[time-nuts] PCB layout question for GPSDO

[jimlux]

On 2/3/20 2:51 PM, Attila Kinali wrote:
> On Thu, 30 Jan 2020 20:20:08 +0000
> Tobias Pluess <tobias.pluess at xwmail.ch> wrote:
> 
> First a general note:
> 
> D is usually used for diodes. For ICs and other complex parts
> usually U is used. In old German schematics, you could find often
> X for ICs. Connectors are usually X or J, depending on the faith
> you believe in. Quite often X is used for crystals too.
> Whatever you use, at least be consistent. Somewhere, there is
> a kind of "standard" (ie a document that lists what is kind of
> common) for naming components in schematics, but I can't find
> it at the moment.
> 
> 
>> First I will explain a bit what is on the board.
>> The connector X2 (top left) is a interface connector where I will connect a
>> flat flexible cable to a front panel board, which is still to be made. In
>> case the GPSDO will be mounted in a benchtop housing or so, one might want a
>> front panel with some status display or even a set of buttons to change
>> settings (e.g. change the PLL filter time constant).
> 
> 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.
> 
> 
>> Next to it are the SMA connectors for the antenna and for the 1PPS output,
>> as well as two 10MHz outputs. The ICs N10 and N3 are output amplifiers which
>> should provide some 40ish dB of isolation between the OCXO and the two
>> outputs (such that the OCXO should not see it when a load is connected or
>> disconnected).
> 
> 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.
> 
> 
>> D10 and D8 are two assembly variants which allow to install different OCXOs,
>> I have currently planned to add an OSA 8663 OCXO. The DAC and filter stuff
>> to control the EFC pin is on the right side.
> 
> That looks fine, though I would align the RF pins (see below: "E")
> 
> 
>> The power supply, which is 15 volts, comes in at the lower left corner;
>> there is a fuse (because the GPSDO will be left on basically forever, I
>> think a fuse is a must, isn't it) and some large input filter cap.
> 
> 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.
> 
>   
>> To generate the 3.3 volts for the logic, a switching regulator (Traco TSR1)
>> is used (N7); the regulator N9 generates 5 volts which is used for both the
>> active antenna supply as well as the 1PPS output.
> 
> You should not use the output of a switching supply to feed your antenna
> directly.
> 
> 

Depending on the power requirement, why not use a good LDO like the 
LT3042 (or LT3045 higher power version).

They are more expensive than a LC filter, it's true ($5)

> 
>> e) anything else important I forgot?
> 
> Quite a few bits, actually...
> 
> Let me start with the simple things: Go for a 4 layer board.

I agree - it's a whole lot easier if you don't have to be routing power 
and ground traces all over.  Put the ground plane as the plane under 
your RF, and keep the RF and analog stuff on the top side. And the 
digital on the bottom side.  So your stackup from top to bottom is:

RF/Analog
Ground
Power
Digital

(or the reverse, what ever makes the layout easier)


> 
> As for the PCB, I've attached an annotated version of your layout, so
> I can point out a few things
> 
> A: Don't feed your antenna from a noisy power supply. You are comming
> from a switched DC/DC with not enough filtering (especially at the lower
> frequencies) and are on the way to the PPS driver chip. You will end up
> with lots of spurs in the power supply that goes to the antenna which
> in turn will lead to intermodulation products. Especially with cheaper
> GPS antennas that have almost no power supply filtering or conditioning.

See above comment on using a LT3042 or LT3045


> 
> E: OCXO like to have a constant 50Ω output impedance on their RF output.
> Going too far on either side will lead to harmonic contents, degraded
> frequency-pull stability and potentially other effects, if the internal
> isolation amplifier is not well designed. Also, you should ensure that
> you have no reflection, ie no impedance jumps or splits. If you want
> to split passively, use a lumped-element Wilkinson spliter or a resisitive
> splitter (adds another 3dB loss of power, ie output is -6dB instead of -3dB).
> As I wrote above, rotate the OCXO such that both RF pins are close as this
> is the most sensitive pin that is hard to control. The power supply can
> be easily stabilized by placing a capcitor close to the power pin (which you
> should do for both footprints). Similarly for the EFC.


Minicircuits has nice SMT power dividers.  Again, $5 instead of 3 
resistors, so you can make the call on board space (wilkinson and 1 
resistor) vs resistive divider (3dB power hit, 3 resistors, but tiny) or 
power divider.





> 
> 
> As for power supply, I would go for a switched + LDO solution.
> The DC/DC switched power supply keeps your losses low and the
> LDO can clean up the noise of the switcher. 


I agree - this has worked well for us - you can run those LDOs with 
something like 0.5-1 Volt drop across them.  Then you don't care about 
IR losses in lumped LC filters too, the LDO takes care of it.


> 
> 
> LM317 is a very poor choice for an LDO, even its
> newer replacement part the LM1117 performs poorly in terms
> of noise and regulation compared to more modern variants.
> My recommendation are the TPS7A45xx series by Ti.
> There are a few others, like LT1764 etc,



> You don't have to go too crazy with the noise levels, as most
> components are pretty resilient (OCXO included).

You could probably find something cheaper than the LD3042 that would 
work well enough.