[Thumbgps-devel] Article -- Macx-1: GPS receiver with standard USB connector and PPS support

[Dave Taht]

A lot of that stuff I'd want to reserve for the Macx-2. Can I keep our
request of navisys limited to merely the supercap and silkscreen?

On Wed, May 9, 2012 at 1:41 PM, Ron Frazier (NTP)
<timekeepingntplist at techstarship.com> wrote:
> Hi guys,
>
> I do see the point TZ was making about the problems of increasing the design
> and production costs.  However, if the supercap turned out to be potentially
> feasible, I have some Circuit Cellar magazines and such that I could rummage
> through for info on micro power conversion components.  I know they exist.
>  I have no idea about specs and efficiency.  How much amperage and at what
> voltage is the unit expected to draw if disconnected from the USB power
> source?  Is it expected to continue tracking?  I assume this will change if
> an external antenna is attached.  How long should it be maintained?
>
> From a user perspective, I'd recommend the following features, if you have a
> viable choice:
>
> * Hi sensitivity internal antenna / chipset that can be used indoors in a
> residential type structure.
> * External antenna port in case you're in a steel building or basement or
> something with no signal.  Use commonly available, generic, possibly
> powered, antennas.
> * GPS status led.  (My BU-353 slowly flashes when it has a fix, shows steady
> with power but no fix.)  The led should be synchronized to the pps.
> * Supercap preferably, or battery which is easily user replaceable and a
> common part number, such as CR2032 for example.  That battery may be too
> big, but you want something readily available.
> * If a battery is involved, a small flashing led to indicate a weak battery
> alert, kind of like what is on a smoke alarm.
> * My BU-353 has a magnetic base.  Something like that might be useful for
> positioning the antenna.
> * May or may not be relevant, but, would 5 - 10 Hz position sensing improve
> accuracy and stability for your purposes?
> * You may wish to expose the pps signal on a screw terminal or connector in
> case you want to attach other instrumentation to that signal.
>
> Here's a possible alternative to providing an external antenna port, and all
> the hassles of piping RF around.  With my BU-353, the "puck" device contains
> both the electronics and the antenna.  The only cord coming from it is the
> USB cable.  So, it might be preferable to have a sealed weather proof unit,
> and just use extension cords for USB rather than extending the RF section.
>  I know you can easily extend USB 10 feet or so.  I think you can go much
> further with powered hubs and such.  Just a thought.
>
> Here's a wild idea, maybe not practical, but, you could attach a solar cell
> to the external antenna and / or the thumb-gps case.  This could feed power
> back to the GPS board and prolong the life of the battery or slow the drain
> of the supercap while powering the antenna if the antenna is in the sun.
>  You could even draw some energy from ambient indoor lighting.  I have a
> solar powered "atomic" watch from Casio that does this.  It never needs a
> battery, never needs charging, and never needs setting.  Very handy.  I also
> have two "atomic" wall clocks that do the same thing.  I think they use a
> rechargeable battery instead of a supercap.
>
> Here's another idea, which may or may not have already been discussed.  Do
> you want to have an on board RTC, which can keep outputting valid time for a
> while, if the satellites are not available due to weather, jamming,
> interference, etc.?
>
> Sincerely,
>
> Ron
>
>
>
>
> On 5/9/2012 2:35 PM, Dave Taht wrote:
>>
>> jeeze guys, all I asked for was an estimate as to the change to the BOM.
>> Could be a Macx-2, might not be worthwhile at all.
>>
>> My concern was that there are 6+ short power flickers a day - forcing
>> a reboot - in places like Nicaragua. These events are *interesting*
>> from a network analysis perspective, as flickers like this cause
>> massive network disturbances.
>>
>> I don't care about battery life longer than about 30 seconds, although
>> a case could be made for 24 hours. Just wanted the analysis of what it
>> would really take, given how little power this thing sucks...
>>
>> and I hate ANYTHING that needs a user-replaceable component, 5 years
>> after manufacture. So I'd prefer the actual analysis based on power
>> draw, etc, before
>> making any decisions.
>>
>>
>> On Wed, May 9, 2012 at 11:21 AM, tz<thomas at mich.com>  wrote:
>>
>>>
>>> It doesn't work that way.  Microampere DC-DC converters?  That isn't a
>>> swap,
>>> it is a redesign, if microamp converters are possible - the base losses
>>> are
>>> high so you won't hit a high efficiency.  You get high efficiencies when
>>> dealing with lots of amps.  For it to last overnight you would need a
>>> very
>>> large (physically) cap.
>>>
>>> There would be no booting problem.  It would merely lose the real-time
>>> clock
>>> and/or location backup so would take a bit longer starting.
>>>
>>> The battery (or cap) only kicks in when the power goes away and is only
>>> needed to make the startup faster, e.g. cycling power on the computer or
>>> router.  USB should be constantly providing 5V 99.9% of the time.
>>>
>>> I could argue that if surviving only a short power disconnect is ok, then
>>> a
>>> supercap might be better, but then it is probably a large engineering
>>> change
>>> since the caps and batteries usually have vastly different footprints.
>>>
>>> Good batteries can last decades - even Supercaps have a finite lifetime.
>>>  It
>>> is a matter of cost, both in redesign, size, and reliability.  But do you
>>> want to add $5-$10 to the cost of the unit to make a cheap ($30-$50) GPS
>>> last over a decade?  Buy a second, seal it hermetically, and put it in
>>> your
>>> freezer.
>>>
>>> If you want to add a few thousand in NRE costs, there are a lot of other
>>> things I would redesign.  Use a USB 2.0 chip to get 125uS jitter.  Allow
>>> for
>>> a detachable or external antenna (which can be run to the window).  Find
>>> a
>>> more optimal chipset.  Maybe a coin-cell port so the battery can be
>>> replaceable.  But the point was to be as cheap as possible so in this
>>> case
>>> it is adding one wire.
>>>
>>> You can always break yours open and if you can find a supercap with the
>>> specs (Sparkfun has a 3.3v available, but don't put more than 3.3v on
>>> it),
>>> you can swap the part.
>>>
>>>
>>> On Wed, May 9, 2012 at 1:50 PM, Ron Frazier (NTP)
>>> <timekeepingntplist at techstarship.com>  wrote:
>>>
>>>>
>>>> Hi all,
>>>>
>>>> (I'm copying this only to the thumb-gps list and to TZ, the most recent
>>>> commenter, as I don't know all the parties in the header of TZ's
>>>> message.
>>>> You guys can forward it to whomever else may need to see it.)
>>>>
>>>> Speaking strictly as a potential user of the thumb-gps device and
>>>> amateur
>>>> interested party, I would rather see a supercap, if feasible.  I hate
>>>> the
>>>> idea of devices having batteries that I have to worry about failing in
>>>> 5-10
>>>> years.  I've had the cmos batteries fail in a few computers, sometimes
>>>> preventing them from booting.  It can get really ugly trying to revive
>>>> them
>>>> A good high efficiency dc-dc converter chip should allow you to drain
>>>> all
>>>> but the last bit of energy from the cap while maintaining whatever
>>>> working
>>>> voltage you need.  I cannot speak to size issues, as I have never
>>>> designed a
>>>> circuit board with one.  The GlobalSat BU-353 that I have has a
>>>> supercap, I
>>>> believe.  Exact backup time is not stated, but I believe it's a few
>>>> days.
>>>>
>>>> Sincerely,
>>>>
>>>> Ron
>>>>
>>>>
>>>> On 5/9/2012 1:12 PM, tz wrote:
>>>>
>>>> For a given size (and circuit board footprint) a supercap will have much
>>>> less capacity and it has an exponential voltage decay curve, so it might
>>>> have plenty of charge but not at a voltage that will hold the data.
>>>>  This
>>>> usually means hours, not days of backup.
>>>>
>>>> A good rechargeable lithium will last several years, maybe longer as
>>>> there
>>>> will be no charge/discharge, maintains voltage until it is nearly
>>>> exhausted,
>>>> and can hold the data for days or weeks.
>>>>
>>>> On Wed, May 9, 2012 at 12:55 PM, Dave Taht<dave.taht at gmail.com>  wrote:
>>>>
>>>>>
>>>>> On Wed, May 9, 2012 at 9:47 AM, Eric S. Raymond<esr at thyrsus.com>
>>>>>  wrote:
>>>>>
>>>>>>
>>>>>> Dave Taht<dave.taht at gmail.com>:
>>>>>>
>>>>>>>
>>>>>>> What would be the change in cost and delay in manufacturing to switch
>>>>>>> to
>>>>>>> using a supercap, rather than battery?
>>>>>>>
>>>>>>
>>>>>> Why would a supercap be better?
>>>>>>
>>>>>
>>>>> Effective lifetime of... forever. no need for replacement. insanely
>>>>> fast recharge. smaller (probably). What's not to like?
>>>>>
>>>>> I am not in a huge hurry to get into manufacturing, and I merely
>>>>> wanted to cost out what what it would do to the bom, any changes to
>>>>> the PCB, and get an estimate for the time it would take to do. I think
>>>>> it will bump the unit cost up slightly,
>>>>> but what price, forever?
>>>>>
>>>>>
>>>>>>
>>>>>> --
>>>>>>                <a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Dave Täht
>>>>> SKYPE: davetaht
>>>>> US Tel: 1-239-829-5608
>>>>> http://www.bufferbloat.net
>>>>>
>
>
>
> --
>
> (To whom it may concern.  My email address has changed.  Replying to former
> messages prior to 03/31/12 with my personal address will go to the wrong
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>
> (PS - If you email me and don't get a quick response, don't be concerned.
> I get about 300 emails per day from alternate energy mailing lists and
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>
> Ron Frazier
> timekeepingdude AT techstarship.com
>



-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://www.bufferbloat.net