[time-nuts] New Subscriber, DIY GPSDO project (yes, another one)

[Attila Kinali]

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

> Following up in the previous post, here's the latest schematics and some 
> screen shots of top and bottom layers. This is a two layer board, btw.

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

-- 
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