[time-nuts] GPS splitter measurements

[Ed Palmer]

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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