[time-nuts] GPS splitter

[Ben Hall]

On 6/17/2017 7:40 AM, Bob kb8tq wrote:
> will do a pretty good job of “antenna detect” signaling. In the more general case
> of “I didn’t design this beast” dc blocks and dc shunts to ground is the best approach.
> This fairly quickly gets you headed in the direction of the HP / Symmetricom
> splitters.

Hi Bob and list,

This is very good timing for this discussion...as I'm in the same boat.

As others have pointed out, the issue with power splitters not designed 
for GPS use, is that they are passive, not DC-blocked, and don't have a 
way to load-down the DC-block ports to "fool" antenna sensing circuitry 
as other here have posted.

For the folks who like to "roll their own", there is this design for a 
DIY GPS antenna splitter:

<http://www.uhf-satcom.com/misc/gps_dist/>

It uses a MAR-6 MMIC, but since I had none of those in stock here, I 
modified it for a PGA-103 amplifier and built onto FR4 using the 
toner-transfer / ferric chloride etch method.  Seems to work okay, but 
doesn't have DC-load on the ports, just DC-block.  I have not taken it 
into work yet to put it on the VNA to see how it does.

I picked up a couple of Mini-Circuits ZC6PD-1900W 6-way splitters on the 
e-place very cheap.  And by "very cheap" I mean that even if I destroy 
all the electronics inside, the enclosures and SMA connectors are worth 
what I paid for them.

I've been toying with the idea of modifying one of the units to add 
internal DC-blocking and resistors to ground for DC-load.  For DC block, 
seems like I could very carefully slice the trace on the board by the 
SMA connector, bridging the gap with an SMA capacitor.  Problem is, when 
I add the capacitor, I'm changing the path length.  I believe I could 
minimize this by using a very small slice and a very small SMA 
capacitor...but I don't know how much this will impact performance. 
Perhaps I need to do some modeling of the design and see what happens. 
Right now I don't have anything very small, with 1206 being the smallest 
I've got.

I also picked up a 40dB PCTEL GPS timing antenna on the e-place, with 
the idea being that even if I do use a passive splitter, I'm starting 
off with 40 dB gain at the antenna versus the typical 26 dB.  If the 
splitters have ~10 dB of loss (what I measured on the VNA at work) I may 
need to add some attenuation to knock it back down to 26 dB-ish.  I 
don't know what sort of dynamic range these GPS receivers have, so that 
might not been needed.

thanks much and 73,
ben, kd5byb