[time-nuts] PCB layout question for GPSDO

[Bob kb8tq]

Hi



> On Jan 30, 2020, at 3:20 PM, Tobias Pluess <tobias.pluess at xwmail.ch> wrote:
> 
> Hi guys
> I have almost finished the layout for my new GPSDO having a TDC. I have attached a screenshot of the PCB layout, for which I have some questions.
> 
> 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).
> 
> 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).
> 
> 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.
> 
> In the lower right corner is a voltage regulator for 12V (I will perhaps use a 7812 for that) and some LC filters.
> 
> X1 is the JTAG connector which allows to download/debug the software to the microcontroller (D1 near the centre).
> 
> D3 is a RS232 interface chip which allows the GPSDO to be connected to a PC COM port for monitoring.
> 
> 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.
> 
> 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.
> 
> The active antenna supply can be selected between 3.3 volts and 5 volts by means of a jumper.
> 
> The GPS module itself, which will be a LEA M8T, is D4 (quite obvious, I guess).
> 
> My questions are the following:
> a) is it acceptable to route some signals under the GPS module? As one can see, the signals I have routed under the GPS module go to the front panel interface, i.e. they are not highspeed signals or so, but I still wonder whether you experts would worry about it (like "oh no, routing anything near the GPS module will disturb it and decreases its sensitivity" or something).

If you look at the layout guidelines for the GPS modules they all pretty much say “don’t do this”. If you have a 8 layer board and the traces are 7 layers down, that
may not be worth noting. If it’s a more normal board, probably worth paying attention to. The RF input obviously needs to be isolated ….


> 
> b) would you worry about the switching regulators? I know linear would be better, but the professional GPSDOs also have switchers, and besides that a linear regulator would not be so nice because my power supply will be at least 15 volts.

There are also commercial GPSDO’s that have major spur issues. It all depends on what you are trying to do. How important are spurs?

> 
> c) is the usage of a RS232 chip a good idea or would you kick that out because they produce such massive amounts of EMI that it will disturb my GPS module or OCXO?

The real question is - what’s the alternative? RS-232 is what people want to use.

> 
> d) is it a wise idea to use an LC filter for the OCXO power supply, or does this lead to another rabbit hole because the inductors will pick up magnetic fields (50 Hz...) and modulate the OCXO?

Mag into the isolation coil? Not an issue. 

> 
> e) anything else important I forgot?

Lots and lots of things to worry about …..

Bob

> 
> Many thanks for some hints to my design.
> 
> Best
> Tobias
> HB9FSX
> 
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