[time-nuts] WTB: GPSDO

Thomas Petig thomas at petig.eu
Wed Mar 22 03:40:42 EDT 2017

Hi Tim,

On Tue, Mar 21, 2017 at 06:36:51PM -0700, Tim Lister wrote:
> [...]
> I have a Symmetricom 58532A GPS antenna which has a N female connector
> but my 3 current GPS receivers all have SMA female connectors. If I
> want to provide capacity for at least 4 receivers fed from the same
> antenna, I was wondering what the best option for a splitter and where
> to do the N-to-SMA conversion. I currently have a Mini-Circuits
> ZAP3PD-2 power splitter which does SMA input to 3 SMA outputs. This
> seems to work but the connected devices all complain of an Antenna
> short, which doesn't seem good. The other popular option seems to be
> the Symmetricom 58536A 1x4 splitter which would then require 3-4
> N-to-SMA cables - it looks like although this has gain, it seems to be
> more of "eliminating loss" than straight gain so would presumably not
> overpower the receivers' frontends. Or maybe there is another more
> suitable SMA splitter in the Mini Circuits confusingly extensive
> catalog ?
Your splitter looks ok, but it has DC pass through on all ports. I would
recommend to put DC-blocks on all but one. The problem is all your
receiver want to power up the antenna and deliver around 3 to 5 V supply
Voltage on the port, your splitter is short-circuit them, which is not
good, especially if the voltage does not match. (The minicircuits
datasheet for the ZA3PD-2+, I guess that is the one you meant,  says
RF+DC on all ports) The short-circuit might damage your receiver.

If you take a splitter without DC pass through, you will need an
additional bias-T to feed some supply voltage again.

The special GNSS splitters forward DC only from one port and provide
some 200 Ohm DC termination together with the DC-block on the other
ports to keep the receiver happy, i.e., it seems current consumption and
therefore believes the antenna is ok. Some have an amplifier to
compensate for the loss of the splitter, but it is not necessary if the
antenna delivers enough.

So the GNSS splitter is electrically, a splitter like you have, a
separate LNA in front and all but one port with a bias-T (in reverse,
this provides the DC-block and ensures the signal is not 200 Ohm
terminated) where the DC connector has 200 Ohm termination.

I personally use a NARDA 4372A-4 (10 Euros on Ebay) with three DC-blocks
from minicircuits to split the GNSS signals and a good outdoor antenna.

   Thomas, DK6KD/SA6CID
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