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

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 4 January 2015 at 02:37, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
>
>> 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?

> 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 bought two of these splitters. One covers 1-30 MHz, the other one
5-200 I think. Anyway, the 1-30 MHz one arrived today, the
Minicircuits ZFSC-16-3.

I had my VNA on, which unfortunately does not drop down to 10 MHz, or
even 30 MHz, as its lower limit is 50 MHz. So I admit these results
are a bit shaky, as I'm using the splitter outside its range. But that
said, There seems to a  "reasonable" amount of isolation at 50 MHz,
even when all other ports are open.

At 50 MHz, the loss from the common port is 12.8 dB, and the isolation
between two ports sets of ports is either 38 or 48 dB, depending on
what ports are chosen. BUT it appears to be improving as one goes
lower in frequency. The Minicircuits data sheet makes it clear there
are two sets of isolation figures. I'm pretty sure internally this is
likely to be a pair of 8-way splitters.

When the other splitter is here, I will use another VNA that covers
300 kHz to 9 GHz, and so make some measurements at 10 MHz on both of
them. But I just happened to have this VNA calibrated in N, and so
done a few quick and dirty measurements.

I've made no allowances for losses of cables.

So although I admit these measurements are not well done, but I'm not
so convinced now that the terminations on the unused ports matters a
huge amount.

I looks to me that this unit might well exceed 40 dB isolation on all
ports at 10 MHz, as all the graphs are sloping in the right direction.

Dave
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