Re: [time-nuts] GPS Disciplined TCXO

[Nick Sayer via time-nuts]

> On Oct 23, 2015, at 9:10 AM, Attila Kinali  wrote:
> 
> On Fri, 23 Oct 2015 07:30:10 -0700
> Nick Sayer via time-nuts  wrote:
> 
> 
>>> BTW: Tom van Baak mentioned in a private mail, that the DOC supply current
>>> is much higher than i thought it was. So it isn't exactly a drop in 
>>> replacement
>>> either.
>> 
>> I’ve run them with the same LDO and they work. The LDO gets warm, of
>> course, but then, so does the oscillator.
> 
> The LM1117 is specced for 800mA output. The DOC is specced with max 2.5W
> at warmup, or 750mA @ 3.3V. Assuming the rest of the circuit does not use
> more than 50mA, that's barely within specs. I would guess that going up
> to 900mA shouldn't fry the LM1117 as long as you can deal with the produced
> heat. Even in the best case, it's too close for my engineers love of
> safety margins :-)

I agree. The only mollification I can offer is that the DOC pulls warm-up 
currently only for a few seconds before ramping down to ~330 mA steady-state.

But in any event, testing with the switcher showed that there was no benefit to 
an LDO for OCXOs. This was confirmed by Connor Winfield. I talked to them about 
ripple and noise tolerance and they said that their OCXOs have separate 
internal regulators for the oscillator, so the 3 mV P-P that I get out of the 
SC189Z is just fine. The extra capacitance on the DAC supply pins brings that 
down a bit more by the time the power gets over there, but the ripple is ~2.5 
MHz, so it’s pretty much gone from the DAC output after the LPF. Any 
non-linearity that might result simply results in the discipline settling on a 
“different” value.

On the other hand, attempting to use the switcher with the DOT050V was a 
disaster. I had to add extra dummy load to bring the ripple down to 3 mV P-P, 
and even then the short term stability of the DOT was an order of magnitude 
worse. The extra load made the efficiency gains moot, and the actual load is 
low enough that the inefficiency of the LDO is hardly an issue. That was about 
the same time I gave up on trying to use the DOC part, so the need for a 
unified design went away. I was left with two boards - an LDO feeding the 
DOT050V and the SC189Z feeding an OH300.


> 
>> When I compare them to the same
>> circuit, but powered with the SC189Z switcher I use for the OH300 variant,
>> their performance is identical. Which is to say, a half order of magnitude
>> worse than the DOT050V at tau 1s.
> 
> Hmm... For a circuit like this, I would use a switched DC/DC converter
> to get down to 3.8V or even 3.5V, then use a modern LDO with high ripple
> rejection and low drop-out (<0.5V). This solution will be more expensive
> then a simple LM1117, but the heating of the circuit would be approximately
> independent of the input voltage and also much lower.
> 

I thought about that, but it turns out that 3 mV P-P is good enough. When I 
compare the DOC with an LDO to the SC189Z, the performance is identical. I 
haven’t tried an OH300 with an LDO, but its performance with the switcher is in 
the ballpark of spec (albeit with sample size 1).

> 
>> The data sheets of both claim a short term stability of 1e-9 at 1s. The DOC
>> is exceeding that, but the DOT is *slaughtering* it. I routinely see 9e-11
>> instead. I asked Connor Winfield about that and they said that they should
>> change the data sheet, as my observations are more in line with reality.
> 
> But no explanation why this might be the case?

Other than their TCXOs are really good? :)

> 
>> Meanwhile, the OH300 (I’ve only tried one of them so far, however) is at
>> around 3e-11 at 1s. So that’s what $100 buys you. :)
> 
> I'm taking notes here :-)
> 
>> The other thing I saw with the DOC020V that I have not seen with either the
>> DOT or OH300 variants is a weird oscillation on the VC pin. I posed the
>> question to Stack Exchange and they suggested that the VC pin internally goes
>> to a varactor which adds too much capacitance for the output of the OP amp. 
> 
> That sounds kind of strange. Yes, too much capacitance could lead to
> oscillations. But given the fact that the datasheet of the AD8538
> has graphs that go up to 1000pF load capacitance (although with huge
> overshots) I would expect it to be stable at least up to 100pF (where
> the overshot graph starts to go up). Most Varicaps are usually in the
> range of 2-20pF, there are a few that go up to 100pF, but it'd be
> surprised if they'd use one of these.
> 
> You can "easily" try whether the varicap theory is right, if you
> add an 2-10k resistor in the path between the AD8538 and the VCO/OCXO.

The oscillations stayed on the oscillator side and disappeared from the amp 
side of the resistor. It took adding capacitance to the VC pin (0.1µF) to damp 
the oscillations away. The final configuration was 0.1µF and 330Ω. With that, 
you could see very small little “ticks” where it was trying to start 
oscillating, but they were only a few hundred µV and 

Re: [time-nuts] GPS Disciplined TCXO

[Nick Sayer via time-nuts]

> On Oct 23, 2015, at 10:18 AM, Nick Sayer  wrote:
> 
> 
>> On Oct 23, 2015, at 9:10 AM, Attila Kinali  wrote:
>> 
>> On Fri, 23 Oct 2015 07:30:10 -0700
>> Nick Sayer via time-nuts  wrote:
>> 
>> 
>> 
 
 In the AD5061 datasheet (Rev C) on page 3, at the bottom,
 section "Reference Input/Output", item "V_REF Input Range".
 Yes, it will not fry the device, but it will not work up to spec either
 (unfortunately, manufacturers never mention what part of the spec gets
 degraded if you violate something).
>>> 
>>> I do see that now. If you look at the footnote, figure 27 which it 
>>> references 
>>> shows that the Vdd headroom with a VRef of 3.3 volts is more like 20 mV or 
>>> so. I would posit that the headroom issue is more likely to result in non-
>>> linearity at high DAC values, perhaps because the output amp may not be 
>>> capable of full rail-to-rail operation. 
>> 
>> It is not clear where the limit comes from. It could be the ouptut buffer,
>> which would then be a no-problem. Or it could be the input buffer for V_ref,
>> which then would lead to an un-buffered, and hence unstable internal 
>> reference
>> voltage. Or it could be a problem of one of the DAC internal stages, that
>> needs the headroom….
> 
> I’ll try asking Analog Devices and see if I can get an answer to the question 
> of what happens when Vdd=Vref=3.3V. All I know is that in practice, it’s 
> working as designed, including meeting the gross tuning slope predictions 
> within 10%.

They got back to me quickly. They said that as you increase the DAC values, 
there will come a point where the output will stop rising as you get to the 
point where the headroom is necessary. That is, the top values will get clipped.

The argument could be made that an alternate configuration could result in 
reclaiming that lost precision, but for now, I think I’m going to let it go. 
The current configuration counts on the midpoint on the input being the same as 
the midpoint on the output. If you reduce Vref, that’s not true anymore. If 
that’s not true, then the tuning range is asymmetric, and that means you’re 
going to throw away part of it. I guess the headroom issue means I’m throwing 
away part of the top end asymmetrically as well, though. *shrug*


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[time-nuts] really Idle Lucent KS24361

[Gerhard Hoffmann]

Is there an easy way to get the Z3811/12 into a state where they
just leave their oscillators alone? I do not mean holdover as when
I just pull the GPS antenna plug; then they will probably try to
correct for drift based on learned history.

I want the "pure" MTI-260s.

And if I build in mechanical switches, they will interfere with
the learning...

regards, Gerhard
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Re: [time-nuts] Unified VCXO Carrier Board

[Charles Steinmetz]

Bruce wrote:

Your statement about the PN of comparators conflicts with my 
measurements. The LTC6957 evaluation board had an 18dBc/Hz lower 
phase noise floor than a comparator circuit with 10MHz 15dBm inputs. 
However I only measured a single comparator circuit. The Holzworth 
sine to CMOS converter had a comparable PN to the LTC6957-4.
I haven't, as yet measured the PN of an optimised Wenzel circuit.My 
setup for this measurement had a PN floor of around -180dBc/Hz.


There are many, many ways of getting unnecessarily poor PN 
performance from comparators (including Wenzel-style squarers) -- one 
has to make sure not to make any of myriad mistakes in both design 
and execution.  You didn't say which comparator you tried, or in what 
circuit, so I'm not in a position to suggest things to check (or to 
confirm that the comparator you tried performs similarly poorly in my 
tests, if that is the case).


One sanity check you can try -- disable the filtering on your 6957 
eval board.  According to the LT data presented in the chart I 
posted, which agrees very closely with my test results, at 
10MHz/15dBm there should be essentially no change in the PN compared 
to the results you obtained with filtering enabled.  If you see a 
significant difference, then something is causing anomalous results.


Best regards,

Charles


ps.  You often respond to one message by replying to a different 
message, as you did in this case.  It would be helpful for someone 
who just joins a thread, and for continuity in general, if you would 
reply to the message to which you are actually responding.  That way, 
readers who are new to the thread will have the context they need, 
and your interlocutor will have his or her previous message 
conveniently available to refer to in any further message.





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Re: [time-nuts] GPS Disciplined TCXO

[Attila Kinali]

On Thu, 22 Oct 2015 21:07:38 -0700
Nick Sayer via time-nuts  wrote:


> > I like the choice of the TXCO. Which allows you to replace it
> > by the pin compatible DOC050V and have an OCXO instead.
> 
> That actually is a downgrade. I’ve tried both, and the DOC020V’s short term 
> stability is much worse. I’m guessing that the OCXO’s selling point is it’s
> medium to long term stability, which is mooted by the GPS discipline here.

BTW: Tom van Baak mentioned in a private mail, that the DOC supply current
is much higher than i thought it was. So it isn't exactly a drop in replacement
either.

> > But I have three things to critisize:
> > 
> > * The AD506x DAC family has a peculiar restriction: the maximum
> >  specced Vref is 50mV below VDD. Ie connecting Vref to VDD means
> >  that the DAC is used outside of specs and thus might or might
> >  not get the speced accuracy/precision.
> 
> I don’t see that anywhere in the datasheet. Can you give me a reference for 
> that? The only thing I see is an absolute maximum of Vdd + 0.3V, but of
> course “absolute maximum” ratings aren’t necessarily what you’re supposed to 
> design to.

In the AD5061 datasheet (Rev C) on page 3, at the bottom,
section "Reference Input/Output", item "V_REF Input Range".
Yes, it will not fry the device, but it will not work up to spec either
(unfortunately, manufacturers never mention what part of the spec gets
degraded if you violate something).


> > 
> > * Using VDD as reference voltage, when VDD is generated by an LM1117
> >  is kind of iffy. Neither the internal reference of the LM1117 nor
> >  the control loop are very stable. There ar LDOs out there, that are
> >  speced for use as ADC/DAC references, but this one isn’t. 
> 
> I’ve considered a separate regulator for the reference, but my thinking is/
> was that it would be bad for the supply voltage of the oscillator to be 
> regulated separately from the reference. The two regulators might drift 
> apart. Connor Winfield’s OCXO application note actually says this too. It’s 
> too bad that these oscillators don’t have a reference voltage output, but I 
> guess that’s part of what makes them cheaper.

Variations in Vdd will change the oscillator frequency slightly (20ppb for 5%).
Changes in Vref will have a higher impact on the frequency. Also, the control
voltage of an oscillator is usually referenced against ground, ie its 
"processing"
is more or less independent of Vdd.


> 
> > 
> > * The DOT050V has an upper limit for the control voltage of 3.0V,
> >  (and a lower 0.3V) but the circuit could potentially supply 3.3V there
> >  losing 10% (or 20%) of precision.
> 
> It shouldn’t be able to do that. The buffer amplifier is in a less-than-unity
> gain configuration with a Vdd/2 virtual ground. It’s designed to reduce the
> swing to 51% for the DOT050V variant or 82% for the OH300 variant.

Oops.. right. Missed that the resistors arent of equal size.


Attila Kinali


-- 
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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Re: [time-nuts] GPS Disciplined TCXO

[Attila Kinali]

On Fri, 23 Oct 2015 07:30:10 -0700
Nick Sayer via time-nuts  wrote:


> > BTW: Tom van Baak mentioned in a private mail, that the DOC supply current
> > is much higher than i thought it was. So it isn't exactly a drop in 
> > replacement
> > either.
> 
> I’ve run them with the same LDO and they work. The LDO gets warm, of
> course, but then, so does the oscillator.

The LM1117 is specced for 800mA output. The DOC is specced with max 2.5W
at warmup, or 750mA @ 3.3V. Assuming the rest of the circuit does not use
more than 50mA, that's barely within specs. I would guess that going up
to 900mA shouldn't fry the LM1117 as long as you can deal with the produced
heat. Even in the best case, it's too close for my engineers love of
safety margins :-)

> When I compare them to the same
> circuit, but powered with the SC189Z switcher I use for the OH300 variant,
> their performance is identical. Which is to say, a half order of magnitude
> worse than the DOT050V at tau 1s.

Hmm... For a circuit like this, I would use a switched DC/DC converter
to get down to 3.8V or even 3.5V, then use a modern LDO with high ripple
rejection and low drop-out (<0.5V). This solution will be more expensive
then a simple LM1117, but the heating of the circuit would be approximately
independent of the input voltage and also much lower.


> The data sheets of both claim a short term stability of 1e-9 at 1s. The DOC
> is exceeding that, but the DOT is *slaughtering* it. I routinely see 9e-11
> instead. I asked Connor Winfield about that and they said that they should
> change the data sheet, as my observations are more in line with reality.

But no explanation why this might be the case?

> Meanwhile, the OH300 (I’ve only tried one of them so far, however) is at
> around 3e-11 at 1s. So that’s what $100 buys you. :)

I'm taking notes here :-)
 
> The other thing I saw with the DOC020V that I have not seen with either the
> DOT or OH300 variants is a weird oscillation on the VC pin. I posed the
> question to Stack Exchange and they suggested that the VC pin internally goes
> to a varactor which adds too much capacitance for the output of the OP amp. 

That sounds kind of strange. Yes, too much capacitance could lead to
oscillations. But given the fact that the datasheet of the AD8538
has graphs that go up to 1000pF load capacitance (although with huge
overshots) I would expect it to be stable at least up to 100pF (where
the overshot graph starts to go up). Most Varicaps are usually in the
range of 2-20pF, there are a few that go up to 100pF, but it'd be
surprised if they'd use one of these.

You can "easily" try whether the varicap theory is right, if you
add an 2-10k resistor in the path between the AD8538 and the VCO/OCXO.

> I asked Connor Winfield about this and they didn’t have an immediate answer. 
> The latest boards have the footprints for another RC LPF on the output to
> counter this, but since I gave up on the DOC part, the cap is no-stuff and
> the resistor is 0 ohms when I build them out (with the other parts).

ACK

> > 
> > In the AD5061 datasheet (Rev C) on page 3, at the bottom,
> > section "Reference Input/Output", item "V_REF Input Range".
> > Yes, it will not fry the device, but it will not work up to spec either
> > (unfortunately, manufacturers never mention what part of the spec gets
> > degraded if you violate something).
> 
> I do see that now. If you look at the footnote, figure 27 which it references 
> shows that the Vdd headroom with a VRef of 3.3 volts is more like 20 mV or 
> so. I would posit that the headroom issue is more likely to result in non-
> linearity at high DAC values, perhaps because the output amp may not be 
> capable of full rail-to-rail operation. 

It is not clear where the limit comes from. It could be the ouptut buffer,
which would then be a no-problem. Or it could be the input buffer for V_ref,
which then would lead to an un-buffered, and hence unstable internal reference
voltage. Or it could be a problem of one of the DAC internal stages, that
needs the headroom

BTW: 20mV or even 50mV below VDD is considered close to rail
(as in rail-to-rail). You need to compare this to the 1-2V (sometimes
even 3V) that opamps needed as headroom (both at input and output) from
the rails before rail-to-rail opams became en-vogue. IIRC any headroom
around/below 100-200mV is considered close to rail.

 
> In practice, the VC output of the analog section is quite stable (more stable 
> > than the analog supply rail), however, because of the LPF on the DAC's 
> output. Remember that the DAC was designed with a maximum clock speed of
> 30 MHz and we’re using it effectively at 10 mHz. So the LPF trades bandwidth 
> and slew rate (neither of which are interesting in this application) for 
> stability. And to some extent you’d be throwing away the pains to which you 
> went to come up with a super-accurate Vref because you then feed the DAC 
> output into an op amp that’s powered by the 

Re: [time-nuts] really Idle Lucent KS24361

[Bob Camp]

Hi

I’d say it’s not 100% clear that they *do* anything to the OCXO once they
go into holdover. I’d watch the DAC in holdover and see ….

Bob

> On Oct 23, 2015, at 2:57 AM, Gerhard Hoffmann  wrote:
> 
> Is there an easy way to get the Z3811/12 into a state where they
> just leave their oscillators alone? I do not mean holdover as when
> I just pull the GPS antenna plug; then they will probably try to
> correct for drift based on learned history.
> 
> I want the "pure" MTI-260s.
> 
> And if I build in mechanical switches, they will interfere with
> the learning...
> 
> regards, Gerhard
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Re: [time-nuts] Unified VCXO Carrier Board

[Bob Camp]

Hi

There are a wide range of OCXO’s listed for this project. I certainly
do not have a sample of ever single one of them. For the ones that 
I *do* have samples of, a properly done CMOS gate sine to square
converter will not degrade the close in phase noise or ADEV of the OCXO.
Based on TimePod measurements, I believe it would be adequate for 
all the ones I’ve seen specs for. 

With far removed phase noise spec’d into the “past 180 dbc/Hz” range on
some parts - no logic is going to handle that. Since the CPLD on the board
will floor out well before that, doing a “perfect” conversion and then degrading
it as soon as you hit the bulk logic does not make a lot of sense. 

Bob

> On Oct 23, 2015, at 5:31 AM, Charles Steinmetz  wrote:
> 
> Bruce wrote:
> 
>> Your statement about the PN of comparators conflicts with my measurements. 
>> The LTC6957 evaluation board had an 18dBc/Hz lower phase noise floor than a 
>> comparator circuit with 10MHz 15dBm inputs. However I only measured a single 
>> comparator circuit. The Holzworth sine to CMOS converter had a comparable PN 
>> to the LTC6957-4.
>> I haven't, as yet measured the PN of an optimised Wenzel circuit.My setup 
>> for this measurement had a PN floor of around -180dBc/Hz.
> 
> There are many, many ways of getting unnecessarily poor PN performance from 
> comparators (including Wenzel-style squarers) -- one has to make sure not to 
> make any of myriad mistakes in both design and execution.  You didn't say 
> which comparator you tried, or in what circuit, so I'm not in a position to 
> suggest things to check (or to confirm that the comparator you tried performs 
> similarly poorly in my tests, if that is the case).
> 
> One sanity check you can try -- disable the filtering on your 6957 eval 
> board.  According to the LT data presented in the chart I posted, which 
> agrees very closely with my test results, at 10MHz/15dBm there should be 
> essentially no change in the PN compared to the results you obtained with 
> filtering enabled.  If you see a significant difference, then something is 
> causing anomalous results.
> 
> Best regards,
> 
> Charles
> 
> 
> ps.  You often respond to one message by replying to a different message, as 
> you did in this case.  It would be helpful for someone who just joins a 
> thread, and for continuity in general, if you would reply to the message to 
> which you are actually responding.  That way, readers who are new to the 
> thread will have the context they need, and your interlocutor will have his 
> or her previous message conveniently available to refer to in any further 
> message.
> 
> 
> 
> 
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Re: [time-nuts] Unified VCXO Carrier Board

[Bruce Griffiths]

On Friday, October 23, 2015 03:27:39 PM Charles Steinmetz wrote:
> Gerhard wrote:
> >What do you consider a run-of-the-mill comparator? LM139, LMV7219,
> >AD8561, ADCMP580?
> 
> For squaring 1-10MHz sine waves, the LT1719 and 1720 are the best
> that I've found.  This is a matter of how much internal hysteresis
> the comparator has, how smoothly the internal hysteresis acts, how
> much gain the part has and how it is structured, how fast it is (both
> risetime and propagation delay), whether it suffers from thermal
> feedback from the output stage to the input stage, how much internal
> ground bounce it has, and a host of other die-level issues.
> 
> The LT1719 uses bipolar input supplies, so ground can be the
> reference voltage and also be in the center of the input common-mode
> range.  This is always quieter than biasing the inputs to the middle
> of a single supply, which is usually done with the 1720 and many
> other comparators.  (Like other single-supply comparators, the 1720
> will work referenced to ground with only a positive supply -- but (i)
> the inputs are then at the very edge of the input common-mode range,
> and (ii) you can only drive the input 100mV below ground, so you have
> to figure out how to clamp the input signal.  It is much easier to
> just use an LT1719 with +/-5v on the input stage, and it works better, 
too.)
> 
> Page 22 of the LT1719 datasheet shows the simplest possible circuit
> and discusses its performance.
> 
> >What optimizations?  I have seen the "Wenzel" circuit in cheapish
> >frequency counter inputs in
> >the late seventies, maybe with a diode bridge added as input 
protection..
> 
> Use medium-speed transistors, bias both bases from the same low-noise
> voltage reference such as an LM329, capacitively couple the emitters,
> and use a higher supply voltage, for starters.  I use MPSH81/MMBTH81s
> and a power supply of around 20v (see attached schematic) for a
> reasonably optimized implementation.  Other transistors can be used,
> but I've found that the H81s work better for squaring 1-10MHz sine
> waves than anything else I've tried -- they hit the sweet spot of the
> bandwidth/gain tradeoff and have a nice flat gain vs. current
> characteristic.
> >so it shows that one can replace filtering by signal power :-)
> 
> It's a matter of the slew rate of the input sine wave at
> zero-cross.  Once you reach the critical slew rate for any particular
> input architecture, the comparator is hard-switched fast enough that
> it doesn't spend significant time in the linear region generating noise.
> 
> >Any objections against the AD9901 phase comparator?
> 
> That should work fine.
> 
> Best regards,
> 
> Charles
Charles

A resistor from point A to ground in the Wenzel style shaper you attached 
has little effect on the output symmetry due to C4. However it does allow 
the output amplitude to be adjusted.

Bruce
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Re: [time-nuts] Unified VCXO Carrier Board

[Bruce Griffiths]

On Friday, October 23, 2015 05:31:46 AM Charles Steinmetz wrote:
> Bruce wrote:
> >Your statement about the PN of comparators conflicts with my
> >measurements. The LTC6957 evaluation board had an 18dBc/Hz lower
> >phase noise floor than a comparator circuit with 10MHz 15dBm inputs.
> >However I only measured a single comparator circuit. The Holzworth
> >sine to CMOS converter had a comparable PN to the LTC6957-4.
> >I haven't, as yet measured the PN of an optimised Wenzel circuit.My
> >setup for this measurement had a PN floor of around -180dBc/Hz.
> 
> There are many, many ways of getting unnecessarily poor PN
> performance from comparators (including Wenzel-style squarers) -- one
> has to make sure not to make any of myriad mistakes in both design
> and execution.  You didn't say which comparator you tried, or in what
> circuit, so I'm not in a position to suggest things to check (or to
> confirm that the comparator you tried performs similarly poorly in my
> tests, if that is the case).
> 
> One sanity check you can try -- disable the filtering on your 6957
> eval board.  According to the LT data presented in the chart If amplier PN 
I split the comparator output and feed it to 2 separa
> posted, which agrees very closely with my test results, at
> 10MHz/15dBm there should be essentially no change in the PN 
compared
> to the results you obtained with filtering enabled.  If you see a
> significant difference, then something is causing anomalous results.
> 
> Best regards,
> 
> Charles
> 

The comparator circuit measured was the front end of David Partridge's 
divider. I merely measured the 10MHz output.

I thought that I had made measurements for various filter settings and 
input levels. If I did, I don't appear to have saved them. I certainly named 
the various TIM files to indicate the filter settings.

I'll try and repeat the measurements for various input levels and filter 
settings.
One thing that I have found is that at low offset frequencies the measured 
PN is substantially reduced when air currents and other sources of thermal 
fluctuations are reduced. Even the effect of a thin piece of paper used as 
an air current shield can be easily seen.
With careful shielding from thermal fluctuations I measure the low 
frequency offset PN to be substantially lower than the datasheet values.
I've seen this effect with everything for which I've measured the PN.
I may machine a custom housing for the evaluation board rather than just 
using an oversize die cast box.
This may take a while as I'll need to check the compatibility of various 
hardware/software with Windows 10 that runs on my Quadcore laptop.
Failing that I do have a quad core PC with water cooled CPU that runs 
Windows 7.

One problem with comparators when attempting to measure their PN is 
that they don't have sufficient output to drive the TimePod input directly. 
An amplifier is required. To reduce the Amplifier PN contribution I split the 
comparator output and drive a separate amplifier from each splitter output 
and then use cross correlation. This makes the amplifier PN much less 
critical. Finding low PN amplifiers with relatively low gain ( ~ 10dB or so) 
with low distortion at 13dBm or so output is somewhat problematic. A low 
noise single transistor discrete amp with 30dB or more reverse isolation 
with a gain of 10dBm ought to be feasible at 10MHz.

> 
> ps.  You often respond to one message by replying to a different
> message, as you did in this case.  It would be helpful for someone
> who just joins a thread, and for continuity in general, if you would
> reply to the message to which you are actually responding.  That way,
> readers who are new to the thread will have the context they need,
> and your interlocutor will have his or her previous message
> conveniently 'available to refer to in any further message.
> 
> 

I dont always have convenient access to my email machine and sometimes 
resort to using a browser to compose a reply via my ISP's interface to my 
email.
This apparently messes up the reply so it is associated with a different 
message to the one I believed I was replying to. This is a fairly recent 
phenomenon.

Bruce
> 
> 
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Re: [time-nuts] GPS Disciplined TCXO

[Nick Sayer via time-nuts]

> On Oct 23, 2015, at 2:09 PM, Gregory Maxwell  wrote:
> 
> On Tue, Oct 20, 2015 at 8:53 PM, Bryan _  wrote:
>> Saw this on the Hackaday site if anyone is interested.
>> https://hackaday.io/project/6872-gps-disciplined-tcxo
> 
> Will this design that uses the output of the DAC directly not run into
> problems with non-monotonicity and/or dead-zones in the DAC output?  I
> would expect a PLL to behave very poorly if there is any
> non-monotonicity in the least significant bit of the DAC.

The datasheet claims the DAC is inherently monotonic. It’s a $7 part, so I 
don’t have much reason to look sideways at that claim.

> 
> My intuition would be to use TPDF dither against a higher resolution
> internal value (perhaps 32 bits) with noise shaping so that the dither
> has no power at DC (and not much near it), followed by a RC low-pass
> filter. With this approach the fact that available DAC parts run at
> speeds far faster than needed for this control helps overcome the fact
> that highly monotone parts are less common (last I looked).  This
> would also help give much finer control without compromising tuning
> range.

That strikes me as familiar - a little like how Arduino fakes analog output by 
running PWM into an LPF.

If you look at the AD5061 datasheet, there is unfortunately a relatively 
significant (to my eyes, at least) update glitch. I suppose it’s quick enough 
that the RC filter would get rid of most of it, but it is an extra noise source 
if you do it frequently, like you’re suggesting.

It’s an avenue I’ve not investigated, but in principle, it’s just a firmware 
change. The one issue you may find is that the DAC update is bit-banged at the 
moment, so it’s much slower than it could be, even with the AVR clocked at 10 
MHz. You could use the SPI facilities built into the ATTiny… except that I 
goofed and used MISO instead of MOSI (or the other way - I forget) for the data 
pin to the DAC. Changing that would require a board redesign and a compile time 
firmware flag, and SPI wouldn’t be usable on currently deployed hardware 
versions.

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Re: [time-nuts] Unified VCXO Carrier Board

[Bob Camp]

Hi

If you are buying the “right” 0.5 ppm / C resistors, they often run well over 
$10 each. You have
a *lot* of them in a circuit….

Bob

> On Oct 22, 2015, at 5:09 PM, Dimitri.p  wrote:
> 
> The price of the resistors is small change compared to the $8 connector for 
> the 10811.
> Then again maybe the plan is not to actually build one board with all the 
> parts that will accomodate any of the OCXOs on the list
> 
> 
> Dimitri
> 
> At 04:44 AM 10/22/2015, Bob Camp wrote:
>> > 2. It provides unified tuning: 0V = lowest possible frequency, 3V3 or 5V = 
>> > highest possible frequency, no matter of the VCXO tuning sense and range.
>> 
>> That immediately gets you into op amps and feedback resistor stability. With 
>> a number of combinations, the required resistors can get pretty expensive. It
>> also gets you into dual supplies with some OCXO’s.
> 
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Re: [time-nuts] GPS Disciplined TCXO

[Nick Sayer via time-nuts]

> On Oct 23, 2015, at 2:25 AM, Attila Kinali  wrote:
> 
> On Thu, 22 Oct 2015 21:07:38 -0700
> Nick Sayer via time-nuts  wrote:
> 
> 
>>> I like the choice of the TXCO. Which allows you to replace it
>>> by the pin compatible DOC050V and have an OCXO instead.
>> 
>> That actually is a downgrade. I’ve tried both, and the DOC020V’s short term 
>> stability is much worse. I’m guessing that the OCXO’s selling point is it’s
>> medium to long term stability, which is mooted by the GPS discipline here.
> 
> BTW: Tom van Baak mentioned in a private mail, that the DOC supply current
> is much higher than i thought it was. So it isn't exactly a drop in 
> replacement
> either.

I’ve run them with the same LDO and they work. The LDO gets warm, of course, 
but then, so does the oscillator. When I compare them to the same circuit, but 
powered with the SC189Z switcher I use for the OH300 variant, their performance 
is identical. Which is to say, a half order of magnitude worse than the DOT050V 
at tau 1s.

The data sheets of both claim a short term stability of 1e-9 at 1s. The DOC is 
exceeding that, but the DOT is *slaughtering* it. I routinely see 9e-11 
instead. I asked Connor Winfield about that and they said that they should 
change the data sheet, as my observations are more in line with reality. 
Meanwhile, the OH300 (I’ve only tried one of them so far, however) is at around 
3e-11 at 1s. So that’s what $100 buys you. :)

The other thing I saw with the DOC020V that I have not seen with either the DOT 
or OH300 variants is a weird oscillation on the VC pin. I posed the question to 
Stack Exchange and they suggested that the VC pin internally goes to a varactor 
which adds too much capacitance for the output of the OP amp. I asked Connor 
Winfield about this and they didn’t have an immediate answer. The latest boards 
have the footprints for another RC LPF on the output to counter this, but since 
I gave up on the DOC part, the cap is no-stuff and the resistor is 0 ohms when 
I build them out (with the other parts). There’s also a footprint for a cap in 
parallel to the feedback resistor on the op amp in case further bandwidth 
limiting is ever desired, but so far adding capacitance there hasn’t had any 
impact on the output, so that too is no-stuff.

> 
>>> But I have three things to critisize:
>>> 
>>> * The AD506x DAC family has a peculiar restriction: the maximum
>>> specced Vref is 50mV below VDD. Ie connecting Vref to VDD means
>>> that the DAC is used outside of specs and thus might or might
>>> not get the speced accuracy/precision.
>> 
>> I don’t see that anywhere in the datasheet. Can you give me a reference for 
>> that? The only thing I see is an absolute maximum of Vdd + 0.3V, but of
>> course “absolute maximum” ratings aren’t necessarily what you’re supposed to 
>> design to.
> 
> In the AD5061 datasheet (Rev C) on page 3, at the bottom,
> section "Reference Input/Output", item "V_REF Input Range".
> Yes, it will not fry the device, but it will not work up to spec either
> (unfortunately, manufacturers never mention what part of the spec gets
> degraded if you violate something).

I do see that now. If you look at the footnote, figure 27 which it references 
shows that the Vdd headroom with a VRef of 3.3 volts is more like 20 mV or so. 
I would posit that the headroom issue is more likely to result in non-linearity 
at high DAC values, perhaps because the output amp may not be capable of full 
rail-to-rail operation. In practice, I expect the actual DAC values to be 
constrained within a relatively narrow range clustered around midpoint. Now, 15 
years of aging may change that, I suppose. But at the moment, out of the dozen 
or so I’ve built so far and played with, I’ve not seen a DAC value (in normal 
operation) outside of ±0x1000 from midpoint.

> 
> 
>>> 
>>> * Using VDD as reference voltage, when VDD is generated by an LM1117
>>> is kind of iffy. Neither the internal reference of the LM1117 nor
>>> the control loop are very stable. There ar LDOs out there, that are
>>> speced for use as ADC/DAC references, but this one isn’t. 
>> 
>> I’ve considered a separate regulator for the reference, but my thinking is/
>> was that it would be bad for the supply voltage of the oscillator to be 
>> regulated separately from the reference. The two regulators might drift 
>> apart. Connor Winfield’s OCXO application note actually says this too. It’s 
>> too bad that these oscillators don’t have a reference voltage output, but I 
>> guess that’s part of what makes them cheaper.
> 
> Variations in Vdd will change the oscillator frequency slightly (20ppb for 
> 5%).
> Changes in Vref will have a higher impact on the frequency. Also, the control
> voltage of an oscillator is usually referenced against ground, ie its 
> "processing"
> is more or less independent of Vdd.

The latest boards do have a designated spot where you can sever the analog 
section supply. I (or you) 

Re: [time-nuts] Unified VCXO Carrier Board

[Gerhard Hoffmann]

Am 22.10.2015 um 22:04 schrieb Charles Steinmetz:
As I have said before, there is very little if any advantage to using 
an LTC6957 at 10MHz (as opposed to using a run-of-the-mill comparator),


What do you consider a run-of-the-mill comparator? LM139, LMV7219, 
AD8561, ADCMP580?


and the LTC6957 is not as good, even with filtering on, as a circuit 
with lower inherent jitter such as an optimized Wenzel-style squarer.
What optimizations?  I have seen the "Wenzel" circuit in cheapish 
frequency counter inputs in

the late seventies, maybe with a diode bridge added as input protection..


The graph compares an optimized 6957 implementation without filtering 
and with optimum filtering.  At an input level of -10dBm, the phase 
noise floor is 7dB lower with filtering, and at an input of +10dBm, 
the improvenent is <2.5dB.  Extrapolating beyond the graph to the 
right, at an input level of +13dBm (= 1Vrms, the customary level for 
frequency references), the improvement with filtering will be very 
near 0dB. 


so it shows that one can replace filtering by signal power :-)


I realize that you did not say you expect to use the board only at 
10MHz, and the LTC6957 with filtering may provide some improvement at 
lower frequencies (compared to the 6957 without filtering). But even 
with filtering on, the 6957 will not outperform an optimized 
Wenzel-type squarer until you get well down into the kHz range. 


But in the KHz range, the filter corner frequency should be much lower 
than in the 6957...


The 100 MHz range would be more interesting. The Wenzel squarer would 
then need
"real transistors" with higher 1/f corner. The PLL would have a sub-Hz 
BW in this application,
so the race for the lowest floor seems not too important. OK, maybe for 
the 1pps.


The 6957 takes one square centimeter including transformer and works for 
all frequencies.

Any objections against the AD9901 phase comparator? I have a tube of them.

regards, Gerhard
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Re: [time-nuts] GPS Disciplined TCXO

[Gregory Maxwell]

On Tue, Oct 20, 2015 at 8:53 PM, Bryan _  wrote:
> Saw this on the Hackaday site if anyone is interested.
> https://hackaday.io/project/6872-gps-disciplined-tcxo

Will this design that uses the output of the DAC directly not run into
problems with non-monotonicity and/or dead-zones in the DAC output?  I
would expect a PLL to behave very poorly if there is any
non-monotonicity in the least significant bit of the DAC.

My intuition would be to use TPDF dither against a higher resolution
internal value (perhaps 32 bits) with noise shaping so that the dither
has no power at DC (and not much near it), followed by a RC low-pass
filter. With this approach the fact that available DAC parts run at
speeds far faster than needed for this control helps overcome the fact
that highly monotone parts are less common (last I looked).  This
would also help give much finer control without compromising tuning
range.

[Disclaimer: I've never built a GPSDO]
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Re: [time-nuts] Unified VCXO Carrier Board

[Charles Steinmetz]

Gerhard wrote:

What do you consider a run-of-the-mill comparator? LM139, LMV7219, 
AD8561, ADCMP580?


For squaring 1-10MHz sine waves, the LT1719 and 1720 are the best 
that I've found.  This is a matter of how much internal hysteresis 
the comparator has, how smoothly the internal hysteresis acts, how 
much gain the part has and how it is structured, how fast it is (both 
risetime and propagation delay), whether it suffers from thermal 
feedback from the output stage to the input stage, how much internal 
ground bounce it has, and a host of other die-level issues.


The LT1719 uses bipolar input supplies, so ground can be the 
reference voltage and also be in the center of the input common-mode 
range.  This is always quieter than biasing the inputs to the middle 
of a single supply, which is usually done with the 1720 and many 
other comparators.  (Like other single-supply comparators, the 1720 
will work referenced to ground with only a positive supply -- but (i) 
the inputs are then at the very edge of the input common-mode range, 
and (ii) you can only drive the input 100mV below ground, so you have 
to figure out how to clamp the input signal.  It is much easier to 
just use an LT1719 with +/-5v on the input stage, and it works better, too.)


Page 22 of the LT1719 datasheet shows the simplest possible circuit 
and discusses its performance.


What optimizations?  I have seen the "Wenzel" circuit in cheapish 
frequency counter inputs in

the late seventies, maybe with a diode bridge added as input protection..


Use medium-speed transistors, bias both bases from the same low-noise 
voltage reference such as an LM329, capacitively couple the emitters, 
and use a higher supply voltage, for starters.  I use MPSH81/MMBTH81s 
and a power supply of around 20v (see attached schematic) for a 
reasonably optimized implementation.  Other transistors can be used, 
but I've found that the H81s work better for squaring 1-10MHz sine 
waves than anything else I've tried -- they hit the sweet spot of the 
bandwidth/gain tradeoff and have a nice flat gain vs. current characteristic.



so it shows that one can replace filtering by signal power :-)


It's a matter of the slew rate of the input sine wave at 
zero-cross.  Once you reach the critical slew rate for any particular 
input architecture, the comparator is hard-switched fast enough that 
it doesn't spend significant time in the linear region generating noise.



Any objections against the AD9901 phase comparator?


That should work fine.

Best regards,

Charles

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Re: [time-nuts] GPS Disciplined TCXO

[Nick Sayer via time-nuts]

> On Oct 22, 2015, at 1:17 AM, Attila Kinali  wrote:
> 
> On Tue, 20 Oct 2015 13:53:54 -0700
> Bryan _  wrote:
> 
>> Saw this on the Hackaday site if anyone is interested.
>> https://hackaday.io/project/6872-gps-disciplined-tcxo
> 
> Oh.. that one looks nice.
> 
> I like the choice of the TXCO. Which allows you to replace it
> by the pin compatible DOC050V and have an OCXO instead.

That actually is a downgrade. I’ve tried both, and the DOC020V’s short term 
stability is much worse. I’m guessing that the OCXO’s selling point is it’s 
medium to long term stability, which is mooted by the GPS discipline here.

> 
> 
> But I have three things to critisize:
> 
> * The AD506x DAC family has a peculiar restriction: the maximum
>  specced Vref is 50mV below VDD. Ie connecting Vref to VDD means
>  that the DAC is used outside of specs and thus might or might
>  not get the speced accuracy/precision.

I don’t see that anywhere in the datasheet. Can you give me a reference for 
that? The only thing I see is an absolute maximum of Vdd + 0.3V, but of course 
“absolute maximum” ratings aren’t necessarily what you’re supposed to design to.

> 
> * Using VDD as reference voltage, when VDD is generated by an LM1117
>  is kind of iffy. Neither the internal reference of the LM1117 nor
>  the control loop are very stable. There ar LDOs out there, that are
>  speced for use as ADC/DAC references, but this one isn’t. 

I’ve considered a separate regulator for the reference, but my thinking is/was 
that it would be bad for the supply voltage of the oscillator to be regulated 
separately from the reference. The two regulators might drift apart. Connor 
Winfield’s OCXO application note actually says this too. It’s too bad that 
these oscillators don’t have a reference voltage output, but I guess that’s 
part of what makes them cheaper.

> 
> * The DOT050V has an upper limit for the control voltage of 3.0V,
>  (and a lower 0.3V) but the circuit could potentially supply 3.3V there
>  losing 10% (or 20%) of precision.

It shouldn’t be able to do that. The buffer amplifier is in a less-than-unity 
gain configuration with a Vdd/2 virtual ground. It’s designed to reduce the 
swing to 51% for the DOT050V variant or 82% for the OH300 variant.

> 
> Taking these things into account, I would have used one of the cheap
> 3.0V refernces like a REF3030 (or even a 2.5V reference) to supply Vref
> to the DAC and changed the opamp circuit to slightly shift the 0V based
> voltage up so the lower limit becomes 0.3V and the upper limit 3.0V.
> 
> 
>   Attila Kinali
> 
> 
> 
> -- 
> 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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Re: [time-nuts] GPS Disciplined TCXO

[Nick Sayer via time-nuts]

It’s not really available as a kit, per se, since it’s all SMD components. It 
comes either as a fully populated and programmed board ($174.99) or with a case 
and power supply (for $50 more). For an extra $100, you can upgrade from the 
DOT050V TCXO to the OH300 OCXO oscillator.

> On Oct 21, 2015, at 11:33 PM, Bob Albert  wrote:
> 
> How much for the kit?
> 
> Bob
> 
> 
> 
> On Wednesday, October 21, 2015 9:03 PM, Nick Sayer via time-nuts 
>  wrote:
> 
> 
> Thanks. That’s me.
> 
> I posted a while ago here, but I’ve made a great deal of progress since then. 
> I’ve logged most of what I’ve discovered and learned on the project itself. I 
> believe I now have reasonable confidence in the performance of the two 
> models. I’d be happy to share my methodology so that others could attempt to 
> duplicate or verify my results.
> 
> It’s going to remain an open source / open hardware project, but I do sell 
> them in my Tindie store too.
> 
> > On Oct 20, 2015, at 1:53 PM, Bryan _  > > wrote:
> > 
> > Saw this on the Hackaday site if anyone is interested.
> > https://hackaday.io/project/6872-gps-disciplined-tcxo 
> > 
> > 
> > -=Bryan=- 
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> 

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