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 <[email protected]> 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" <[email protected] <javascript:;>>
To: <[email protected] <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.
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