The HP / Symmetricom 58536A has a gain spec of 0 +-3 db so something is
clearly wrong. Did you power it through a spare port? Also, the
isolation spec is 26 db @ L1. The 50 db spec is for L1 +- 40 MHz.
I looked inside mine and found that there's an input filter(-1db), 21 db
amp, 4 way resistive splitter(-12db), output filter(-4db), and 4 db
pad. That matches your measurement of 28 db isolation - no power is
required for that.
Ed
On 10/11/2012 3:20 PM, Magnus Danielson wrote:
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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