[time-nuts] PCB layout question for GPSDO

[Mark Goldberg]

If you are concerned about phase noise, choose your regulator carefully.
Even many linear regulators are noisy enough to degrade phase noise. There
are lots of low noise regulators to choose from.

Regards,

Mark

On Thu, Jan 30, 2020, 1:18 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).
>
> 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.
>
> 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?
>
> 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?
>
> e) anything else important I forgot?
>
> Many thanks for some hints to my design.
>
> Best
> Tobias
> HB9FSX
>
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