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 output, > >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.). Bruce
> > Best regards, > > Charles > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the > instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.