[time-nuts] Yet Another GPSDO design - Timing on the move

djl djl at montana.com
Mon Jun 26 21:09:08 UTC 2017


d'oh never mind
don

On 2017-06-26 14:43, djl wrote:
> I'd really like to have a look at the schematic, but trying to read it
> leads to some app requiring me to bare my machine's soul to an unknown
> app developer. Could  plain .pdf be put somewhere not involving
> Google?
> Thanks
> don
> 
> On 2017-06-26 14:17, William H. Fite wrote:
>> On Monday, June 26, 2017, Tom Van Baak <tvb at leapsecond.com> wrote:
>> 
>>> 
>>> Be careful about that. Act like there are no outliers: every point is
>>> trying to tell you something.
>> 
>> 
>> ARRRRRGHHHH! Your resident statistician just had a sudden, stabbing 
>> pain in
>> the head. Before an audience of statisticians, you would find that
>> statement extremely difficult to justify. Indeed, some would say it is
>> inherently self-contradictory.
>> 
>> Perhaps in this context it does not matter. Your knowledge is vastly
>> greater than mine in the TF domain.
>> 
>> 
>> 
>>> /tvb
>>> 
>>> ----- Original Message -----
>>> From: "Jan-Derk Bakker" <jdbakker at gmail.com <javascript:;>>
>>> To: <time-nuts at febo.com <javascript:;>>
>>> Sent: Monday, June 19, 2017 3:44 PM
>>> Subject: [time-nuts] Yet Another GPSDO design - Timing on the move
>>> 
>>> 
>>> > Dear all,
>>> >
>>> > After a hiatus of seven years I have finished the first version of my
>>> GPSDO
>>> > design. The full schematic can be found at
>>> https://drive.google.com/file/d/
>>> > 0B7mNymXfcKMqaFcyRXdURC1KMXM/view?usp=sharing (Google Drive seems to
>>> guess
>>> > the file type wrong; Acrobat opens the file just fine). Its first use
>>> will
>>> > be in the telemetry system of my students' solar-powered boat (
>>> > http://cleanmobility.info/voertuigen/solar-2015/ ), on a trip from
>>> > Amsterdam to Monaco.
>>> >
>>> > The design objectives are, in decreasing order of importance:
>>> >
>>> > 1) Providing a reference frequency for a SDR system in the 868MHz ISM
>>> band,
>>> > having a frequency drift over a day no worse than 10% of the maximum
>>> > Doppler shift at a relative speed of 100km/h, while consuming at most 2W
>>> in
>>> > steady-state from a 24V net
>>> >
>>> > 2) Testing/teaching platform for the evaluation of different design
>>> choices
>>> > in a GPSDO, including alternative phase detectors, EFC generation by DAC
>>> vs
>>> > PWM, FLL/PLL algorithms, timing vs navigation receivers, and OCXO choices
>>> >
>>> > 3) When equipped with a timing receiver, having ADEV/MDEV at most 10x
>>> worse
>>> > than a Thunderbolt in the interval between 1s and 1d.
>>> >
>>> > (Yes, objective 1 could be met with a quality OCXO, but where's the fun
>>> in
>>> > that?)
>>> >
>>> > Where possible I tried to stick to the suggested design criteria listed
>>> in
>>> > https://www.febo.com/pipermail/time-nuts/2007-April/025597.html .
>>> >
>>> > The primary phase detector is a TDC7200, which almost feels like cheating
>>> > after all the trouble I went through last time (
>>> > https://www.febo.com/pipermail/time-nuts/2010-August/049347.html ). The
>>> > '7200 is used in Mode I, which needs at least 12ns between START and
>>> STOP.
>>> > A fairly vanilla synchronizer handles this. As I expect the phase offset
>>> in
>>> > lock to be zero (modulo hanging bridges), the first flip-flop is clocked
>>> > with an inverted copy of the main clock to further reduce the possibility
>>> > for metastability. U16/U17 latch the lower bits of clock divider U19/U18
>>> to
>>> > get around synchronizer uncertainty in the microcontroller. A second
>>> > TDC7200 channel is added to ease comparison with other references or
>>> > timestamp external events. (I have a mains ZCD in the works just for
>>> this).
>>> > Both channels have a simple flip-flop as an alternate phase detector; the
>>> > second channel can be wired to be driven by the GPS PPS as well.
>>> >
>>> > The microcontroller board holds a 32MHz ATXMega256A3U. While this board
>>> > cannot use the 10MHz oscillator for its main clock, both 10MHz and PPS
>>> > inputs are available as event channels. The microcontroller board also
>>> has
>>> > a microSD socket for standalone phase data logging and a charger for a
>>> > small LiIon cell that can provide power when the boat's systems are
>>> powered
>>> > down. U21 is a 128KB SRAM chip for scratch space, U13 is a FeRAM chip to
>>> > store EFC settings (as EEPROM would wear out too fast with regular
>>> writes,
>>> > and I cannot guarantee having enough energy after detecting a brownout to
>>> > only write to EEPROM in such conditions). The other systems for the boat
>>> > already include a GPS module (Venus 6) which is used for PPS in normal
>>> > circumstances; a footprint for a small Venus8-board offers an alternative
>>> > in standalone use ( until I can get my hands on a 3v3 timing receiver ).
>>> > The microcontroller measures system temperature, OCXO current and the
>>> > voltages on the raw power nets.
>>> >
>>> > The EFC is based on a pair of 16-bit DACs plus a 24-bit ADC in a PID
>>> loop,
>>> > inspired by Linear/Jim Williams' AN86 ( http://www.linear.com/docs/4177
>>> ).
>>> > The DACs are fairly noisy, an RC with a few film caps plus a quiet
>>> follower
>>> > should take care of that. Plan B for the EFC is a pair of PWM outputs
>>> from
>>> > the microcontroller followed by a 4-pole filter. Both 1in2 OCXOs with and
>>> > without internal reference can be used, as well as cheaper
>>> Connor-Winfield
>>> > DOC/DOT-series XOs.
>>> >
>>> > What else? Status LEDs, a heavily filtered synchronized switch-mode
>>> supply
>>> > (necessary to hit the power consumption target), an isolated serial debug
>>> > line.
>>> >
>>> > Things I have yet to figure out:
>>> > - how to apply the temperature measurements to the EFC. I guess I can
>>> > measure the holdover performance of the GPSDO in a climate chamber, but
>>> > does such a temperature curve stay mostly-constant over the life of the
>>> > OCXO?
>>> > - similar to the previous point: how to apply the current measurements to
>>> > the EFC. Is there any way to measure/estimate the resistance in the
>>> shared
>>> > path between heater GND and EFC GND? (I've done my best to directly refer
>>> > the filter and the ADC to the GND pin of the oscillator to reduce this
>>> > effect on the external path).
>>> > - how to properly do outlier removal in a mobile platform which may get
>>> > bumped (leading to OCXO jumps). My starting point looks like
>>> > https://www.febo.com/pipermail/time-nuts/2006-December/022689.html , but
>>> > I'm not sure how to make this robust enough without throwing too much
>>> > useable data away. Possibly monitoring (changes in) the satellites the
>>> > GPSDO uses for its solution might help
>>> > - in relation to the previous point: when to switch between FLL
>>> > (aquisition) and PLL (tracking), and when to switch PLL time constants.
>>> > (Maybe I should have added an accelerometer to detect jolts)
>>> > - how to measure ADEV/MDEV beyond hooking my Thunderbolt and/or my
>>> FE-5680
>>> > to the auxillary input
>>> > (- plus all unknown unknowns)
>>> >
>>> > We're building ten of these to start with. (With a good OCXO, the BOM
>>> cost
>>> > is a bit over the eBay price of a Thunderbolt). Some stay in Amsterdam
>>> over
>>> > the summer to collect phase data; for those coming to Monaco I'm still
>>> > undecided whether I'll try a simple version of my FLL/PLL or to just use
>>> > this trip for logging.
>>> >
>>> > Thoughts?
>>> >
>>> > JDB.
>>> > _______________________________________________
>>> > time-nuts mailing list -- time-nuts at febo.com <javascript:;>
>>> > To unsubscribe, go to https://www.febo.com/cgi-bin/
>>> mailman/listinfo/time-nuts
>>> > and follow the instructions there.
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com <javascript:;>
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>>> mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>> 

-- 
Dr. Don Latham
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304




More information about the Time-nuts_lists.febo.com mailing list