[time-nuts] NIST isolation amplifiers

Bob Camp kb8tq at n1k.org
Tue Nov 25 19:54:02 EST 2014


Harmonics are (in general) the least of your issues on a distribution amp. There is very little difference in ADEV or instrument performance at -20 dbc versus -120 dbc.  Since filtering is relatively easy, adding another inductor or two is about all it takes. 


If you are going with the NIST approach rather than gates, remember that there are a few issues:

1) These circuits tend to “sing like a bird” at UHF if built from leaded parts. Often it’s tough to spot due to the output filter. 

2) Past a handful of outputs, the input impedance of the circuit will become an issue. You will need a more complex approach. 

3) The isolation you achieve is far more dependent on the layout than on the circuit. You need a *very* good layout to achieve the numbers commonly tossed around for the circuit. That’s much easier to do with SMT parts.

4) Any (hopefully) low noise circuit needs a quiet supply. This one is no different. That’s not just the regulator, the rest of the feed (ground loops etc) matters as well. 

5) There is a tradeoff between filter bandwidth and temperature induced ADEV issues. Going crazy on filtering will likely degrade your ADEV. 

6) The amp(s) as shown are not matched either at the input or the output. That may or may not be an issue to you. If it is, you will need to do some mods to the circuit. I’d suggest at least a 3 to 6 db pad on the input and output. 


> On Nov 25, 2014, at 6:45 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:
> On 25 Nov 2014 23:10, "Bob Camp" <kb8tq at n1k.org> wrote:
>> Hi
>> For a modern build, the PZT3904’s and PZT2222’s are a pretty good way to
> go with this amp.
>> For normal distribution to instruments, there’s really no need to do
> anything this complex.
>> Bob
> I am also thinking about the construction of a  distribution amplifier with
> 15 or so outputs.  One thing that came to my mind, is that there may be
> some point in  having one or two outputs where more money is spent. Then if
> one thinks an item might be particularly sensitive to some aspect of the
> reference,  one can use that.
> One could for example have one or two outputs which have harmonics
> suppressed 100 dB, without going to unnecessary expensive on all outputs.
> Dave
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