Re: [time-nuts] Cheap GPSDO's

[Nick Sayer via time-nuts]

> On Mar 2, 2016, at 6:00 PM, Keith Loiselle  wrote:
> 
> Below are some comments from Said:
> 
>>> The TCXO variant is powered by an LDO rather than a switcher, FWIW.
> 
> The published schematics only showed the DC-to-DC switcher version, sounds
> like the TCXO version should have better PN and spur performance with an
> LDO.

You might be behind. Go to 
https://hackaday.io/project/6872-gps-disciplined-tcxo and check in the “files” 
section. There are PDF schematics of the 1.5.3 (DOT050V) and 1.6.2 (OH300) 
hardware versions which are presently shipping.

I had plans for the next released variant to run at 20 MHz, which requires the 
digital section to be powered at 5 volts (with a separate dedicated 3.3v LDO 
for the GPS module), with the controller offering variable output frequencies 
by dividing the 20 MHz clock. That variant is going to be superseded by the one 
with the hardware phase detection. It will still run the digital section at 5 
volts, but with a 10 MHz oscillator and no output frequency selecting. One big 
gain from powering the digital section at 5 volts is that the 3.3 volt supply 
now only powers the oscillator and the analog section - further isolating it 
from variances caused by the digital section. It also gives greater output 
amplitude, FWIW. The output frequency selection infrastructure had to go 
because I had to switch to a different controller (the ATTiny841), and it 
doesn’t have enough pins (and the feature isn’t sufficiently valuable to fuss 
over it. The original justification for the 20 MHz variant was the h
 igher clock frequency afforded greater phase detection granularity, but that’s 
mooted by the hardware phase detector).

> 
>>> In my bench experiments with the OH300, its stability did not differ when
> powered by the switcher as opposed to a massively heat-sunk LDO.
> 
> Stability is likely not affected by the design issues I had mentioned (even
> though using the LDO or switcher as a voltage reference for the DAC will
> give poor tempco and thus influence stability over temperature especially
> when using DOCXOs as opposed to using a good, dedicated voltage reference).
> What the issues that I had flagged will affect most are phase noise and
> spur performance. If the switcher happens to operate at 2.50025MHz for
> example, you will see large and wide spurs at 1KHz, 2KHz, 3KHz, etc offsets
> in the output spectra of the 10MHz output as the fourth harmonic of the
> switcher frequency beats with the crystal output.
> 
>>> Still, most of the items of which you have expressed concern would, in
> principle, have an impact on low tau
>>> ADEV measurements, and my results there are in line with the expectations
> set forth by Connor Winfield.
> 
> For low-taus of less than 1s, yes. The items listed mostly affect phase
> noise and spurs. I am not familiar with the expectations that CW sets, but
> what we likely should use as a benchmark instead are some low-cost
> secondary market alternatives such as Z3801A’s, Thunderbolts, etc.

Those are easily at least an order of magnitude more stable than the OH300, 
which is itself an order of magnitude more stable than the DOT050V.

The OH300 has a 1s tau ADEV of around 1e-11. The DOT050V datasheet claims 1e-9, 
but in my own testing I’ve seen closer to ~2e-10. I e-mailed Connor Winfield 
about this and they acknowledged that the average performance is far better 
than the claim on the datasheet. For the OH300, I’m seeing performance on a par 
with their claim, with the caveat that my TIA’s own measurement abilities peter 
out at around that level (it can’t distinguish an OH300 from a Thunderbolt, for 
instance).

I do not pretend at all to make a GPSDO that’s as good as those alternatives. 
What I offer is a less expensive GPSDO (comparing prices new rather than used), 
and that’s open hardware and has open source firmware. I believe the rest of 
the hardware performs in a way commensurate with the performance of the 
oscillator(s). If the oscillators were better, then I agree that the rest of 
the hardware probably would need to be improved to keep pace. I believe the 
market for oscillators at that level is already well served by Jackson Labs and 
Trimble and a host of others with whom I don’t believe I can reasonably compete.


> Those
> units have good to superb phase noise performance. Have you taken Phase
> Noise measurement of the output?

I have not. My TIA (53220A) lacks the ability to do so (unless there’s a 
capability in there I haven’t found yet - which wouldn’t surprise me in the 
least).

> With a good PN system it should be
> possible to fix these issues interactively.
> 
> Keith
> 
> 
> 
> Keith
> 
> On Wed, Mar 2, 2016 at 1:17 PM, Nick Sayer via time-nuts > wrote:
> 
>> 
>>> On Feb 25, 2016, at 12:41 PM, Keith Loiselle 
>> wrote:
>>> 
>>> Below is a message Said asked me to forward to the group:
>>> 
>>> 
>>> Gents,
>>> 
>>> 
>>> 
>>> 

Re: [time-nuts] Cheap GPSDO's

[Keith Loiselle]

Below are some comments from Said:

>>The TCXO variant is powered by an LDO rather than a switcher, FWIW.

The published schematics only showed the DC-to-DC switcher version, sounds
like the TCXO version should have better PN and spur performance with an
LDO.

>>In my bench experiments with the OH300, its stability did not differ when
powered by the switcher as opposed to a massively heat-sunk LDO.

Stability is likely not affected by the design issues I had mentioned (even
though using the LDO or switcher as a voltage reference for the DAC will
give poor tempco and thus influence stability over temperature especially
when using DOCXOs as opposed to using a good, dedicated voltage reference).
What the issues that I had flagged will affect most are phase noise and
spur performance. If the switcher happens to operate at 2.50025MHz for
example, you will see large and wide spurs at 1KHz, 2KHz, 3KHz, etc offsets
in the output spectra of the 10MHz output as the fourth harmonic of the
switcher frequency beats with the crystal output.

>>Still, most of the items of which you have expressed concern would, in
principle, have an impact on low tau
>>ADEV measurements, and my results there are in line with the expectations
set forth by Connor Winfield.

For low-taus of less than 1s, yes. The items listed mostly affect phase
noise and spurs. I am not familiar with the expectations that CW sets, but
what we likely should use as a benchmark instead are some low-cost
secondary market alternatives such as Z3801A’s, Thunderbolts, etc. Those
units have good to superb phase noise performance. Have you taken Phase
Noise measurement of the output? With a good PN system it should be
possible to fix these issues interactively.

Keith



Keith

On Wed, Mar 2, 2016 at 1:17 PM, Nick Sayer via time-nuts  wrote:

>
> > On Feb 25, 2016, at 12:41 PM, Keith Loiselle 
> wrote:
> >
> > Below is a message Said asked me to forward to the group:
> >
> >
> > Gents,
> >
> >
> >
> > While I haven’t posted here for a while, I have been following Time Nuts.
> > One recent post caught my attention, and here are my comments.
> >
> >
> >
> > There are some serious basic design issues with Nick Sayers’ GPSDO hack
> > mentioned by Attila in the previous post. Since Nick is asking up to $275
> > for the kit, and there are various ways to get professional-grade
> brand-new
> > GPSDO’s for significantly less than that. Here are some comments:
> >
> >
> >
> > 1.   The design is based on a 3.3V switching regulator. The output of
> > this is used essentially without any significant filtering to power the
> > DAC, the DAC opamp, the RF output drivers, and the OCXO/TCXO. All of
> these
> > items should be powered by a low-noise LDO and the DAC should be driven
> by
> > a high-stability voltage reference. The noise floor and spur levels will
> be
> > quite bad, and since the switcher is running at 2.5MHz, its fourth
> harmonic
> > and higher harmonics (at 10MHz, 20MHz, etc) will beat with the TCXO/OCXO.
>
> The TCXO variant is powered by an LDO rather than a switcher, FWIW.
>
> In my bench experiments with the OH300, its stability did not differ when
> powered by the switcher as opposed to a massively heat-sunk LDO.
>
> None of the Connor Winfield oscillators I’ve been using have a separate
> reference voltage output, which would be the best driver for the DAC. In
> lieu of that, I chose to reference the DAC directly from the oscillator
> supply voltage, in keeping with Connor Winfield’s own recommendations.
>
>
> >
> >
> >
> > Any switching noise on the 3.3V supply will also be directly transferred
> > into the opamp positive input pin via R17 and will be amplified to find
> its
> > way to the EFC pin of the OCXO. There should have been a cap in parallel
> to
> > R16 to filter this noise out. Additional low-pass filtering at the output
> > of the opamp would have also helped.
>
> There are footprints in the most recent board for an RC filter on the
> output of the OP amp, and for an additional cap across the feedback
> resistor.
>
> My experiments have shown no benefit to populating them, but they’re there
> if you want to have a go.
>
> >
> >
> >
> > 2.   The opamp positive input is driven by a 50K Ohms equivalent
> > impedance. This high resistance will cause resistor thermal noise to pass
> > right through the opamp, and will pass right into the EFC pin and
> modulate
> > the OCXO. Same issue with the negative input pin and the high-value
> > resistors R14 and R15 connected to that input pin
> >
> >
> >
> > 3.   The DAC only has ~200ppt resolution (16 bits). If using a DOCXO
> > with stability in xE-012, this will cause issues as the LSB resolution is
> > 10x to 100x worse than what the DOCXO could achieve in stability
>
> The DAC has an ~200 ppt resolution with the DOT050V variant. With the
> OH300, the resolution is closer to 20 ppt.
>
> Given the stabilities of the two oscillators, I believe this 

Re: [time-nuts] Cheap GPSDO's

[Nick Sayer via time-nuts]

> On Feb 25, 2016, at 12:41 PM, Keith Loiselle  wrote:
> 
> Below is a message Said asked me to forward to the group:
> 
> 
> Gents,
> 
> 
> 
> While I haven’t posted here for a while, I have been following Time Nuts.
> One recent post caught my attention, and here are my comments.
> 
> 
> 
> There are some serious basic design issues with Nick Sayers’ GPSDO hack
> mentioned by Attila in the previous post. Since Nick is asking up to $275
> for the kit, and there are various ways to get professional-grade brand-new
> GPSDO’s for significantly less than that. Here are some comments:
> 
> 
> 
> 1.   The design is based on a 3.3V switching regulator. The output of
> this is used essentially without any significant filtering to power the
> DAC, the DAC opamp, the RF output drivers, and the OCXO/TCXO. All of these
> items should be powered by a low-noise LDO and the DAC should be driven by
> a high-stability voltage reference. The noise floor and spur levels will be
> quite bad, and since the switcher is running at 2.5MHz, its fourth harmonic
> and higher harmonics (at 10MHz, 20MHz, etc) will beat with the TCXO/OCXO.

The TCXO variant is powered by an LDO rather than a switcher, FWIW.

In my bench experiments with the OH300, its stability did not differ when 
powered by the switcher as opposed to a massively heat-sunk LDO.

None of the Connor Winfield oscillators I’ve been using have a separate 
reference voltage output, which would be the best driver for the DAC. In lieu 
of that, I chose to reference the DAC directly from the oscillator supply 
voltage, in keeping with Connor Winfield’s own recommendations.


> 
> 
> 
> Any switching noise on the 3.3V supply will also be directly transferred
> into the opamp positive input pin via R17 and will be amplified to find its
> way to the EFC pin of the OCXO. There should have been a cap in parallel to
> R16 to filter this noise out. Additional low-pass filtering at the output
> of the opamp would have also helped.

There are footprints in the most recent board for an RC filter on the output of 
the OP amp, and for an additional cap across the feedback resistor.

My experiments have shown no benefit to populating them, but they’re there if 
you want to have a go.

> 
> 
> 
> 2.   The opamp positive input is driven by a 50K Ohms equivalent
> impedance. This high resistance will cause resistor thermal noise to pass
> right through the opamp, and will pass right into the EFC pin and modulate
> the OCXO. Same issue with the negative input pin and the high-value
> resistors R14 and R15 connected to that input pin
> 
> 
> 
> 3.   The DAC only has ~200ppt resolution (16 bits). If using a DOCXO
> with stability in xE-012, this will cause issues as the LSB resolution is
> 10x to 100x worse than what the DOCXO could achieve in stability

The DAC has an ~200 ppt resolution with the DOT050V variant. With the OH300, 
the resolution is closer to 20 ppt.

Given the stabilities of the two oscillators, I believe this is sufficient.

> 
> 
> 
> There are additional issues in this simple hw design. This design is not
> professional grade in my opinion, and at $275 not really priced as an
> amateur kit either.

Well, alrighty then.

> 
> 
> 
> For example at $220 in single piece quantities for a brand new LTE-Lite kit
> you could also purchase a very good MTI or Morion DOCXO for about $25 on
> eBay that can be gluelessly disciplined by the LTE-Lite. That solution
> would still cost a bit less than Nicks kit overall and have DOCXO stability.
> 
> 
> 
> There is one big advantage to Nicks design - access to the firmware for
> tweaking and expanding. That fw may be useful as a basis to design, or
> modify the hardware to follow the above design suggestions.

One thing I anticipate adding in the near future is a hardware phase detector 
(and full credit here goes to Jim Harman). The goal of that is to improve the 
responsiveness by a couple orders of magnitude, which will have a lot of impact 
on the 10^1-10^3 ADEV, if all goes according to plan.

Still, most of the items of which you have expressed concern would, in 
principle, have an impact on low tau ADEV measurements, and my results there 
are in line with the expectations set forth by Connor Winfield.

> 
> 
> 
> Bye,
> Said
> 
> 
> Keith
> 
> On Thu, Feb 11, 2016 at 12:34 AM, Attila Kinali  wrote:
> 
>> On Thu, 11 Feb 2016 00:13:20 -0700
>> Joseph Gray  wrote:
>> 
>>> I searched the list archives and found some discussion mid-to-late
>>> last year about several inexpensive GPSDO's made by bg7tbl. It seems
>>> that all of the better models (according to discussion on the EEVBlog
>>> forum) are gone.
>> 
>> Have a look at [1] especially at last two paragraphs.
>> 
>> If you want something cheap but ok GPSDO, get one of Nick Sayers [2].
>> He sells them on tindie [3]. The TCXO version goes for $175 and the
>> OCXO version for $275. If you already have a good OCXO 

Re: [time-nuts] Cheap GPSDO's

[Keith Loiselle]

Below is a message Said asked me to forward to the group:


Gents,



While I haven’t posted here for a while, I have been following Time Nuts.
One recent post caught my attention, and here are my comments.



There are some serious basic design issues with Nick Sayers’ GPSDO hack
mentioned by Attila in the previous post. Since Nick is asking up to $275
for the kit, and there are various ways to get professional-grade brand-new
GPSDO’s for significantly less than that. Here are some comments:



1.   The design is based on a 3.3V switching regulator. The output of
this is used essentially without any significant filtering to power the
DAC, the DAC opamp, the RF output drivers, and the OCXO/TCXO. All of these
items should be powered by a low-noise LDO and the DAC should be driven by
a high-stability voltage reference. The noise floor and spur levels will be
quite bad, and since the switcher is running at 2.5MHz, its fourth harmonic
and higher harmonics (at 10MHz, 20MHz, etc) will beat with the TCXO/OCXO.



Any switching noise on the 3.3V supply will also be directly transferred
into the opamp positive input pin via R17 and will be amplified to find its
way to the EFC pin of the OCXO. There should have been a cap in parallel to
R16 to filter this noise out. Additional low-pass filtering at the output
of the opamp would have also helped.



2.   The opamp positive input is driven by a 50K Ohms equivalent
impedance. This high resistance will cause resistor thermal noise to pass
right through the opamp, and will pass right into the EFC pin and modulate
the OCXO. Same issue with the negative input pin and the high-value
resistors R14 and R15 connected to that input pin



3.   The DAC only has ~200ppt resolution (16 bits). If using a DOCXO
with stability in xE-012, this will cause issues as the LSB resolution is
10x to 100x worse than what the DOCXO could achieve in stability



There are additional issues in this simple hw design. This design is not
professional grade in my opinion, and at $275 not really priced as an
amateur kit either.



For example at $220 in single piece quantities for a brand new LTE-Lite kit
you could also purchase a very good MTI or Morion DOCXO for about $25 on
eBay that can be gluelessly disciplined by the LTE-Lite. That solution
would still cost a bit less than Nicks kit overall and have DOCXO stability.



There is one big advantage to Nicks design - access to the firmware for
tweaking and expanding. That fw may be useful as a basis to design, or
modify the hardware to follow the above design suggestions.



Bye,
Said


Keith

On Thu, Feb 11, 2016 at 12:34 AM, Attila Kinali  wrote:

> On Thu, 11 Feb 2016 00:13:20 -0700
> Joseph Gray  wrote:
>
> > I searched the list archives and found some discussion mid-to-late
> > last year about several inexpensive GPSDO's made by bg7tbl. It seems
> > that all of the better models (according to discussion on the EEVBlog
> > forum) are gone.
>
> Have a look at [1] especially at last two paragraphs.
>
> If you want something cheap but ok GPSDO, get one of Nick Sayers [2].
> He sells them on tindie [3]. The TCXO version goes for $175 and the
> OCXO version for $275. If you already have a good OCXO you can
> modify it to get the EFC voltage out. Maybe you'll have to adjust
> the firmware for it, which you can find on github [4].
>
>
> Attila Kinali
>
>
>
> [1] http://www.ke5fx.com/gpscomp.htm
> [2] https://hackaday.io/project/6872-gps-disciplined-tcxo
> [3] https://www.tindie.com/products/nsayer/gps-disciplined-ocxo/
> [4] https://github.com/nsayer/GPS-disciplined-OXCO
> --
> 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
> ___
> time-nuts mailing list -- time-nuts@febo.com
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[time-nuts] Cheap GPSDO's

[Alan Ambrose]

>>> For more money, yixunhk has some nicely packaged units and AFAIK he has a 
>>> good reputation.

BTW I have a '58503A' from him. It did and does (a year later) still work, 
although it took me a while to get it going. But it was a bit iffy - obviously 
in a re-made case and the eBay images were not actually of this unit although 
he had obviously taken care to tell a good story with pictures of the inside 
etc ... all the way along to a shot of it getting GPS lock. Except the pictures 
were not of the unit of he sent me - actually when I looked more closely they 
were of at least three other units. The responses I got were not encouraging 
and of the 'sorry I don't understand' kind. There's been previous reports of 
yixunhk sending out units with rust inside so I started to be suspicious when I 
saw the 'cal' sticker protecting it (there's no cal on these units of course). 
Here's an extract:

Yixunhk: "Sorry for the inconvenience. Do you mean the unit we sent to you is 
different from the pictures we have listed on ebay, or is different from other 
pictures of Z58503A on ebay? Pls advise. Thank you."

Me: "To be clear - pictures 2 and 4 in the eBay description are not of this 
device. Picture 2 shows a top view of the inside of the device but clearly 
shows 4 BNC connectors. However, both the device sent and picture 4 have 2 BNC 
connectors. Also, picture 4 shows a back view with a serial number of 3710xxx 
and is marked for BTS power, but the device sent has a sticker with Z3801A on 
it and a serial number 3625xxx and is marked for BSC power (see attached 
image). It looks to me that there are mixed pictures of at least 3 different 
devices. So, my question is - what exactly has been sent?"

Yixunhk: "This unit was fully tested and it's functional before shipped out. 
This Z3801A is upgarded to 58503A by us and the feature is the same as 58503A. 
The connectors are RS-232 but not BNC. Pls refer to the service manual of 
58503A to connect to the RS-232 connector which is different from the connected 
way of Z3801A. We have one more unit in stock and the pictures listed on ebay 
is another one for reference."

Blah blah blah ... as I said I did get it working in the end.

Alan
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[time-nuts] Cheap GPSDO's

[Joseph Gray]

I searched the list archives and found some discussion mid-to-late
last year about several inexpensive GPSDO's made by bg7tbl. It seems
that all of the better models (according to discussion on the EEVBlog
forum) are gone.

I do find two similar units currently listed:

http://www.ebay.com/itm/252162780444

http://www.ebay.com/itm/181948031179

If these have already been discussed, please point me in the right
direction. Otherwise, does anyone know anything about these particular
units?

Then, there is this listing, which looks like factory-made and not homemade:

http://www.ebay.com/itm/181389532308

Comments on this one?

Thanks,

Joe Gray
W5JG
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Re: [time-nuts] Cheap GPSDO's

[John Green]

That last one sure looks interesting. I have one that is similar to the
Trimble units that have been discussed, but was made by Symmetricom. About
all I can say right now is that it *seems to work. I'll know more after I
have compared it to the Z3801.*

On Thu, Feb 11, 2016 at 1:13 AM, Joseph Gray  wrote:

> I searched the list archives and found some discussion mid-to-late
> last year about several inexpensive GPSDO's made by bg7tbl. It seems
> that all of the better models (according to discussion on the EEVBlog
> forum) are gone.
>
> I do find two similar units currently listed:
>
> http://www.ebay.com/itm/252162780444
>
> http://www.ebay.com/itm/181948031179
>
> If these have already been discussed, please point me in the right
> direction. Otherwise, does anyone know anything about these particular
> units?
>
> Then, there is this listing, which looks like factory-made and not
> homemade:
>
> http://www.ebay.com/itm/181389532308
>
> Comments on this one?
>
> Thanks,
>
> Joe Gray
> W5JG
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> time-nuts mailing list -- time-nuts@febo.com
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Re: [time-nuts] Cheap GPSDO's

[Azelio Boriani]

The last you mention is a FEI PicoSync with the even second (PP2S)
output. Usually, for time-nuts purposes (and if it will be your first
GPSDO), it is better to have a pulse-per-second (PPS) output,

On Thu, Feb 11, 2016 at 8:13 AM, Joseph Gray  wrote:
> I searched the list archives and found some discussion mid-to-late
> last year about several inexpensive GPSDO's made by bg7tbl. It seems
> that all of the better models (according to discussion on the EEVBlog
> forum) are gone.
>
> I do find two similar units currently listed:
>
> http://www.ebay.com/itm/252162780444
>
> http://www.ebay.com/itm/181948031179
>
> If these have already been discussed, please point me in the right
> direction. Otherwise, does anyone know anything about these particular
> units?
>
> Then, there is this listing, which looks like factory-made and not homemade:
>
> http://www.ebay.com/itm/181389532308
>
> Comments on this one?
>
> Thanks,
>
> Joe Gray
> W5JG
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Re: [time-nuts] Cheap GPSDO's

[Attila Kinali]

On Thu, 11 Feb 2016 00:13:20 -0700
Joseph Gray  wrote:

> I searched the list archives and found some discussion mid-to-late
> last year about several inexpensive GPSDO's made by bg7tbl. It seems
> that all of the better models (according to discussion on the EEVBlog
> forum) are gone.

Have a look at [1] especially at last two paragraphs. 

If you want something cheap but ok GPSDO, get one of Nick Sayers [2].
He sells them on tindie [3]. The TCXO version goes for $175 and the
OCXO version for $275. If you already have a good OCXO you can
modify it to get the EFC voltage out. Maybe you'll have to adjust
the firmware for it, which you can find on github [4].


Attila Kinali



[1] http://www.ke5fx.com/gpscomp.htm
[2] https://hackaday.io/project/6872-gps-disciplined-tcxo
[3] https://www.tindie.com/products/nsayer/gps-disciplined-ocxo/
[4] https://github.com/nsayer/GPS-disciplined-OXCO
-- 
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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