Re: [time-nuts] DAC performance

[Richard (Rick) Karlquist]

On 7/17/2017 1:41 PM, Attila Kinali wrote:

On Mon, 17 Jul 2017 12:07:29 -0700
"Richard (Rick) Karlquist"  wrote:


On 7/17/2017 10:54 AM, Azelio Boriani wrote:

This implies that in a Rb or Cs there is not a voltage reference source?



Yes, that's right, there is no voltage reference with a material
effect on stability or accuracy.


How about the temperature and C-field control? Aren't those based
on keeping the output of a sensor stable compared to a voltage reference?

Attila Kinali



Good oven design is typically done with thermistor/resistor bridges 
which are ratiometric and definitely don't depend on a reference.

In the 5071A, the C field is controlled with
a Zeeman splitting check, hence no reference involved.  In
older Cs standards, the C field accuracy depended on a reference
but it didn't require a very good one to get sufficient accuracy
in the C field.  In Rb standards, the C field is so crude it
is often adjusted using a pot.

Rick N6RK
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Re: [time-nuts] DAC performance

[Charles Steinmetz]

Attila wrote:


Charles Steinmetz  wrote:



how about the LTC1650?
 * * *
[it] is nearly 100x (40dB) quieter (30nV/sqrtHz vs. 280) [than the 1650]



I was about to ask the same question :-)


Note:  I divided 280 by 30 and got "nearly 100x".   D'Oh!  Of course, it 
is ~10x, or 20dB, not ~100x or 40dB as I stated.  Still, a ~20dB noise 
advantage is substantial.


One point I didn't mention previously -- the 1650 is only marginally 
more expensive than the 1655.



I am sure there are ways to compensate for the charge injection
by using multiple switches, but I have not been able to come up
with a good way. Does anyone have a good idea?


I've tried several approaches to cancel the glitch energy of analog 
switches, but I never hit on anything I was really happy with -- partly 
because the glitch energy is more random than you'd like and partly 
because the "kludge factor" of multiple-switch solutions exceeds my 
tolerance pretty quickly.  Even if you accept the high kludge factor, 
you find that like input bias current cancellation, glitch cancellation 
is most effective (only really effective??) if it is done on-chip.


Best regards,

Charles


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Re: [time-nuts] unknown GPSDO

[Richard Solomon]

The July 1998 QST had an article on a GPSDO he designed.

That may help.


73, Dick, W1KSZ


Sent from Outlook

From: time-nuts  on behalf of Volker Esper 

Sent: Monday, July 17, 2017 2:47:42 AM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] unknown GPSDO

Hello dear fellow time-nuts,

Last weekend I bought a selfmade GPSDO at the fleamarket for a very
adequate price. Now I need some more information about it to operate it.
It seems to work fine, but I don't understand the displayed numbers, the
menu, the lamps, the DIP switches...

Can you help to identify it? I couldn't find it on the internet. Here
are some fotos...

When powering up, the display shows "Brooks Shera GPSVCXO 2014".
Enthough that might give me a clear hint to identify it, I only found
the starting docs from 1998 or so, but not the actual board.

Thanks a lot!

Volker


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[time-nuts] Quantic GPS receiver EFC control (was: Papers on timing for lunar laser ranging)

[Attila Kinali]

On Mon, 17 Jul 2017 10:24:42 -0700
Brooke Clarke  wrote:

> The Quantic Timing GPS receiver makes use of patent 5440313
> which results in a 48 bit DAC.  How does this idea compare?
> http://www.prc68.com/I/Q5200.shtml
> https://www.google.com/patents/US5440313
> PS the above patent cites 4582434 i.e. the Heathkit GC1000 HFDO.

Figure 4 is nothing but an elaborate way of describing a PWM generator.
Figure 5 uses an DDS to generate an offset frequency relative to the
GPSDO output and uses that to phase lock the GPSDO to an external
frequency reference .

I don't think that Fig 5 gives any advantage over a proper DAC.
If anything, I would guess the spurs of the DDS generate a lot
of noise on the EFC. It's advantage is, though, that you can lock
the 10MHz on any reference that is close to 10MHz (or far off, for
that matter) and use the stability of that reference for the output
of the GPSDO.

There are IMHO better ways to achieve that, though.

Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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[time-nuts] TAPR GPS Kit end plates

[Steve - Home]

A few weeks ago we wrapped up the end plate project. We had only 1 non-payer in 
the end, Tom Vojtek K1ADR, who hadn't responded to emails regarding payment so 
Tom's set and the extra sets of plates we ordered for engineering assessment 
were spoken for and I've had a few people ask about a second run of plates. 

Jerry, the machinist who deserves the time-nuts Volunteer of the Year award for 
the time and effort he put into the end plate project, agreed to do a second 
run of plates. 

I will place another order under the same conditions as the first order:
• Cost based on final quantity ordered. Cost will be higher as the numbers will 
be smaller so the quantity discount will be less. Plate cost and all shipping, 
plus PayPal fees, should be paid to me once the final cost is known
• From the above I will pay Jerry for the shipping from him to you so there 
will only be one PayPal fee
• If we don't have enough orders to meet the required $100 minimum I'll let 
everyone know and those who missed out will need to find alternate ways to 
close up the box and mount the connectors. 

I would ask those of you who had contacted me about a second purchase of plates 
to please contact me again now that the door is open for another order. 

Thanks,

Steve
WB0DBS



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Re: [time-nuts] DAC performance [WAS: Papers on timing for lunar laser ranging]

[Attila Kinali]

On Sun, 16 Jul 2017 14:24:24 -0400
Charles Steinmetz  wrote:


> Well, to name just the first one that comes to mind, how about the 
> LTC1650?  Like the 1655, it is available in SO and DIP packages.  Its 
> differential nonlinearity is >2x better than the 1655, it settles 5x 
> faster (4uS vs. 20uS to 1 LSB) and is nearly 100x (40dB) quieter 
> (30nV/sqrtHz vs. 280).  The 1650 has substantially lower glitch energy, 
> as well (1.8nV-S for the 1650, 12nV-S for the 1655).

I was about to ask the same question :-)

BTW: I am planning to do something similar with an LT1650 and
thought about adding an CMOS switch to minimize the glitch energy.
Though even low charge injection switches like the ADG5212/5213
give something in the order of 0.5pC.. which turns out to be in the
order of magnitude of the LTC1650's worst case glitch energy, if put
into a complete circuit.

I am sure there are ways to compensate for the charge injection
by using multiple switches, but I have not been able to come up
with a good way. Does anyone have a good idea?

> I have not surveyed the field to see what other "SO or easier to solder" 
> DACs are available with better performance than the 1655, but I'm sure 
> there are others.

There is also the AD5060 family from Analog. Though their performance
is slightly worse then the LTC1650, they are much cheaper. Unless you
need the high specs, they are a cheap alternative.
 
Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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Re: [time-nuts] DAC performance

[Attila Kinali]

On Mon, 17 Jul 2017 12:07:29 -0700
"Richard (Rick) Karlquist"  wrote:

> On 7/17/2017 10:54 AM, Azelio Boriani wrote:
> > This implies that in a Rb or Cs there is not a voltage reference source?
> > 
> 
> Yes, that's right, there is no voltage reference with a material
> effect on stability or accuracy.

How about the temperature and C-field control? Aren't those based
on keeping the output of a sensor stable compared to a voltage reference?

Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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Re: [time-nuts] DAC performance [WAS: Papers on timing for lunar laser ranging]

[Richard (Rick) Karlquist]

On 7/17/2017 10:54 AM, Azelio Boriani wrote:

This implies that in a Rb or Cs there is not a voltage reference source?



Yes, that's right, there is no voltage reference with a material
effect on stability or accuracy.

Rick N6RK
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Re: [time-nuts] DAC performance [WAS: Papers on timing for lunar laser ranging]

[Azelio Boriani]

This implies that in a Rb or Cs there is not a voltage reference source?

On Mon, Jul 17, 2017 at 3:49 AM, Bob kb8tq  wrote:
> HI
>
> This is a limitation on an OCXO based GPSDO. That’s really the bottom line 
> here.
> It’s a limitation in an OCXO based part, but not in one based on an Rb or a 
> Cs. If
> the added component costs far more than a Cs, it’s not an answer.
>
> Bob
>
>> On Jul 16, 2017, at 7:25 PM, Chris Albertson  
>> wrote:
>>
>> What about josephson standards?   After all, this is "Time Nuts" and we are
>> allowed to propose silly-complex solutions to simple problems if it
>> improves performance even a little.
>>
>> But seriously I thought the issue of making a perfect voltage standard was
>> solved because the Volt is defined to be whatever the Josephson array
>> produces. Yes expensive because to runs at nearly absolute zero.
>>
>> On Sun, Jul 16, 2017 at 4:10 PM, Bob kb8tq  wrote:
>>
>>> Hi
>>>
 On Jul 16, 2017, at 6:33 PM, Richard (Rick) Karlquist <
>>> rich...@karlquist.com> wrote:



 On 7/16/2017 1:51 PM, Bob kb8tq wrote:
> Hi
> One gotcha with any ADC or DAC is going to be the reference. There, you
>>> are in the same
> “get what you pay for” dilemma. Stable and noisy, can do. Quiet and not
>>> very stable, can do.
> Both stable and quiet, not so easy if you want it cheap.
> Noise can also be the sigma delta ADC’s weak point. Even at slow rates,
>>> some of them need
> a lot of averages to quiet down.

 The reference initially used in the E1938A turned out to be too
>>> noisy/unstable.  It was non trivial to find an upgrade.  The
 HP Smart Clocks of 20 years ago were limited in their performance
 by the reference used.

 Has there been much improvement in references in the intervening
 20 years?
>>>
>>> They still don’t seem to have the hysteresis problem licked. Yes, you can
>>> do an oversized reference
>>> and take care of the issue. More or less that’s what you would have done
>>> 20 years ago.
>>>
>>> Bob
>>>
>>>

 Rick N6RK
>>>
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>>>
>>
>>
>>
>> --
>>
>> Chris Albertson
>> Redondo Beach, California
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>
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Re: [time-nuts] Papers on timing for lunar laser ranging

[Brooke Clarke]

Hi:

The Quantic Timing GPS receiver makes use of patent 5440313which results in a 
48 bit DAC.  How does this idea compare?
http://www.prc68.com/I/Q5200.shtml
https://www.google.com/patents/US5440313
PS the above patent cites 4582434 i.e. the Heathkit GC1000 HFDO.

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html

 Original Message 

On Sat, 8 Jul 2017 11:42:44 -0700
Tim Lister  wrote:


Forgive the ignorance, but why is there a large disparity between ADC
and DAC capabilities ?
For example, Linear Technology sell a 24 bit ADC for ~$7 but an 18 bit
DAC is $30-50...

Much simplified, it boils down to it being easier to measure voltage
differences by averaging than keeping a voltage constant.

E.g. in those >20bit ADC's you will usually find a delta-sigma ADC,
usually 3rd to 5th order with a 1.5 to 5 bit ADC/DAC inside. The ADC
and DAC can be laser trimmed to be in the order of 0.1% of their
ideal values. With a few additional tricks you can get the most of
the remaining non-linearity out. These tricks also help to remove
errors due to DC-offsets in the signal path. But the biggest
improvement comes from averaging over many "samples" to get the
white noise out. If you look at the usual sample rates at which
those ADC reach their "full" performance, it is around
1-30 (output) samples per second.

On the other hand, on a DAC you need to keep the output voltage
stable. You can do the same delta-sigma approach as with the ADC
with much the same result, but you have one big problem:
it is not easy to build an analog low pass filter that has a corner
frequency down at 10Hz. This means, you have to work at a much higher
frequency to have a low pass filter that can be realized (let's say 1kHz
if you are building a discrete filter, higher if it's integrated).
But that means that you have several orders of magnitude more (white) noise.

Additionally, a lot of people expect to do a couple of 1000 samples
per second at least, to have a usefull DAC. But that contradicts the
need to have a narrow band low pass filter to get the noise out.


Attila Kinali


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