[time-nuts] Raspberry Pi 4 oscillator replacement (Avamander)

[Tim S]

I've been working on something similar with a CM4 (finally got a few this
week).  The reason to me for having the GNSS discipline is to not need a
local rubidium - but the oscillator can be improved still.

Using this 54MHz VCTCXO for the CPU:
https://www.digikey.com/product-detail/en/connor-winfield/TB514-054.0M/CW808-1-ND/4311758

...and this 25MHz VCTCXO for the IEEE-1588 NIC:
https://www.digikey.com/en/products/detail/connor-winfield/T604-025.0M/3757210

...a couple of these 3mm tall mezzanine headers:
https://www.mouser.com/ProductDetail/798-DF40HC30100DS451/

...and some 3mm working envelope pogo-pins to take the clocks from the
baseboard to the module without soldering:
https://www.digikey.com/en/products/detail/mill-max-manufacturing-corp/0900-0-15-20-76-14-11-0/663496

This gets around the package size limits for the oscillator, gives a very
stable temperature compensated clock, and some control to discipline the
long term frequency.  The extra z-height of the connectors allows the
oscillators to sit between the two boards, protecting them more from
circulating air - also allows for cleaner power supply for the clocks.

A Spartan-3 runs the 20-second phase counters, takes the GPS out of standby
about 5 seconds before the GPS needs to have a valid 1PPS, and runs the
PDM-dither DAC.  This cuts the GPS power about in a third (application is
vehicle-borne) when navigation is not needed. Some low jitter clock
distribution amps allow a free-running GPS 1PPS to keep the clocks aligned
even if the CM4 is powered down.  2x Ublox F9T GNSS modules in RTK mode,
dual antennas measured antenna distance, same cable length (calibrated).

This module was my inspiration: https://www.teradak.com/products/115.html

Board design is about done, I'll probably put up a webpage for it around
summer.  The general baseboard layout is going to become my core design for
a few different devices, including a CCTV IP-camera (self time/date setup,
GPS location watermarking, global shutter video frames and audio samples
aligned to time).

-T

On Thu, Feb 4, 2021 at 9:42 AM <time-nuts-request at lists.febo.com> wrote:

> Date: Thu, 4 Feb 2021 12:20:23 +0200
> From: Avamander <avamander at gmail.com>
> To: time-nuts at lists.febo.com
> Subject: [time-nuts] Raspberry Pi 4 oscillator replacement
> Message-ID:
>         <CAPLrxsE1nWESw6+ZcaTDw62fAbmBf54YQkB-yy0zza9_sA_=
> OQ at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> Hi,
>
> I was wondering if anyone here has replaced the 54 MHz oscillator on the
> Raspberry Pi 4 with a GNSS-disciplined rubidium standard? An overkill
> upgrade, but is technically doable? What hardware would it take in addition
> to a GNSS-disciplined rubidium standard and a Pi 4?
>
> Here's where I got my inspiration from, someone replacing the oscillator on
> a Pi 3 with a TXCO:
>
> https://raspberrypi.stackexchange.com/questions/74482/switch-out-the-x1-oscillator-on-a-rpi-2-3
>
>
> Yours sincerely,
> Avamander
>
>