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
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