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

[Matthias Welwarsky]

On Dienstag, 3. März 2020 14:14:37 CET Attila Kinali wrote:
> N'achmittag!
> 
> > 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.
> 
> Ok.. I'm surprised. Is the PCB able to dissipate this much heat?

Well, it probably means that, yes. But I've not characterized the power 
consumption fully. I was in a bit of a hurry to get the prototype up and 
working so I didn't really make a power or thermal budget before, I just put 
everything together and hoped for the best. Since the numbers cannot lie, 
either the heat transfer through the thermal vias is much better than 
anticipated or the actual power consumption is much less than the estimate :)

I'll know that soon.

> 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.

That's going to roast the LDO for 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.
> > 
> > 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.
> 
> 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.

Thanks for the pointers, I'll have a look at them.

> > 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.
> 
> 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?

Hm, the origin of that circuit was a connector board I designed for the LPRO 
as a standalone frequency reference. I was a bit afraid to damage the Rb by 
connecting an incompatible power supply. I wasn't too worried about surge 
protection but actually overvoltage and reverse polarity. I just copied and 
pasted that part into the GPSDO.

> > 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.
> 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/0982
> 07.html

Thanks, I'll have a look.

In the meantime, I'm thinking of another modification to make: getting rid of 
the HC390 ripple counter. I think I might be able to use the STM32 as a 
frequency divider. I just have to figure out how to program the timers. What 
might work is the following:

- Feed the 10MHz clock into the external trigger input for the timer, 
configure the timer to count the trigger pulses instead of the internal clock 
source.
- Set the timer to auto-reload the desired divisor, e.g. 20 (-1),
- Set the capture/compare register to the same value, set the output mode to 
"toggle"

The result should be a 250kHz signal at the timers output pin. I have no idea 
though if the jitter is going to be better than with the HC390. If anything in 
the above scheme is linked to the internal AHB clock, the expected jitter will 
be around 20ns. That would definitely not improve the situation ;)

BR,
Matthias

> 
> 
> 			Attila Kinali