[time-nuts] PCB layout question for GPSDO

[Attila Kinali]

Hoi Tobias,

Sorry for the late answer. Took me some time to look at your
schematics.

On Tue, 4 Feb 2020 09:11:29 +0000
Tobias Pluess <tobias.pluess at xwmail.ch> wrote:

> not quite. Having D for diodes is maybe some US or whatever standard, but 
> there is also an IEC and DIN standard. However, the newest version of it is 
> very weird and not really useful (sorry, I have only a german link: 
> https://de.wikipedia.org/wiki/EN_81346). At the time I learnt my job,
> the previous version of that standard was the https://de.wikipedia.org/wiki/DIN_40719-2
> which I still use. (The new standard suggests using R even for diodes, and 
> transistors may be R or Q, depending on their function, nonsense...). 
> (Actually I just found out that I should have called my OCXOs G and not 
> D :-)).

Woha... It's been so long since I last saw anyone using this standard
that I had completely forgotten about it. Sorry about that!

> > FPC are not very hobbyist friendly, as they are usually only available
> > in a limited number of wires and lenght combinations, with no way for most
> > people to change their length. This might or might not be a problem with
> > your circuit. Unless you need the density, I would avoid them.
> 
> You are maybe right, but I have a lot of those connectors here because I used 
> them for another project a while ago where I made a little brushless motor 
> controller, and those motors had such connectors. Maybe if I find a "nicer" 
> connector I will use that, no big deal hear. I think it does not really 
> matter, does it :-)

I usually use standard header pins if possible. They are easy to solder and
available everywhere.


> > You want to keep the antenna input as far away from the PPS signal as
> > possible. The steep slopes that you want for the PPS output mean that it
> > will be high in harmonic content. Depending on the exact type of D2, it
> > could be harmonics up to 100MHz or it could be up to a few GHz.
> 
> D2 should be an 74ACT541 or 74AC541, depending on whether it is powered by 5 
> volts or by 3.3 volts (can be selected). Therefore, the slopes will indeed be 
> quite steep. Maybe I will add another ground plane around the 1PPS. But I am 
> afraid it is not really possible to move the 1PPS output to the other side, 
> but maybe I can shift it closer to the OCXO (which I also don't really 
> like...).

You can use the AC for both 5V and non-5V. The T is for the input voltage
level compatibility. As you are not using any TTL components, this is of
no concern here. 

> > What are the requirements on the power supply?
> > With OCXO that can use up to 1.5A (Morion MV89), having a fuse is a
> > good idea.
> 
> The OCXO takes min. 8 watts during the warmup phase and at least 2.5 watts 
> when operating normally. Plus some margin. So the warmup current is some 
> 700ish mA.

Hmpf.. I forgot that the 8663 spec suck at this point. 
I guess they wanted to make their life easier while specifying
for the different supply voltage levels.

So the design would be for ~1A power supply.. With LDOs that will be some
dissipated power. A TO-220 can safely dissipate around half a watt without
heatsink.
 
> > You should not use the output of a switching supply to feed your antenna
> > directly.
> 
> Yup, I already  changed that, because actually the power requirements on the 
> 5 volt rail are not that high. I use now a linear regulator. Further, there 
> is an LCR filter for the antenna. (But actually, I would expect that a 
> commercial active antenna also has some filtering built-in in its power 
> supply, doesn't it? at least it would make sense, but maybe to save some 1 or 
> 2 cents, the manufacturers save every single resistor :-)).

I know manufacturers who spend days discussing with their suppliers to
safe 1/100s of a cent on resistor prices... so yes, they are going to
be cheap, whenever they can. The good (and expensive) antennas will have
smaller filters, but not too fancy and definitely not going far down
(makes filter big and expensive). The cheap antennas basically don't have
any filtering to speak of.



> > No (see below: "J"). I guess ublox forgot to mention it, because by now
> > most people should know it.
> 
> I wonder from where people should know that if they don't read this mailing
> list :-) because the uBlox manual really reads nothing about having traces
> under the modules. (However it does so for the older versions! maybe the
> newer modules are less sensitive to this?). But I learnt it now from this
> list and avoided all signals below the GPS module.

Yeah.. That requirement was on the list since before the LEA line
of ublox modules. I wonder why it dropped out. I would guess the
modules are less sensitive these days (they integrate more and more
of the RF electronics into the ASIC which each version), but I would
not rely on it.


> I only used 100nF X7R as blocking caps. Depending on the voltage, I used 0603 
> size on the 3.3V rail, 1206 on the 5V rail and 1210 on the 12V rail. This is 
> because the capacitance is voltage dependent and the effect is smaller the 
> larger the caps are.

I would say: Use more caps! :-)
I might be one of those people who put too many capacitors everywhere, but
I had some very bad experiences and I like to err on the too much side
rather than on the too little side.


> Actually, this trace is a 50 ohms microstrip. But for shielding reasons, I 
> changed that now to a 50 ohms coplanar waveguide, which has lower losses and 
> better shielding. Because the frequency is so low for GPS, I didn't want to 
> make too many vias because it is simply not necessary I think. (Distance 
> between two adjacent vias should be smaller than lambda/10 as a rule of 
> thumb, which still gives some 10mm! so a via fence would be quite pointless I 
> think?!).

Not really. While it is true that the wavelength is much larger, it is not
true that waves cannot propagate in the space between the metal layers.
Having a few vias there doesn't cost you anything and might help you avoid
problems that are hard to debug later.


> OK I have attached the schematics. The only thing I have changed there is 
> that I added an EEPROM which I will use to store some settings (i.e. PLL loop 
> time constant, the last value of the DAC, maybe the GPS position?).

The schematics look nice. A few comments:

* connect the USB pins of the ublox module to a header 3-pin
  (no need for 5V). This would allow you to access the module
  directly for debugging.

* you might want to consider adding a low-capacitance TVS diode on
  the antenna input. To prevent from surges destroying stuff.
  Unfortunately, these special TVS diodes are kind of volatile
  and whether you can get one of them or not depends largely which
  cell phone manufacturer is currently using which.. or rather not
  using. I have used the PESD5V0X1UB, which Digikey seems to have
  on stock currently.

* connect the PPS pulse to the spare transmitter of the RS-232 transceiver.
  This way you can feed your PC with a PPS pulse

* turn N5/ADG779 around and connect the other side of the switch to GND.
  You want to switch between Vref and GND and not between Vref and open. 
  I am a little bit concerned that the ADG779 does not list a value for the
  charge injection. So keep an eye on that. I would also use some kind of
  buffer between N5 and the voltage reference, to avoid influencing the
  reference voltage while switching for the benefit of the rest of the circuit.
  As you already have a 5V supply, I would supply N5 from these 5V as it
  lowers the on resistance and makes it more constant over the whole
  source/drain voltage range.

* R24/R25 are not needed. The feedback resistor of the inverter will do
  that job already. I might use a comparator instead of a schmitt trigger
  here, for lower phase noise/jitter.

* As you are using two opamps for the PWM output filter N6, I would switch
  to an MFB architecture instead of Sallen-Key, as MFB has better high frequency
  supression. 

The whole design looks very nice. Thanks for sharing with us!

			Attila Kinali
-- 
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
        -- Richard W. Hamming, The Art of Doing Science and Engineering