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

[Ron Frazier (NTP)]

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
>>>>          


-- 

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Ron Frazier
timekeepingdude AT techstarship.com