[time-nuts] GPS splitter

Bob kb8tq kb8tq at n1k.org
Wed Jun 21 16:37:08 EDT 2017


Hitting the inductance required is relatively easy. Doing so and not hitting self resonance is a bit more tricky. 
Even a zero ohm reactance likely will work ok (in series with a 150 ohm resistor).  It’s tough to know 
what you have done without a network analyzer. It is even possible that your resistor has enough stray C
to mess things up. Of course it might also have enough stray L to get you back in business. 

A somewhat less critical approach is to use ferrite beads on both sides of the resistor. They generally are a 
bit less critical (= more likely to do what the data sheet claims). The trick there is to find some that are set up
to work at L band …. they aren’t common in my junk box. 


> On Jun 21, 2017, at 4:16 PM, Attila Kinali <attila at kinali.ch> wrote:
> On Wed, 21 Jun 2017 15:03:54 -0400
> Bob kb8tq <kb8tq at n1k.org> wrote:
>> The typical answer is: The largest one I could that 1) Would handle the short circuit current expected and
>> 2) was self resonant at or above 1.6 GHz.
> I found out, by experiment, that any "large enough" inductance is ok.
> A simple "a few dozen windings of AWG30, air core and a tiny bit of
> epoxy to keep the windings apart, yet stable" does the trick quite well.
> At least it does for the bias-T I used it in. As a rule of thumb,
> at the frequency you are using it, the inductor should have an
> impedance much larger then the 50Ohm. And for 1.5GHz you get there
> pretty quickly. E.g. 20-30 windings on 3mm spread to a length of 20mm
> give you something in the order of 150 to 350nH, which translates
> to 1k5 to 3k5 impedance at 1.5GHz.
> I recommend reading [1] and [2] for the design of bias-T's
> (there are probably better sources, but these are those
> that I stumbled upon, some time ago)
> 			Attila Kinali
> [1] "Design of Bias Tees for a Pulsed-Bias, Pulsed-RF Test System
> using Accurate Component Models", by Baylis, Dunleavy, Clausen, 2006
> http://www.microwavejournal.com/articles/3912-design-of-bias-tees-for-a-pulsed-bias-pulsed-rf-test-system-using-accurate-component-models?v=preview
> [2] "Wideband Bias Tee". by Johnson, 2008
> http://wb9jps.com/Gary_Johnson/Bias_Tee_files/Bias_Tee_Design_V2R.pdf
> -- 
> You know, the very powerful and the very stupid have one thing in common.
> They don't alters their views to fit the facts, they alter the facts to
> fit the views, which can be uncomfortable if you happen to be one of the
> facts that needs altering.  -- The Doctor
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