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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jan 4 05:43:22 EST 2015

On Sunday, January 04, 2015 03:39:48 AM Charles Steinmetz wrote:
> Dave wrote:
> >I was looking to make a 10 MHz distribution amp to feed test 
equipment with
> >the output of a GPSDO.
> >
> >      *   *   *
> >
> >16-way Minicircuits splitter on eBay which I got for $40. I guess the loss
> >is around 12 dB.
> >
> >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 
> >but is it just a lot less hassle to split a higher power amp.
> It all depends on what you need.  If the job is feeding the time base
> inputs of test equipment from a GPSDO, the splitter approach should
> work fine if you bear a few things in mind.
> First, the splitter achieves its rated isolation only if it is
> matched well at the input and *all* outputs.  That at least means
> using dummy loads on unused outputs, but consider that the REF inputs
> of most instruments are not 50 ohm loads (usually ~ 1k ohm).  So, you
> would need to provide proper termination for the in-use outputs,
> too.  Ideally, it would be in the form of a 52.6 ohm pass-through
> load at the instrument, so the coax is terminated in 50 ohms at the
> load (this assumes the instrument has a 1k ohm input -- if not, the
> pass-through terminator would need to be recalculated -- but note
> that only 50 and 75 ohm pass-through terminators are likely to be
> available as commercial items).
> Second, the amplifier should put out about 25dBm from a 50 ohm
> source, so each load will receive the standard 1Vrms = 13dBm (10dBm
> would probably work OK for most instruments, but they usually are fed
> 13dBm).  If the amplifier is not naturally matched to 50 ohms, there
> will be additional loss in the matching network.  Amplifiers that put
> out 25-35dBm with harmonics below, say, -50dBc are not trivial to
> design or build.  (Not terribly difficult, but not trivial.)
> I used the splitter topology for the multicoupler that multiplexes
> antennas to eight receivers.  In that case, an amplifier with
> extremely low noise and high dynamic range, with a natural 50 ohm
> output -- the sort of amplifier people use these days for post-mixer
> amps in HDR receivers -- works very well.  But the radios are all 50
> ohm loads to begin with, so I didn't need to muck about with odd
> value pass-through terminators.
> For distributing 10MHz to test equipment, I find it much easier to
> get good results with a distributed amplifier approach.  The circuit
> I posted back on Nov. 26 is about as simple as it gets, but can be
> made even simpler.  As drawn, each stage uses a 1:1:1 transformer,
> with the output taken from one of the windings.  If the output
> winding is deleted, the transformer becomes a simple 1:1 and the
> output is taken through a 10nF blocking capacitor straight from the
> collector.
A 10nF cap connected to the emitter winding avoids capacitively coupling 
collector power supply noise to the output (assuming that the collector 
supply isnt ground.).

> Best regards,
> Charles
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