[time-nuts] Any reason not to use one power amplifier and splitter for distribution amplifier?

[Bob Camp]

Hi

> On Jan 3, 2015, at 10:37 PM, paul swed <paulswedb at gmail.com> wrote:
> 
> Dave
> I am sure there will be many answers. But yes indeed it will work fine. All
> of the outputs should have the same delay also and that may be useful.
> 
> There really are the two approaches. The big power amp thats a 1/2 Watt you
> are talking and the many small amps as in the distributed mode.
> 
> Whats interesting is the telcos always do the big amp splitter and the test
> equipment manufactures use precision distributed distribution amps. I guess
> its a pick your poison. Or maybe the test equipment manufacturers needed
> more isolation port to port. Or heavens maybe they could just sell them for
> more money. Would they do that?

The telco types were used to the need to terminate all the lines correctly. It’s been part of their DNA for > 100 years. They also are used to designing cable plants rather than just tossing wires around. They also have some  needs like > 20 year MTBF, so orderly is a good idea. 

If I see a properly terminated single drop standard line in a real working lab, it will be the first time I see one. BNC Tee’s seem to be a popular item when hooking up instruments in a typical lab. (Note - in this case the definition of “properly terminated”  is > 40 db return loss, so it’s as good as the splitter).

More or less - the instrument guys opted for a “tap off multiple instruments” approach. The telco guys opted for a “individual run to each instrument” approach. Did every single outfit all over the world read the exact same book? Certainly not, the real world is never that orderly. Telco’s are not 100% fixed, labs are not 100% random wired.  

So what happens:

Telco has a problem, it’s a fixed plant. They track it down and do what’s needed. The problem (hopefully) does not come back. 

Lab has a problem, It’s Bob over on the third bench from the end. They try to track it, Bob moves on in his troubleshooting. They never find it. Much oratory, very little labor. How often does that happen? Roughly three of four times a week where I work… We find it and fix it about half the time. The rest of the time it “just goes away” or people get tired of looking. Most common issue - can’t run a clean phase noise / ADEV / vibe plot with that silly gizmo running on (name deleted to protect Bob)’s bench. Less common issues - can’t get a clean counter reading on standard line #XXX. 

So yes, a real lab does go for more isolation and better protection that a telco. Undiagnosed / random problems from shorts and opens are very real in the lab, not so much in the telco. 10 MHz threat signals and 10 MHz standards are common in the lab, less so in the telco. Different systems, different needs, different solutions. 

At least in my basement, I’m a lot closer to a lab than a telco. I move stuff around, I turn gear on and off. I set things up and tear them down. I do indeed work on a lot of stuff that runs at 10 MHz. I have multiple 10 MHz sources running all the time. Troubleshooting every cable connection for issues is not much fun. Isolation does indeed matter in a situation like that.  Even with all my junk, I don’t have a *need* for a 16 output system. I doubt I ever have more than a dozen taps running on my Spectracom at any one time. 16 boxes is a *lot* of stuff. 

Are there some exotic situations that come up - sure. What’s exotic to me might be normal to you. My typical might be your exotic. Do close in spurs matter to you? They do to me. Does a “burp” (phase shift) in all the standards when a cable is unplugged matter to you? It does to me. Do you run stuff 24/7 365 days a year? I do sometimes (yes yell at me for “sometimes and 365”). Do you need -175 dbc/Hz  phase noise on your standard lines? I don’t.  Do you need a range of standard frequencies (100K, 1M, 5M, 10M)? I do. Do you have some stuff (= almost everything) that’s fine at 1x10^-11 at 1 second wired one way and other stuff wired another? I do. How much do you listen to WWV at 5 or 10 MHz? Me - not so much.  Again, different needs, will drive different solutions. 

Bob 


> Humor aside each has a very good reason for doing the distribution and its
> driven by the requirements.
> 
> I have several of those spitters and picked them up for $ 0 at hamfests.
> Seems no one had a use for them when all of the 900 Mhz gear came out of
> the sites. Mostly gone at this point. A 1/2 watt 10 Mhz amp is not that
> hard to build look at the many Ham sites we have a band close to 10 Mhz/ 30
> Meter.
> Regards
> Paul
> WB8TSL
> 
> On Sat, Jan 3, 2015 at 6:09 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
> drkirkby at kirkbymicrowave.co.uk> wrote:
> 
>> I was looking to make a 10 MHz distribution amp to feed test equipment with
>> the output of a GPSDO.
>> 
>> I see this
>> 
>> http://m.ebay.com/itm/201244302355
>> 
>> 16-way Minicircuits splitter on eBay which I got for $40. I guess the loss
>> is around 12 dB.
>> 
>> I actually bought another for $35 which was similar but  one of them, the
>> isolation data made no sense,  so given their low cost I just bought both.
>> 
>> I suspect internally these 16 way units might have a pair of 8 way dividers
>> as there are two isolation figures,  depending on what ports one is
>> measuring between
>> 
>> Is there any reason not to just drive that with 22 dBm or so of power to
>> get 10 dBm at each of 16 ports?
>> 
>> Is 10 dBm an optimal value?
>> 
>> I see several distribution amp designs witb one amplifier on each output,
>> but is it just a lot less hassle to split a higher power amp.
>> 
>> I have a range of Minicircuits amps in my junk box in little enclosures,
>> which means a distribution amp can be built from just 3 main components
>> 
>> * PSU
>> * Power amplifier
>> * 16 way splitter.
>> 
>> That seems a *lot* simpler than many designs I see.
>> 
>> I was looking to feed it with an HP 58503A or similar device.
>> 
>> I do have an amplifier in my junk box which will produce 27 dBm. If I
>> combined that with 16 x 5 dB attenuators I could improve the isolation by
>> 10 dB, but I am unlikely to find the attenuators cheaply, and buying new
>> would add at least $200-$300 to the price, for what I suspect is no
>> significant benefit.
>> 
>> Dave
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