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

Chuck Harris cfharris at erols.com
Wed Dec 24 13:07:26 EST 2014

I agree that in some ways it is not too swift, but it is
as I said, a direct DC path... not even a resistor, nor, as
far as I can tell, any diode clamp.  The GPS RF path does
have a blocking capacitor, and all sorts of protection
diodes and bandpass filters.  Because the antenna of the
40dB preamplified variety, the receiver doesn't have much
in terms of on board gain.  [Remember, once you have loss,
you can't get your nice low noise figure back.  It always
pays to put your best low noise preamp up at the antenna
end of the coax.]

As far as I could determine, the ADC0808 is used for only
two things:

1) To measure the voltage at the antenna feed.  This allows
    the GPS to detect shorts and opens.  The antenna preamp
    power supply has a small resistor in series with its 5V
    output, so the ADC can measure how much the antenna
    preamp's current draw drops the voltage across that
    resistor, and calculate the feed line current.

2) To measure the temperature of the Rb oscillator's heat
    sink.  Rb oscillators Achilles heel is temperature
    sensitivity.  The MGPS controller takes the Rb oscillator's
    temperature, and tweaks the control voltage, hopefully
    compensating for changes in ambient temperature.

With the ADC having a 5V maximum input voltage, and a direct
connection to the feed line, all one would have to do to
kill it is to attach a long feed line to the antenna connector
with an open on one end.  A nearby lightning strike could do
the deed too.

I am quite pleased that the RF section of the MGPS unit didn't
get affected by whatever killed the ADC0808.

-Chuck Harris

Azelio Boriani 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.

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