Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Bill Hawkins]

What kind of capacitor is used for the 10 µF cap - electrolytic or film?

Bill Hawkins 

-Original Message-
From: Nick Sayer via time-nuts
Sent: Monday, September 26, 2016 11:18 PM

With a short TC loop filter, the PLL does lock up, but obviously the
jitter of the Venus’ 10 MHz output comes through.

With a longer TC, the PLL never locks - or at least if it does lock,
it’s locking significantly off frequency.

That’s with a 10 µF cap and varied resistors between 10k and 1M. The
best I got was at 200k - a TC of 2s. That resulted in this video. Unlike
other videos I’ve made comparing two GPSDOs, this one is not a
time-lapse. The reference is an OH300 based GPSDO.

https://www.youtube.com/watch?v=hiHRp0dCJ64


A time constant of 10s (1M resistor) just doesn’t work at all.


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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[David]

Is there a schematic showing what you attempted?

I went back through the discussion thread and it was not real clear to
me which design you implemented.

On Mon, 26 Sep 2016 21:18:25 -0700, you wrote:

>...
>
>The results aren’t very good.
>
>With a short TC loop filter, the PLL does lock up, but obviously the jitter of 
>the Venus’ 10 MHz output comes through.
>
>With a longer TC, the PLL never locks - or at least if it does lock, it’s 
>locking significantly off frequency.
>
>That’s with a 10 µF cap and varied resistors between 10k and 1M. The best I 
>got was at 200k - a TC of 2s. That resulted in this video. Unlike other videos 
>I’ve made comparing two GPSDOs, this one is not a time-lapse. The reference is 
>an OH300 based GPSDO.
>
>https://www.youtube.com/watch?v=hiHRp0dCJ64 
>
>
>A time constant of 10s (1M resistor) just doesn’t work at all.
>
>But the real nail in the coffin here is that the price of the PLL chip is 
>still more expensive than the microcontroller and all of the components it 
>replaces.
>
>...
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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Nick Sayer via time-nuts]

At the risk of inviting everyone to say “I told you so,” I’ll report here my 
experimental results from trying this concept out.

Since there was a great deal of doubt about the outcome, I hedged my bet a bit 
and designed for the DOT050V rather than the OH300. If it worked out for the 
TCXO, then I could try with the more expensive one.

The results aren’t very good.

With a short TC loop filter, the PLL does lock up, but obviously the jitter of 
the Venus’ 10 MHz output comes through.

With a longer TC, the PLL never locks - or at least if it does lock, it’s 
locking significantly off frequency.

That’s with a 10 µF cap and varied resistors between 10k and 1M. The best I got 
was at 200k - a TC of 2s. That resulted in this video. Unlike other videos I’ve 
made comparing two GPSDOs, this one is not a time-lapse. The reference is an 
OH300 based GPSDO.

https://www.youtube.com/watch?v=hiHRp0dCJ64 


A time constant of 10s (1M resistor) just doesn’t work at all.

But the real nail in the coffin here is that the price of the PLL chip is still 
more expensive than the microcontroller and all of the components it replaces.

In the end, I’m glad I tried, but I don’t think I’m going to invest any more 
time in the design. I could try configuring the venus for a 10 kHz output and 
see if it’s better able to phase lock with a divided TCXO output, but I don’t 
think I have any reason to believe that would be more likely to succeed.
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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Nick Sayer via time-nuts]

> On Sep 16, 2016, at 1:13 PM, Lars Walenius  wrote:
> 
> 
> 
> My experience with the Venus838-T is only 2 weeks but disappointing. This can 
> also be guessed from the datasheet ADEV curve, that I guess is sawtooth 
> corrected values as it starts at 3E-9 at 1s, but is only 1E-11 at 1000s a 
> factor 10 worse than I get with the LEA-6T with the same antenna and setup. 
> If anyone have ADEV-MDEV curves to share I would be glad to see what can be 
> achieved with the venus838-T. My conclusion is also that sawtooth correction 
> is useless on my 838-T.

Are you talking about the PPS output or the frequency output (10 MHz by 
default)?

I haven’t attempted to get ADEV plots of the PPS output mainly because I’m not 
sure the best experimental setup. 

I could (try to) capture time differences between the PPS output of a 
thunderbolt and the PPS output of the Venus, but would taking the ADEV of that 
give correct results?

> 
> Lars
> 
> 
>> Nick wrote:
> 
>> Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this 
>> problem, or the 10 kHz output of the >Jupiter is substantially better than 
>> the Venus’ 10 MHz output, or the design doesn’t give the results time-nuts 
>> expect >from a GPSDO. Which of those applies?
> 
> 
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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Mark Spencer]

Hi Lars.   I own a G3RUH (Miller) GPSDO.

>From what I can recall  While testing mine with  my collection of 
>HP5370B's and my assortment of references the performance of mine while locked 
>was generally comparable to the one in the link you provided.   I never tested 
>it "un locked."  

I couldn't measure phase noise and the performance of my test setup at taus of 
less than 100 seconds or so was probably a limitation as well.   My references 
(BVA OCXO, PRS10 rb and a cherry picked Z3805 GPSDO) probably weren't really 
good enough either.

It is a nice unit that I have currently loaned out to a local amateur radio 
operator.   Acquiring it lead to me spending a sizeable amount of time and 
money on time nuts pursuits over the years.

Sent from my iPhone

> On Sep 16, 2016, at 1:13 PM, Lars Walenius  wrote:
> 
> Hi Nick,
> 
> Jim Millers design is very clever and as I see can give results as good as a 
> digital approach but it has the same limitations:
> The GPS jitter and the oscillator jitter in combination with the loop 
> bandwidth.
> 
> The only ADEV I have seen for the Miller GPSDO is this one  
> http://www.leapsecond.com/pages/gpsdo/  .
> Can anyone point me to other tests?
> 
> The ADEV I see on leapsecond.com indicates for me a time constant of around 
> 200seconds. That is what you get with a OCXO range of 0.25ppm. You don´t need 
> to have a large RC-filter. The original R1-C1 time constant with 16 seconds 
> time constant will work as long as the internal oscillator in the Jupiter-T 
> is at least some Hz away from a multiple of 10kHz. If that is true the 
> sawtooth out of the Jupiter will have enough high frequency to be easily 
> filtered by the RC. This is of course a risk. See for example: 
> https://www.febo.com/pipermail/time-nuts/2005-August/019106.html . If you get 
> a hanging bridge you have at least as much trouble as a non sawtooth 
> corrected GPSDO and probably much worse as the RC filter is only 16s. In the 
> digital approach the prefilter may filter away the hanging bridge (in best 
> case).
> 
> As I understand the Miller GPSDO with a OCXO with a 0.25ppm range and 10kHz 
> into an XOR phase detector and 16s RC will be very similar to a digital 
> approach with a TIC (Time interval counter ) with +-25us range followed by a 
> prefilter with time constant 16s and a loop with just the P-term with a time 
> constant of 200secs. As it has no I-term it will have slightly less noise but 
> the output phase will drift as soon as the OCXO drifts (as a FLL). The 
> resolution of the XOR phase detector (TIC) will be limited by the noise but 
> as 0.1mV is 1ns the resolution can be seen as better than 1ns is my 
> conclusion. A problem might be that the output of the XOR drifts with the 5V 
> supply but this can be seen as the same problem as the DAC drift in a digital 
> approach.
> 
> My conclusion (without testing) is that the Jupiter-T 10kHz is very good but 
> not better than the sawtooth corrected outputs from M12 or LEA6T receivers. 
> That is the ADEV can be approximated by 1E-9/Tau (1E-12 at 1000s) in good 
> conditions.
> 
> My experience with the Venus838-T is only 2 weeks but disappointing. This can 
> also be guessed from the datasheet ADEV curve, that I guess is sawtooth 
> corrected values as it starts at 3E-9 at 1s, but is only 1E-11 at 1000s a 
> factor 10 worse than I get with the LEA-6T with the same antenna and setup. 
> If anyone have ADEV-MDEV curves to share I would be glad to see what can be 
> achieved with the venus838-T. My conclusion is also that sawtooth correction 
> is useless on my 838-T.
> 
> Lars
> 
> 
>> Nick wrote:
> 
>> Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this 
>> problem, or the 10 kHz output of the >Jupiter is substantially better than 
>> the Venus’ 10 MHz output, or the design doesn’t give the results time-nuts 
>> expect >from a GPSDO. Which of those applies?
> 
> 
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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Lars Walenius]

Hi Nick,

Jim Millers design is very clever and as I see can give results as good as a 
digital approach but it has the same limitations:
The GPS jitter and the oscillator jitter in combination with the loop bandwidth.

The only ADEV I have seen for the Miller GPSDO is this one  
http://www.leapsecond.com/pages/gpsdo/  .
Can anyone point me to other tests?

The ADEV I see on leapsecond.com indicates for me a time constant of around 
200seconds. That is what you get with a OCXO range of 0.25ppm. You don´t need 
to have a large RC-filter. The original R1-C1 time constant with 16 seconds 
time constant will work as long as the internal oscillator in the Jupiter-T is 
at least some Hz away from a multiple of 10kHz. If that is true the sawtooth 
out of the Jupiter will have enough high frequency to be easily filtered by the 
RC. This is of course a risk. See for example: 
https://www.febo.com/pipermail/time-nuts/2005-August/019106.html . If you get a 
hanging bridge you have at least as much trouble as a non sawtooth corrected 
GPSDO and probably much worse as the RC filter is only 16s. In the digital 
approach the prefilter may filter away the hanging bridge (in best case).

As I understand the Miller GPSDO with a OCXO with a 0.25ppm range and 10kHz 
into an XOR phase detector and 16s RC will be very similar to a digital 
approach with a TIC (Time interval counter ) with +-25us range followed by a 
prefilter with time constant 16s and a loop with just the P-term with a time 
constant of 200secs. As it has no I-term it will have slightly less noise but 
the output phase will drift as soon as the OCXO drifts (as a FLL). The 
resolution of the XOR phase detector (TIC) will be limited by the noise but as 
0.1mV is 1ns the resolution can be seen as better than 1ns is my conclusion. A 
problem might be that the output of the XOR drifts with the 5V supply but this 
can be seen as the same problem as the DAC drift in a digital approach.

My conclusion (without testing) is that the Jupiter-T 10kHz is very good but 
not better than the sawtooth corrected outputs from M12 or LEA6T receivers. 
That is the ADEV can be approximated by 1E-9/Tau (1E-12 at 1000s) in good 
conditions.

My experience with the Venus838-T is only 2 weeks but disappointing. This can 
also be guessed from the datasheet ADEV curve, that I guess is sawtooth 
corrected values as it starts at 3E-9 at 1s, but is only 1E-11 at 1000s a 
factor 10 worse than I get with the LEA-6T with the same antenna and setup. If 
anyone have ADEV-MDEV curves to share I would be glad to see what can be 
achieved with the venus838-T. My conclusion is also that sawtooth correction is 
useless on my 838-T.

Lars


>Nick wrote:

>Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this 
>problem, or the 10 kHz output of the >Jupiter is substantially better than the 
>Venus’ 10 MHz output, or the design doesn’t give the results time-nuts expect 
>>from a GPSDO. Which of those applies?


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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Charles Steinmetz]

Bryan wrote:


 From the Jupiter-T TU60-D120 datasheet
 Figure 1-3 (next page) shows the typical 1PPS performance of the Jupiter-T GPS 
receiver. The 10 kHz output is also available from the receiver and is phase 
coherent with the 1PPS signal. This output is made available for functions such 
as phase locking of crystal oscillators, frequency synthesisers, and similar 
applications.


Yes, but  The devil is in the details.

The fact that one thing is phase-locked to another does not necessarily 
mean it puts out a good, clean signal.  At short time scales (tau less 
than ~100 seconds), the PPS signal from any GPS receiver is noisy.  At 
tau = 1second, it is shockingly noisy (~5e-9), and it decreases by a 
nominal factor of 10 per decade as the averaging time (tau) is made 
longer.  By tau = 1000 seconds, it is pretty respectable (~2e-12 if the 
GPS rx designers did their job well).


So, the trick is to use the noisy source (GPS) to discipline the clean 
source (OCXO) very gently and very slowly.  That way, the OCXO remains 
in control of the output at short tau (< 100 to 1000 seconds), while it 
is kept on-frequency over the long term by the GPS.  This requires a PLL 
control loop with a very long time constant (equivalently, a very low 
cutoff frequency, in the microHertz to milliHertz region).  It is not 
practical to build analog filters with time constants that long, so one 
must design a digital filter (far from impossible, but not the sort of 
thing most hobbyist GPSDO designers are willing to undertake).


If you use a control loop with a short time constant, then the quiet 
OXCO just follows the noisy reference source and doesn't improve anything.


If a GPS rx puts out a phase-locked audio or RF frequency (10kHz or 
10MHz in the examples we've been discussing), the question becomes 
whether that output has better stability (lower jitter) over short 
averaging times than the PPS.  The usual way to do this would be to use 
a clean local oscillator disciplined in a very slow loop -- the same 
thing discussed above, only at 10kHz rather than 10MHz.  To my 
knowledge, the Jupiter receivers don't have internal OCXOs devoted to 
this, so if the 10kHz output really does have better stability at short 
tau than the PPS, it isn't exactly obvious how the designers did it. 
Presumably, they would need a very high-Q resonator of *some* sort.


Best regards,

Charles

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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Hal Murray]

csteinm...@yandex.com said:
> The fact that one thing is phase-locked to another does not necessarily
> mean it puts out a good, clean signal.  At short time scales (tau less  than
> ~100 seconds), the PPS signal from any GPS receiver is noisy.  At  tau =
> 1second, it is shockingly noisy (~5e-9), and it decreases by a  nominal
> factor of 10 per decade as the averaging time (tau) is made  longer.  By tau
> = 1000 seconds, it is pretty respectable (~2e-12 if the  GPS rx designers
> did their job well). 

and then there are hanging bridges.  :)


-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Charles Steinmetz]

Nick wrote:


Ok. Is it the case that the loop filter bandwidth is related to the time 
constant?


They are two ways of expressing the same thing, with the proviso that TC 
applies strictly only to first-order, linear time-invariant systems -- 
in electronics, circuits with with one real pole.


T = RC, where "T" is the time constant and "R" and "C" are resistor and 
capacitor values.


So, a single-pole LP filter consisting of a series 10k resistor and a 
shunt 100nF capacitor has T = .001 seconds (=1mS).  Its frequency 
response (i.e., the -3dB or "corner" frequency) is given by:


 f = 1/(2 x pi x T), or 159.16 Hz.

Checking, we note that the reactance of a capacitor is given by x = 
-1/(2 x pi x f x C).  When f = 159.16 Hz, the reactance of a 100nF 
capacitor is -10k ohms, which is the value that produces a -3dB response 
of 159.16 Hz in the LP filter with a 10k resistor.



Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this 
problem, or the 10 kHz output of the Jupiter is substantially better than the 
Venus’ 10 MHz output, or the design doesn’t give the results time-nuts expect 
from a GPSDO. Which of those applies?


I have no idea how the Jupiter generates its 10k output -- it was 
speculation on my part that its stability must be no better than the 
PPS.  I have not looked at the Miller circuit for a long time (also, 
note that Mr. Miller published at least two different GPSDO designs, so 
one also has to be careful to know which one is under discussion).  I 
recall that I did not have much hope that the better of the two would 
give TN-quality results, and was very surprised to find that when Tom 
tested one, it performed quite well (although, the loop time constant 
was clearly way too low -- as it is in the Tbolt with factory default 
settings).  I suppose the answer to the question is to be found in ADEV 
plots of the Jupiter's 10kHz output.  Perhaps there are some already 
published on the web?


There was some discussion of this on the list back in mid-February 2008.

Best regards,

Charles


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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Bryan _]

>From the Jupiter-T TU60-D120 datasheet
Figure 1-3 (next page) shows the typical 1PPS performance of the Jupiter-T GPS 
receiver. The 10 kHz output is also available from the receiver and is phase 
coherent with the 1PPS signal. This output is made available for functions such 
as phase locking of crystal oscillators, frequency synthesisers, and similar 
applications. 

-=Bryan=-

> Date: Mon, 12 Sep 2016 21:00:51 +
> From: b...@evoria.net
> To: nsa...@kfu.com; time-nuts@febo.com
> Subject: Re: [time-nuts] A new take on the all-hardware GPSDO concept
> 
> Nick said:
> "Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this 
> problem, or the 10 kHz output of the Jupiter is substantially better than the 
> Venus’ 10 MHz output, or the design doesn’t give the results time-nuts expect 
> from a GPSDO. Which of those applies?"
> Hi Nick,
> You have to be very careful about the receiver you select.  I think it's the 
> case that the Jupiter's internal oscillator is phase locked to the GPS 
> signal.  In that case, you can use the output from the receiver to directly 
> drive a PLL giving a high quality 10MHz or whatever you like.  But, the low 
> cost receivers on the market now, including those from Ublox, do not phase 
> lock their oscillators to the GPS signal.  In the case of Ublox, they 
> specifically say that the quantization value is only available for the 1PPS 
> output.  So, yes, you can get a 10KHz pulse train from a Ublox or any other 
> modern inexpensive GPS receiver, but it will suffer from whatever 
> quantization error the manufacturer specifies.
> 
> bob
>  -
> AE6RV.com
> 
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> 
> 
>
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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Charles Steinmetz]

Tim wrote:


You know Nick, the loop time constant typically used with the HMC1031 loop
filter is typically 5 milliseconds. I'm sure some bigger R's and C's can
used for a longer time constant, and I'm sure that'll help clean up the
awful 10MHz output of the Venus838LPx-T. But it is hardly what I'd call a
"GPSDO".


One point that may not be obvious to people thinking of designing GPSDOs 
is this:  The requirement for a very long time constant control loop 
(hundreds to thousands of seconds) has nothing to do with the frequency 
of the reference signal from the GPS (that is, it is not due to the 
reference being a 1PPS signal).  Rather, it is determined by the tau at 
which the stability of the GPS signal becomes better than the stability 
of the local oscillator (generally an OCXO).  Unless the designer 
chooses a very bad local oscillator, this will be in the region of tau = 
100-10kS (maybe only 20-50S if using a TCXO instead of an OCXO).


The stability of any higher-frequency signal from a GPS that does not 
have a disciplined, high-stability LO (e.g., 10kHz in the case of the 
obsolete Navman Jupiter GPS receivers, or 10MHz in the case of the 
Venus) will be no better than the stability of the PPS, so the 
high-stability local oscillator you add will still need to be 
disciplined with the same slow loop you would use with PPS discipline if 
you want the sort of results time-nuts expect from a GPSDO.


Best regards,

Charles


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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Bob Stewart]

Nick said:
"Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this 
problem, or the 10 kHz output of the Jupiter is substantially better than the 
Venus’ 10 MHz output, or the design doesn’t give the results time-nuts expect 
from a GPSDO. Which of those applies?"
Hi Nick,
You have to be very careful about the receiver you select.  I think it's the 
case that the Jupiter's internal oscillator is phase locked to the GPS signal.  
In that case, you can use the output from the receiver to directly drive a PLL 
giving a high quality 10MHz or whatever you like.  But, the low cost receivers 
on the market now, including those from Ublox, do not phase lock their 
oscillators to the GPS signal.  In the case of Ublox, they specifically say 
that the quantization value is only available for the 1PPS output.  So, yes, 
you can get a 10KHz pulse train from a Ublox or any other modern inexpensive 
GPS receiver, but it will suffer from whatever quantization error the 
manufacturer specifies.

bob
 -
AE6RV.com

GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info


   
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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Nick Sayer via time-nuts]

> On Sep 12, 2016, at 1:27 PM, Charles Steinmetz  wrote:
> 
> Tim wrote:
> 
>> You know Nick, the loop time constant typically used with the HMC1031 loop
>> filter is typically 5 milliseconds. I'm sure some bigger R's and C's can
>> used for a longer time constant, and I'm sure that'll help clean up the
>> awful 10MHz output of the Venus838LPx-T. But it is hardly what I'd call a
>> "GPSDO".
> 
> One point that may not be obvious to people thinking of designing GPSDOs is 
> this:  The requirement for a very long time constant control loop (hundreds 
> to thousands of seconds) has nothing to do with the frequency of the 
> reference signal from the GPS (that is, it is not due to the reference being 
> a 1PPS signal).  Rather, it is determined by the tau at which the stability 
> of the GPS signal becomes better than the stability of the local oscillator 
> (generally an OCXO).  Unless the designer chooses a very bad local 
> oscillator, this will be in the region of tau = 100-10kS (maybe only 20-50S 
> if using a TCXO instead of an OCXO).
> 
> The stability of any higher-frequency signal from a GPS that does not have a 
> disciplined, high-stability LO (e.g., 10kHz in the case of the obsolete 
> Navman Jupiter GPS receivers, or 10MHz in the case of the Venus) will be no 
> better than the stability of the PPS, so the high-stability local oscillator 
> you add will still need to be disciplined with the same slow loop you would 
> use with PPS discipline if you want the sort of results time-nuts expect from 
> a GPSDO.

Ok. Is it the case that the loop filter bandwidth is related to the time 
constant? Are there methods for minimizing the loop filter bandwidth that might 
be useful here? I would hazard a guess that obtaining >100s of TC equivalent in 
pure hardware would be at best difficult. At the same time, it seems like 
having a microcontroller act as an averager in software between an ADC and a 
DAC would be just silly.

Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this 
problem, or the 10 kHz output of the Jupiter is substantially better than the 
Venus’ 10 MHz output, or the design doesn’t give the results time-nuts expect 
from a GPSDO. Which of those applies?

> 
> Best regards,
> 
> Charles
> 
> 
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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Nick Sayer via time-nuts]

> On Sep 12, 2016, at 1:01 PM, Bob Camp  wrote:
> 
> Hi
> 
> The noise performance of the HMC1031 close in is pretty horrible. It’s 
> actually *worse* than the GPS signal noise. In a normal GPSDO the idea is to 
> use an oscillator that is cleaner at 0.01 to 10 Hz than the GPS. 

I don’t see where you get that.

To generate the phase noise figures (8,9,11,12) they used a Crystek CVHD-950. I 
don’t think it necessarily follows that a better quality OCXO can’t do better 
than that, does it?

> 
> Bob
> 
> 
>> On Sep 12, 2016, at 2:41 PM, Nick Sayer via time-nuts  
>> wrote:
>> 
>> I was talking with someone at AD about a question I had about one of their 
>> TinyDACs when they mentioned their HMC1031 chip. It looks like the ideal 
>> building block for a clean-up oscillator.
>> 
>> It struck me just a touch later that the Venus838LPx-T has by default a 10 
>> MHz output that’s phase locked to GPS time. It’s not good quality, but I 
>> wonder if it’s good enough to be the reference for this particular chip. 
>> They do talk about the ability to be driven by a “noisy” or "jittery” 
>> reference.
>> 
>> I think I’m going to take a crack at an OH300 GPSDO based on this design 
>> concept. Actually, first I’m going to actually try to quantify the jitter on 
>> the 10 MHz output from the Venus. From the HMC1031 datasheet it appears that 
>> if it’s not confined to a ±3 ns corridor that the lock indicator may not 
>> work (or work well). That would be a bummer.
>> 
>> I can foresee a GPSDO with the miniDIN 4 jack presenting the PPS and serial 
>> I/O from the GPS module and two LEDs on the front - the “FIX” LED from the 
>> GPS module and the lock LED from the PLL along with two BNC jacks. It would 
>> have some downsides. For one, I believe in the absence of GPS reception, it 
>> wouldn’t be able to properly hold-over at all. But it’ll be interesting to 
>> see if it can work as well as the micro-controller driven variant does.
>> ___
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>> and follow the instructions there.
> 

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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Bob Camp]

Hi

The noise performance of the HMC1031 close in is pretty horrible. It’s actually 
*worse* than the GPS signal noise. In a normal GPSDO the idea is to use an 
oscillator that is cleaner at 0.01 to 10 Hz than the GPS. 

Bob


> On Sep 12, 2016, at 2:41 PM, Nick Sayer via time-nuts  
> wrote:
> 
> I was talking with someone at AD about a question I had about one of their 
> TinyDACs when they mentioned their HMC1031 chip. It looks like the ideal 
> building block for a clean-up oscillator.
> 
> It struck me just a touch later that the Venus838LPx-T has by default a 10 
> MHz output that’s phase locked to GPS time. It’s not good quality, but I 
> wonder if it’s good enough to be the reference for this particular chip. They 
> do talk about the ability to be driven by a “noisy” or "jittery” reference.
> 
> I think I’m going to take a crack at an OH300 GPSDO based on this design 
> concept. Actually, first I’m going to actually try to quantify the jitter on 
> the 10 MHz output from the Venus. From the HMC1031 datasheet it appears that 
> if it’s not confined to a ±3 ns corridor that the lock indicator may not work 
> (or work well). That would be a bummer.
> 
> I can foresee a GPSDO with the miniDIN 4 jack presenting the PPS and serial 
> I/O from the GPS module and two LEDs on the front - the “FIX” LED from the 
> GPS module and the lock LED from the PLL along with two BNC jacks. It would 
> have some downsides. For one, I believe in the absence of GPS reception, it 
> wouldn’t be able to properly hold-over at all. But it’ll be interesting to 
> see if it can work as well as the micro-controller driven variant does.
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

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Re: [time-nuts] A new take on the all-hardware GPSDO concept

[Tim Shoppa]

You know Nick, the loop time constant typically used with the HMC1031 loop
filter is typically 5 milliseconds. I'm sure some bigger R's and C's can
used for a longer time constant, and I'm sure that'll help clean up the
awful 10MHz output of the Venus838LPx-T. But it is hardly what I'd call a
"GPSDO".

Tim N3QE

On Mon, Sep 12, 2016 at 2:41 PM, Nick Sayer via time-nuts <
time-nuts@febo.com> wrote:

> I was talking with someone at AD about a question I had about one of their
> TinyDACs when they mentioned their HMC1031 chip. It looks like the ideal
> building block for a clean-up oscillator.
>
> It struck me just a touch later that the Venus838LPx-T has by default a 10
> MHz output that’s phase locked to GPS time. It’s not good quality, but I
> wonder if it’s good enough to be the reference for this particular chip.
> They do talk about the ability to be driven by a “noisy” or "jittery”
> reference.
>
> I think I’m going to take a crack at an OH300 GPSDO based on this design
> concept. Actually, first I’m going to actually try to quantify the jitter
> on the 10 MHz output from the Venus. From the HMC1031 datasheet it appears
> that if it’s not confined to a ±3 ns corridor that the lock indicator may
> not work (or work well). That would be a bummer.
>
> I can foresee a GPSDO with the miniDIN 4 jack presenting the PPS and
> serial I/O from the GPS module and two LEDs on the front - the “FIX” LED
> from the GPS module and the lock LED from the PLL along with two BNC jacks.
> It would have some downsides. For one, I believe in the absence of GPS
> reception, it wouldn’t be able to properly hold-over at all. But it’ll be
> interesting to see if it can work as well as the micro-controller driven
> variant does.
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> and follow the instructions there.
>
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[time-nuts] A new take on the all-hardware GPSDO concept

[Nick Sayer via time-nuts]

I was talking with someone at AD about a question I had about one of their 
TinyDACs when they mentioned their HMC1031 chip. It looks like the ideal 
building block for a clean-up oscillator.

It struck me just a touch later that the Venus838LPx-T has by default a 10 MHz 
output that’s phase locked to GPS time. It’s not good quality, but I wonder if 
it’s good enough to be the reference for this particular chip. They do talk 
about the ability to be driven by a “noisy” or "jittery” reference.

I think I’m going to take a crack at an OH300 GPSDO based on this design 
concept. Actually, first I’m going to actually try to quantify the jitter on 
the 10 MHz output from the Venus. From the HMC1031 datasheet it appears that if 
it’s not confined to a ±3 ns corridor that the lock indicator may not work (or 
work well). That would be a bummer.

I can foresee a GPSDO with the miniDIN 4 jack presenting the PPS and serial I/O 
from the GPS module and two LEDs on the front - the “FIX” LED from the GPS 
module and the lock LED from the PLL along with two BNC jacks. It would have 
some downsides. For one, I believe in the absence of GPS reception, it wouldn’t 
be able to properly hold-over at all. But it’ll be interesting to see if it can 
work as well as the micro-controller driven variant does.
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