[time-nuts] GPS splitter measurements

[Magnus Danielson]

Dear fellow time-nuts,

I have fooled around with the network analyzer on a set of GPS splitters 
in order to illustrate the differences.

The goal has been to measure:

1) Gain (input to port 1 and input to port 2)
2) Group delay (input to port 1)
3) Port isolation (port 1 to port 2)

I have measured these devices:

A) GPS networking passive splitter

This is a passive broadband splitter.

B) GPS source S14WI

This is a ruggidized "smart" 4-way splitter.

C) Symmetricom 58536A

A "smart" 4-way splitter.

I used a 5 V supply on a spare port for the B and C splitters.

Please see the attached files.
For each I used a 1.1-1.7 GHz wide-band sweep, and I did a 1.57542 GHz 
102,3 MHz wide sweep. The later sweep range was chosen as 10,23 MHz is 
the basic unit for GPS modulation, so you can see how many multiples the 
skirts cover. Narrow-band C/A only is only 2.046 MHz wide, where as 
Narrow-band C/A+P(Y) is usually around 20.46 MHz wide. Further, for the 
narrow sweeps I focused on the group delay read-outs. This is not an 
ultra-clean, all calibrated, full compensated measurement.

The GPS Networking splitter has a fairly flat gain with about 4.5 dB 
loss. The isolation is about 25 dB in L1 and 20 dB in L2. Group delay is 
very flat at about 1.1 ns. This is expected. I didn't power this one, as 
I knew it was passive.

The GPS Source S14WI splitter I expected to have gain, but found the 
loss to be about 20 dB in L1 and about 26 dB in L2. The isolation is 
about 48 dB in L1 and about 58 dB in L2. Group delay is a little rocky, 
but is around 3 ns.

The Symmetricom 58536A splitter I expected to have unity gain, but found 
loss to be about 14 dB in L1, and L2 is suppressed to 54 dB loss, as 
expected. It is clear that this is a filtered product, and this is also 
stated in the datasheet. The isolation for L1 is around 28 dB, which is 
high from the expected 50 dB. The group delay clearly shows to peaks at 
26,129 ns and 21,889 ns, which translates to Q values of about 124 and 
104 respectively (withdrawing 1 ns for "electrical delays").

This is the results of a quick-and-dirty measurements, leaving many 
details done sloppy, like unused ports left open instead of terminated, 
not following up on the gain structure misses, not doing full 
calibration etc. It's to show what some modern available splitters 
perform like. I really don't have the time to play these games this 
week, but I wanted to get started at least.

I hope you enjoy the results never the less.

Getting 50 dB isolation is possible. It seems open ports remains an 
isolation issue thought.

The Symmetricom 58536A provides filtering, leaving a quite noticeable 
group delay. It does have the benefit of providing improved suppression 
of side-band signals.

So, you milage will vary. I hope this little lab-report encourage you to 
look more at your splitters, measure them and get your hands dirty.

Cheers,
Magnus
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