[time-nuts] Any reason not to use one power amplifier and splitter for distribution amplifier?
csteinmetz at yandex.com
Sun Jan 4 03:39:48 EST 2015
>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.
More information about the time-nuts