[time-nuts] Ball/Efratom MFS-209 Rubidium GPSDO...

Azelio Boriani azelio.boriani at gmail.com
Wed Dec 24 04:58:37 EST 2014


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
> afternoon.
>
> 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
> know...
>
> 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.
>
> And...
>
> The system status changed to GPS Lock!  And reports normal
> operation.
>
> 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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