's a 1 Hz repetition rate, but the bandwidth depends upon the rise time.  
If the rise time is ballpark of 1 ns, the bandwidth will be ballpark of 1 
GHz.  So, yes, you want low inductance.  That includes the power to the chip 
as well as the resistors.  Surface mount is your friend.  So are ground/power 
planes.

How good is your scope?


> Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out, not
> counting the gate impedance on the hex inverter... 

The FIVE is 6-1.  The one is for isolation.  The 5 is the rest of the 
package.  You might as well use them all as drivers.  You don't want to use 
them for another signal (even if it is supposed to be identical) or you will 
get minor crosstalk when you do things like plug or unplug a cable.

I don't know why you are saying "gate impedance".  That's over on the input 
side.  I would have said "output impedance" or "driver impedance" of the chip 
or section.

I'm not sure why they picked 100 ohms.  Assume the net source impedance is 25 
ohms.  Suppose the far end is terminated with 50 ohms.  There won't be any 
reflections so the source value doesn't matter.  25 ohms will provide a 
higher voltage at the far end than 50.  If you have a CMOS driver and a CMOS 
receiver, 1/2 the voltage at the receiver is nasty.  It might be OK if you 
have HT type receivers.

I would suggest a bit of lab work.  What are you going to use on the far end?

Lots of gear has 1000 ohms rather than 50 so a 50 ohm source impedance takes 
care of the reflections and leaves (almost) the full voltage at the receiver.


caut...@montac.com said:
> thus I can only include that I need to use something slightly more than 250
> Ohms on a 5 gate parallel setup)

More than 250 divided by 5 and rounded up a bit for the output impedance of 
the chip will be more than 50 ohms.  I'd do some experiments.


-- 
These are my opinions.  I hate spam.



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[time-nuts] Hints on PPS Buffer design...

[Clay Autery]

Trying to pin down a reasonably optimal buffer design for bringing PPS
out...  I've looked at all the references, like the i3detroit.org site
et al.

Of the few schematics and devices I see, most are using a hex inverter
(1 into the other 5 paralleled with series resistors for "balance" and
setting output impedance?

Q:  Why does everyone pick FIVE x 100 Ohm resistors?  That's 20 Ohm out,
not counting the gate impedance on the hex inverter...

Q2:  Anyone have a reference to the math for choosing the resistors for
setting a 50 Ohm nominal out INCLUDING determining and including the
gate impedance of a particular part.
(Right now, I am going to use the TI SN74AC04 Hex Inverter)  I saw a
refernence in the archive referring to a 4 gate setup using a different
part needing 187 Ohm resistors... thus I can only include that I need to
use something slightly more than 250 Ohms on a 5 gate parallel setup)

Q3: It's only a 1Hz frequency, but is low inductance a desired trait of
the chosen resistors?

I'm sure there are others...

Thanks!

-- 
__
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389

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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Hal Murray]

time-nuts@febo.com said:
> Not too surprising to read locking two crystal oscillators together without
> using a physical cable is difficult to impossible.  Essentially what I am
> looking for is the phase alignment accuracy (and phase noise) one would get
> PLL’ing one oscillator to the other using a cable, but over a longer
> distance.

Have you investigated getting a fiber connection?

I think you said the target distance was a mile.  That's easy for a fiber 
without any repeaters.  If you (try to) get the fiber from the local giant 
telephone company, they will probably run to the CO and back to the other 
site because that's the way they do things.  You might get through to 
somebody who will understand and cooperate.

Maybe you have a cooperative municipality or small telco or a site out in the 
woods where you can run your own fiber.  Or...

I didn't say it would be cheap.

You can prototype it in the lab.


-- 
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Lifespeed via time-nuts]

I guess that is the obvious answer, and sort of how the problem is currently 
addressed up to the limits of realistic cable length, which actually cannot be 
very long for my application.  I am trying to come up with a better way.  What 
you describe quickly becomes impractical.  Again, apologies for not spelling 
out the entire system.

Lifespeed

-Original Message-
From: Mike Cook [mailto:michael.c...@sfr.fr] 

Why not just have ONE frequency generator locked to GPS if you want, and just 
distribute the output with equal length cables. It would mean a cable roll to 
store at one end, but you would be assured of phase coherency at both. No? 



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Re: [time-nuts] GPS splitter

[Mike Cook]

> Le 17 juin 2017 à 02:26, Tim Lister  a écrit :
> 
> On Fri, Jun 16, 2017 at 4:40 PM, Gregory Beat  wrote:
>> I have reached the point that I need a 4-port splitter for my GPS antenna 
>> (outdoor 5 volt).  Any recommendations of models (HP/Symmertricom/Microsemi) 
>> to acquire OR to avoid??
> 

I use Mini-Circuits ZN4PD1-50-S+  with added DC blocking  as they are not 
specifically GPS splitters.  Cheap and effective.



>> greg
>> ---
> 
> Cheers,
> Tim
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George Bernard Shaw

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Re: [time-nuts] GPS splitter

[Charles Steinmetz]

Clay wrote:


If sharing an active GPS antenna, do you have to DC block all but one
receiver port to prevent multiple receivers trying to supply current to
the antenna?


Yes.  Note that you may need to put DC loads on the other receivers to 
fool them into thinking they have an antenna attached (otherwise they 
may report an antenna fault and refuse to operate).



On say a 26dB antenna (ignoring line loss, power divider insertion loss,
et al), what is the effective gain to each receiver?


Splitters often have amplifiers so they provide unity gain to all output 
ports.


Best regards,

Charles


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Re: [time-nuts] ***SPAM*** Re: GPS splitter

[Mike Cook]

> Le 17 juin 2017 à 01:40, Gregory Beat  a écrit :
> 
> I have reached the point that I need a 4-port splitter for my GPS antenna 
> (outdoor 5 volt).  Any recommendations of models (HP/Symmertricom/Microsemi) 
> to acquire OR to avoid??
> 
> greg
> —

I use Mini-Circuits ZN4PD1-50-S+  with added DC blocking  as they are not 
specifically GPS splitters.  Cheap and effective.

>> I've decided I need more clocks :)
>> 
>> Dave
> 
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George Bernard Shaw

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Re: [time-nuts] ***SPAM*** Re: GPS discipline oscillator vs phase lock

[Mike Cook]

> Le 17 juin 2017 à 07:55, Lifespeed via time-nuts  a écrit 
> :
> 
> Not too surprising to read locking two crystal oscillators together without 
> using a physical cable is difficult to impossible.  Essentially what I am 
> looking for is the phase alignment accuracy (and phase noise) one would get 
> PLL’ing one oscillator to the other using a cable, but over a longer 
> distance.  Some modest phase noise degradation might be acceptable, but not 
> an order of magnitude.  Clearly not a trivial problem. Yes, the jitter (phase 
> noise) typically accomplished from a PLL phase comparing at 100MHz is better 
> than what one could get “locking” to GPS.  It was just a thought, apparently 
> not a realistic one.  Thanks for disabusing me of that notion.
> 
> 
> 
> Sorry I can’t go into a lot of detail about the overall system block diagram, 
> but this one aspect of the design does just reduce to phase-locking two 
> oscillators over a distance.
> 

Why not just have ONE frequency generator locked to GPS if you want, and just 
distribute the output with equal length cables. It would mean a cable roll to 
store at one end, but you would be assured of phase coherency at both. No? 

> 
> 
> Bob, I think I understand your post processing method refers to the reality 
> that all broadcast signals from which phase information could be extracted 
> are modulated, introducing complications that would not be present with a 
> simple carrier.
> 
> 
> 
> Lifespeed
> 
> 
> 
> Hi
> 
> 
> 
> 
> 
> On Jun 16, 2017, at 7:24 PM, life speed  wrote:
> 
> 
> 
> That sounds like phase-locking the oscillators to a local radio transmitter.  
> Not sure there is any difference post-processing vs. real time.
> 
> 
> 
> The advantage is that you capture a much wider bandwidth signal than you can 
> lock to. That lets you extract better “instantaneous phase” information. With 
> the narrow band loop normally used for locking, loop dynamics get into the 
> picture. That on top of the RF propagation issues is a bit of a mess. It also 
> is quite possible to capture multiple radio (or TV or …) transmissions and 
> post process against all of them.
> 
> 
> 
> The bottom line is still that “many degrees” at 100 MHz is far more practical 
> than “tenth of a degree”. There are very few options if your application 
> really does need roughly a tenth of a degree. 
> 
> 
> 
> Bob
> 
> 
> 
> 
> 
> 
> 
> 
> 
> - Lifespeed
> 
> 
> 
>  _  
> 
> Hi
> 
> 
> 
> A far more common approach is to let the two oscillators free run and to 
> record something like a local broadcast station. 
> 
> You then post process all of the data to give you the phase accuracy. One of 
> several gotcha’s is the stability of any 
> 
> radio link at the level you are looking for.
> 
> 
> 
> Bob
> 
> 
> 
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have not got it. »
George Bernard Shaw

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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Charles Steinmetz]

life-sp...@yahoo.com wrote:


Perhaps I could implement an ISM band radio link for the purpose of locking the 
two oscillators.  Of course that wouldn't reach a couple miles either.


There appears to be some amount of talking past each other going on here.

First, I think you may have a fundamental misconception of phase locking 
as it applies in your proposed case.  If there are two GPSDOs, the 
oscillators are *already* phase locked -- each one to the GPS network as 
received at its actual location.  If you were to try to do some other 
phase-locking, at least one of them wouldn't be a GPSDO any more.  (That 
may not be a bad thing, if common-view GPSDOs can't achieve the required 
accuracy.)


The two GPSDOs would, ideally, produce clock "ticks" identical to each 
other within picoseconds, which would be plenty sufficient for the vast 
majority of applications.   Of course, there are inevitably various 
errors, so in reality we do not achieve the full theoretical precision 
of the system.


The largest contributors to the differential errors (i.e., the phase 
difference between the two oscillators) are (or should be)  (i) mismatch 
in the cable delays due to differences between the lengths of the coax 
connecting each receiver to its antenna and/or the propagation 
velocities of the antenna cables (including the temperature coefficients 
of the cables), and (ii) differences in the GPS "solutions" in use at 
the two locations, which includes differences between the satellite 
constellations being used moment-to-moment by the two GPS receivers and 
the local reception conditions (quality of sky view, multipath, etc.).


Then there is (iii) the jitter of each GPSDO, which is not synchronous 
one to the other.  This includes the ionospheric path distortion [maybe 
this should be its own item], the GPS receiver electronics, and the 
locked oscillators themselves (including noise on the EFC line and the 
different instabilities of the two OCXOs).


Item (i) cable delay differences due to the cable lengths and/or 
isothermal propagation velocities result in a static offset.  Most 
timing-grade GPS receivers have a "cable length" setting that allows one 
to compensate for the cable delay (although that will not correct for 
the cable temperature coefficients).  This is all avoided if you use 
integrated GPSDOs (GPSDO built into an antenna housing).


It may be possible to reduce item (iI) by operating the GPS receivers in 
"single satellite" mode, both looking at the same satellite.  If long 
observations are required (such that the satellite in use must be 
changed during the measurements), this would become messy.  The tradeoff 
in "single satellite" mode is that while you eliminate errors due to the 
different satellite constellations being used moment-to-moment by the 
two receivers, but the the RF path errors and noise may increase, giving 
back at least some of the gain.


To reduce item (iii) errors, use identical GPSDOs with the very best 
OCXOs available.  You may need to select samples of the GPSDOs to 
minimize these errors.


All that will hopefully get you down to a differential phase of a few 
nS, at least for substantial stretches of time (using single-frequency 
receivers).  Times of day with low ionospheric distortion will produce 
lower differential phase than times with higher ionospheric distortion.


If you need better differential phase than this, you may want to 
consider solutions like White Rabbit (note that there may be issues with 
portable-type [movable] applications).


Best regards,

Charles


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