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

Bob Camp kb8tq at n1k.org
Sat Jan 3 21:37:36 EST 2015


> On 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?

no, 13 to 15 is better.

> 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

the reason for that hassle is to better isolate the outputs. The splitter has good isolation only when all the ports are properly matched. In the case of a “40 db isolation splitter” that can mean the terminations all have 40 db return loss.

> 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

Roughly 99.9% of all those little amps degrade the phase noise of the input signal significantly when driven near their “rated” output power.

If you are after 13 dbm after 12 db loss you need 25 dbm clean signal. If you need to run the amp 10 db below it’s rated output to keep it clean, that means an amp rated at 35 dbm output. Most of those amps are 10% efficient, so you will be putting 20W into the device. 

The next layer is the return loss on the output of the amp. If you are after the full isolation numbers it needs to be matched as well as the terminations. If you need a pad to achieve that match, it comes out of your power budget. 

> * PSU
> * Power amplifier
> * 16 way splitter.
> That seems a *lot* simpler than many designs I see.

Since your test gear typically does *not* provide a 50 ohm match (they are high Z inputs), making this sort of setup work is fairly complex. You need to terminate at each device and terminate each unused output on the splitter. Even then 

> 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.

For under $20 in active parts you can build a distribution box that will do a far better job than a big amp driving a multi port splitter.


> Dave
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