[time-nuts] NIST isolation amplifiers

[Bruce Griffiths]

Another issue is that if even one output needs high reverse isolation and 
low crosstalk, then even those outputs that arent so critical will also need 
high reverse isolation and low crosstalk to avoid degrading the crosstalk 
to the critical output.

Bruce
 
On Tuesday, November 25, 2014 07:54:02 PM Bob Camp wrote:
> Hi
> 
> 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.
>

The small resistors in series with each CB stage emitter are useful in 
suppressing such parasitics as is a low inductance ground connection for 
each base.
> 2) Past a handful of outputs, the input impedance of the circuit will 
become
> an issue. You will need a more complex approach.
> 
A low noise input amp driving a splitter can be useful in resolving that 
issue.
> 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.
> 
Shielding each individual amp from the others (SMT or not) may be 
necessary. 
> 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.
> '
Input pads will increase the PN floor.
With slight modifications up to 6 such isolation amp inputs can be driven 
by a single 50 ohm source.  
> Bob
> 
> > 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
> > _______________________________________________
> > time-nuts mailing list -- time-nuts at 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 at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the
> instructions there.