[time-nuts] Building a DC Block Thingy....

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jul 27 17:50:13 EDT 2007

Jason Rabel wrote:
> I have a NTS-200 on the way, but one thing about them is their GPS power
> output is 12V instead of the usual 5V. I really didn't want to modify the
> board any in an irreversible fashion so I was hoping to build an inline
> coupler that I could block / sink the 12V current.
> I have a Symmetricom SmartSplitter, and I know that *should* do the job as
> long as a lower numbered port is supplying power, but I really don't want to
> take the accidental risk of sending 12V to my antenna.
> Doing the math to sink 25ma of current would require a 480 ohm resistor...
> That should make the receiver happy and think all is well. But that's as far
> as my knowledge goes. What size capacitor would I need to use? Do I need to
> add an inductor in series with the resistor?
> Jason

Using a resistor may not be such a good idea.
It all depends on the antenna preamp circuit configuration.
The safest and quietest way is to use a 5V series regulator 
inconjunction with a couple of inductors and a few capacitors.
In principle a series zener (shunted by an RF bypass capacitor 
 >=100pF??) to drop the voltage to around 5v despite the inevitable 
spead and temperature dependence of the antenna amplifier bias current. 
It may just be simpler to use a bias T and an auxiliary 5V supply for 
the antenna.


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