[time-nuts] Ball/Efratom MFS-209 Rubidium GPSDO...
kb8tq at n1k.org
Wed Dec 24 09:29:04 EST 2014
The ADC0808 is a (relatively) slow multi-input 8 bit ADC. It’s a good candidate for built in test chores on a board that does not have a MCU with built in ADC capability (or where you have used all the pins). They are cheap and easy to use.
Current limiting the antenna supply is a pretty good idea. They do short from time to time. Monitoring the voltage going to the antenna is a quick way to see if the supply is limiting. So far a great idea. Get a bit of over voltage on the antenna and the ADC probably isn’t going to last very long …Over voltage might be lightning, it could be other things.
> On Dec 24, 2014, at 4:58 AM, Azelio Boriani <azelio.boriani at gmail.com> wrote:
> An ADC0808 directly connected to the GPS antenna? Not even a DC
> blocking capacitor? So it receives also the 5V antenna supply?
> On Wed, Dec 24, 2014 at 7:42 AM, Chuck Harris <cfharris at erols.com> wrote:
>> It's Alive!!!
>> The MGPS module was saying there was an A-to-D fault
>> and a Feedline fault. Suspicious of the common element,
>> I traced the signal path from the GPS antenna connector
>> to the input of the ADC, and it was a straight DC path.
>> Sure, there were a few chokes, and capacitors to ground
>> to filter out any stray 1.5GHz signal, and to keep the DC
>> circuitry from loading the antenna signal.
>> I ordered up a new ADC converter, which was an ADC0808 by
>> National and TI. A small 28 pin quad J lead surface mount
>> package. It finally came, and I swapped it out this
>> I was rather worried, as the failure mode could have been
>> simply a faulty converter, or it could be that something
>> happened on the antenna line that killed the converter.
>> It was not evident how the GPS module knew there was an
>> A-to-D fault. It could have been something like a lack
>> of an end of conversion flag, or they could have been
>> using some extras of the 8 analog inputs to measure things
>> like power, ground, resistor dividers... I really don't
>> Powering the GPS module on the bench showed an immediate
>> improvement: The A-to-D converter fault condition was
>> no longer set! It was still showing an antenna line fault,
>> but since I didn't have an antenna connected, that was to
>> be expected.
>> I put the GPS module back into the MGPS chassis, and
>> installed it in the MFS-209, and as soon as I connected
>> a good antenna, the antenna line fault indicator went
>> away.... So far so good.
>> Some satellites started appearing on the display, and after
>> waiting for what seemed like forever, and the almanac was
>> finally updated.
>> The system status changed to GPS Lock! And reports normal
>> Now to let it cook for a few weeks and see how well this
>> ancient GPS RbDO performs.
>> Thanks to all that offered tips and moral support.
>> -Chuck Harris
>> Chuck Harris wrote:
>>> And as I feared, the MGPS display is telling the truth, there
>>> is something wrong with the GPS module's ability to check the
>>> feed line for shorts and opens, and something wrong with the
>>> motherboard's ADC module.
>>> At least the communication between the MGPS and the GPS engine
>>> is working properly...
>>> -Chuck Harris
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