On Donnerstag, 27. Februar 2020 19:16:22 CET ew via time-nuts wrote:

I'm in Germany.

BR,
Matthias

Please contact me direct off list
In a message dated 2/27/2020 4:46:32 PM Eastern Standard Time, time-nuts@welwarsky.de writes:

On Donnerstag, 27. Februar 2020 19:16:22 CET ew via time-nuts wrote:

I'm in Germany.

BR,
Matthias

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On Samstag, 29. Februar 2020 15:01:13 CET Matthias Welwarsky wrote:

And I hit the send button too early, here's the missing PCB screenshots.

On Sat, 29 Feb 2020 15:01:13 +0100
Matthias Welwarsky time-nuts@welwarsky.de wrote:

I had a quick look at your design. It looks okish, but there are
a few things that need to be improved.

First of all, the power supply seems to be under-dimensioned.
You are going down from 24V, which means you are burning around 7
times the power consumption of your electronics in the LDOs.
Even if you assume everything is low power and the electronics only
use around 200-300mW, that means you burn another 1.4-2.1W (Watt, no milli!)
in the LDOs. Something in the order of 500-700mW consumption in the
electronics would be probably closer to the truth. I.e. you should
plan for ~4W that are dissipated by the LDOs. Or better still,
calculate how much power each component uses and design the power
supply accordingly.

The bulk (ie a bit less than half) of the power is dissipated in U3,
which is a 0.5A spec'ed LDO in D3PAK... if it's properly cooled.
Your PCB doesn't show any heatsink and will dissipate into the board.
Assuming that you can dump more than 100-200mW at a single spot
into a densly populated PCB without some thermal design is asking
for trouble. You will need a heatsink. If we are going by the above
4W number, you need to get rid of at least 2W in U3. Assuming a
20°C above ambient case temp is ok, this means a maximum thermal
resistance of 10K/W, better 5K/W. That's doable with a large D3PAK
heatsink, but even that is probably pushing it, unless you add a fan

The next problem is U12. I don't know which one of the ublox
modules you choose, but be warned: they consume a lot of power!
At the lower end of the spectrum we have the power safe mode
of an NEO-6 spec'ed at typically (not max!) 11mA during tracking.
It can, and will go up to 47mA(typ) during aquisition. Max
current consumption is spec'ed at 67mA. The LEA/NEO-M8T are
spec'ed at 32mA average, 67mA max. A LEA-5 is known to go up to
180mA during aquisition, while the datasheet only says only
150mA max. And all this is not yet including the antenna power
supply, which is another 10-30mA. So, assuming you have a modern
ublox module, you are in for around 100mA current consumption
during aquisition (can last for several minutes) and around 50mA
during tracking. That means poor little U12 has to dissipate 2W
during aquisition and 1W during tracking. There is no way a tiny
SC-73/SOT-223 case is going to handle that.

The next two LDO, U4 isn't any better off. It comes in even
tinier SOT-23-5 cases, which I wouldn't trust beyond 100mW
dissipation. That limits the current going through it to
about 10mA. Yet there are quite a few components on it that
definitely draw more alone. Heck, U8 alone probably draws 40-50mA.
Guestimating, I would say there is a total 60-100mA on U4,
which would result in something around 0.5-0.8W of dissipated power.

Only U9 and U10 have low enough loads that I would say that they
can do their job. Though you should calculate the maximum load
and dissipated power, just to be sure

I would also recommed using a DC/DC switched power supply to go
down from 24V to 5V, to get around the big bulk of waste heat
production.

Going forward.. or rather backward. You have Q1 presumably as
reverse polarity protection. Unfortunately it doesn't work that
way. The way you put the FET in, it will act as a source follower.
Meaning the voltage at the source will be the voltage at the
gate minus the threshold voltage. Now the gate is being pulled
up by a 8.4V Zener diode, which means the gate is supposedly
8.4V-ish below the source, but that's more than the thresold
voltage, so the FET closes off and doesn't conduct. For this
kind of thing to work, you would have to turn the FET around,
the source pointing at the power source. But then, while
the FET sure does not conduct during reverse polarity,
the body/protection diode of the FET will conduct. Hence you
don't get any protection there. It would be a better idea
to just use a 1N4001 instead. Simpler, and can withstand
100V reverse polarity :-) D4/D5 are probably meant as over
voltage protection. While this definitely works, it's kind of
crude. At least at these voltages. If this would be a 1kV system,
then using a triac would be fine, but for low voltage electronics,
the time it takes to fire a triac and get it switching will not
prevent the downstream electronics from getting fried. A better
approach is to use an appropriately rated TVS diode. Overall
simpler and better (aka faster) protection.

Going further back: You use two decade counters to devide the 10MHz
signal from the rubidium by 2000. and use this than as a stop signal
for the TD7200. Yes, this idea works but be aware that the 74390 is
a ripple counter. This means that each division stage is clocked by
the previous stage. This means the jitter of each stage add up. If
you want to stay with this topology, I would suggest switching to
two 74161/74163 which are synchronus counters. I would also suggest
that you go further down in frequency than 500kHz. With 500kHz you
only have a 2µs window within which you can alias free determine
the phase. If you ever have any GPS outage, +-1µs phase drift is
accumlated quickly in just a few hours, even with a rubidium. Of
course, If you don't mind that you have phase discontinuity during
outages, that's fine.

If you don't mind going to another architecture, I would suggest
to use a half-Nutt interpolator and timestamp the PPS pulse. Then
you can use the timer unit of your µC to do the coarse counting.
Tobias' and my GPSDO design both do that. There are two advantages
for doing this: 1) You have a shorter measurement window of <200ns,
which in which the TDC7200 is better than 100ps accuracy (typical
better than 60ps). 2) You don't get any problems with measurement
windows as you have absolute timestamps.

Overall, I would recommend you have a look at Tobias Plüss' schematic.
He has done a very nice job for the design. Another very good
example is Nick Sayer's GPSDO, which is not only well made, it's
one of the most simple designs out there, while still reaching
very good performance (the only two simpler designs, I am aware
of, are the ones by Brooks Shera and James Miller).

		Attila Kinali

--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.

Hi Attila,

thanks for the review, but I think you're a bit too pessimistic about the
power consumption. But let's get through your points:

On Samstag, 29. Februar 2020 20:08:13 CET Attila Kinali wrote:

I'll check the total power consumption when I assemble the next two PCBs, but
I think you're probably not far off. The bulk of it is however the Ublox
module, definitely the single most prominent consumer.

No heatsink, just a couple of thermal vias into the ground plane and it gets a
bit warm, but not more than 35°C (case temperature). I just checked with a
thermocouple.

Yep, this one was more of an afterthought when I realized that I couldn't
route the 12V rail through to the Ublox module and that the 12V regulator
would never be able to cope with the additional load thermally. So I decided
to put another 5V regulator in front of the Ublox module and pray. It keeps
thermal equilibrium of around 46°C when the receiver is in HOLD mode, but
certainly gets pretty toasty while it's acquiring satellites. If I do a
revision, I'm definitely going to put a buck converter here, there's some
pretty nice synchronous designs from TI with very good efficiency.

Yes, but you're overestimating the power consumption. The complete digital
part has no more than, say 30mA total consumption on the 3.3V rail. U4 gets a
bit warm, but 40°C case temperature is OK'ish. Still a lot, but apparently the
thermal conductivity is enough. However - this is with no load on the 10MHz
output. The output driver is connected to the same power rail. This might push
U4 over the edge ;)

Keep in mind also that the ambient temperature is not 20°C, the whole PCB sits
in front of the LPRO-101 with a surface temperature of about 40°C.

These two are fine, they don't need source a lot of current.

For the GPS pre-regulator definitely. For the rest of the electronics - maybe.
But I wanted the power supply of U6 at more than 5V and I had to balance the
power dissipation somewhat to not burden everything onto U3, hence the 12V
intermediate voltage. Still, U3 could be replaced by a buck converter down to,
say, 6V, that would take the stress off of all downstream LDOs. I just need to
find something that has an appropriate footprint. I don't have a lot of PCB
area. I seem to remember that synchronous buck converters can be had in
SOT-23-6 package ;) I just need to find something with a high enough switching
frequency so that the inductor can be very small.

You probably missed that Q1 is a p-channel mosfet. The circuit around Q1 is
basically a textbook approach to reverse polarity protection. If you google
for the term "reverse polarity p-channel mosfet", this is what you find. With
a bit of effort I could probably find it in the "Art Of Electronics" I have on
my desk: The idea is to have the body diode source the initial current until
the source voltage raises above the threshold voltage to switch the FET on. In
reverse polarity, the diode will not conduct and the FET will stay off. The
Zener is only to limit the gate-source voltage to around 10V.

On the idea of using a TVS diode instead of the SCR crowbar - forget it. I
tried. By the time the polyfuse trips, the TVS has released the magic smoke.
The LPRO-101 draws about 1.7A during initial heat-up, the fuse has to be rated
accordingly. A TVS diode with a breakdown voltage of 27V would have to
dissipate, say, about 50 Watts for a couple of seconds at least. I used a
LDP24A, Besides from being of enormous size, I didn't trust it not causing a
fire when the protection trips.

Yep, that's a good idea. Though, holdover stability wasn't really a design
goal - yet. I simply didn't consider what would happen when the GPS coverage
returns after a prolonged outage. So, yes, TIC could wrap meanwhile. But since
I don't output a 1PPS signal, maybe I wouldn't care.

I actually tested the TIC circuitry stand-alone with a coherent start/stop
(sourced from the same reference clock) and the performance was fine. Standard
deviation of less than 30ps over 1000 measurements or so. The 56ps granularity
of the TDC7200 was clearly visible in the results. Anyway, 100ps is better
than necessary. I did a couple of tests with the GPSDO simulator on
Leapsecond.com, resolution below 1ns or so didn't (seem to?) make a huge
difference on the performance. If you look at what a GPS receiver typically
delivers, you'll need to average over many, many, many seconds anyway ;)

Using the TDC7200 in mode 2 seems to be "good enough".

The G3RUH GPSDO is indeed very simple. Basically an XOR PLL with a very tight
loop bandwidth. Brooks Shera I don't know yet, but the one by Nick Sayer is an
analog interpolator with a 4046 as a phase discriminator switching a FET
charging a capacitor feeding an ADC. Similar to Lars Walenius' GPSDO. That
design is even a bit simpler than the one from Nick Sayer.

Do you have some schematics online of your design? I've seen Tobias schematics
on the list, but I only looked at the last month of postings so far.

Best regards,
Matthias

With all the recent discussions on GPSDO's allow me to add my two cents worth. Being a Ham and at the time subscribing to QST the July 1998 Brooks Shera GPSDO article was what I needed since I had for 10 years controlled my FRK with a 12 bit DAC and did have 60 KHZ Tracor M100 and Loran C. It started with emails with Brooks to adapt his circuit to Rb, changing the input to 100 MHz etc. Visiting him since I had friends in Albuquerque I did have a chance to meet him personally.  Over lunch we had a great time and when I offered to  sign a non disclosure agreement he laughed so hard saying with my lack of understanding of code not necessary. I was made aware of all code changes. After passing away from cancer, starting with melanoma going to his brain his wife gave me permission to share his work with time nuts.

Parallel to that work I got to know Richard McCorkle when shortly after joining time nuts I declared I thought I could do a D/M along with counters but needed help on PIC Code, one response was that's why we call our self time nuts but Richard offered help. This developed in a long relationship with many applications including PIC TIC and GPSDO. Most the time response to a question because of the time difference to Alaska, was code. He was a genius when it came to code and we still do new designs with his code. We disagreed only twice one had to do with his GPSDO where he was using PIC TIC type interface to GPS, in my opinion an overkill. More to it later. My last message from him was that he had fallen down stairs and prostate cancer was detected. He would get with me when better, never did.

Lars and I connected also on his code capabilities. We discussed multiple projects but not much his GPSDO.  He never had an opportunity to get AV tests done. Magnus I do not know if he contacted you his health may have prevented it. We skiped regularly till 4 weeks before his passing. Early last year the subject came up and somehow Jim Hamman who had worked with Lars contacted me.  He had been in contact with Lars worked with him on his code and his input circuit. Jim shared his work with Nick Saya but also continue his refinement. Attached the three versions. I offered to run tests on his unit and he send me a picture. I was getting worried and asked him to make it more user friendly. He complied and I did post the excellent results on time nuts. We continue to work together with my focus mainly on board designs and procurement of some of the critical components. Recently we had a great time when Jim came to Florida for 3 weeks 20 miles from me. We call them Snow Birds.

I call the GPS side the dirty side since the output of most GRS receivers have a lot of Jitter. Antenna and field of view is the main contributor to a good signal output. I spend a lot of time of that part of the circuit and it is mainly an improvement of Brooks with later circuits and 100 MHz OCXO. remember well Brooks explanation of sampling.

The NIST paper

TEST 76120S

Test Folder Number: 288073-16

Test performed for EndRun Technologies

tells me a lot about GPS , particular the last plot between NIST and USNO common view.

The filter is where you do the music. In reality that is where 90% of your contribution is. Can not say much about it because there is where I know the least. I consider for convenience the DAC part of the filter but for obvious reasons make it part of the Frequency source.

The third one is the Frequency source. There has been lately discussion on OCXO's but few have resources other than buying what the budget allows. I like to refer to SRS FS740 manual that clearly demonstrates what a filter can do. It can not improve on the source but maintains it capability. All Telecom GPSDO's diminish Frequency sources because the change frequency to correct for time. That is where the hump comes from and if you have equipment like we do you can see it. The NIST paper shows it.

Based on the January visit Jim is working on some refinements that will be part of the next board design, there may well be also a TDC7200 version and a performance comparison will be done. One thing we agreed on there will be separate board with OCXO and LTC1655 with some adoption of the page 11 circuit. Also regulator. Last year I published two universal OCXO boards, plan to add the necessary circuits to the one with output amplifier. There will also be a Rb board Juerg and I plan to use but also for my Maser performance work.

Attached is a sketch from Jim showing the three input circuits starting with Lars. Also his prototype design using cardboard but his final circuit.

Lars along with Brooks have left a legacy

 
Bert Kehren

N'achmittag!

On Sun, 01 Mar 2020 01:38:11 +0100
Matthias Welwarsky time-nuts@welwarsky.de wrote:

Ok.. I'm surprised. Is the PCB able to dissipate this much heat?

40°C is not just OKish, it's totally OK. I wouldn't worry until you
hit something like 60°C. As long as the die is safely below 100°C
it will be fine.

And yes, the 10MHz output will add a lot of additional current.
At 3.3V that is about 25-30mA going into the connector.

There are parts are meant as a replacement for 78xx. Ie fit
in a TO-220 footprint, like e.g. the OKI-78SR series from Murata.
They are usually available in 3.3V, 5V and 12V... some manufacturers
also have values in-between.

GAH! Indeed I did! I take back everything I said!

Oh...kay? The input circuitry is usually meant as a protection against
surges, not against having a power supply with the wrong voltage attached.
So I am a litte bit surprised that you try to protect against that.
Do you see it likely that your power supply goes up to a voltage that
would break the LDOs downstream?

Not yet. I'm working on a write-up that explains all the components
and what design decisions lead to them. For now, you can find the
key components in the mails I wrote as an answer to Tobias:
http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-October/097962.html
http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-November/098207.html

		Attila Kinali

--
<JaberWorky> The bad part of Zurich is where the degenerates
throw DARK chocolate at you.