[time-nuts] high rev isolation amps

jimlux jimlux at earthlink.net
Sun Mar 27 10:00:25 EDT 2016


On 3/27/16 4:40 AM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
> On 27 Mar 2016 03:00, "jimlux" <jimlux at earthlink.net> wrote:
>>
>> On 3/26/16 3:25 PM, Bruce Griffiths wrote:
>>>
>>> On Saturday, March 26, 2016 09:30:30 PM Rob Sherwood. wrote:
>>>>
>>>> You cannot add the directivity and gain.  Doesn't work that way.  Rob,
> NC0B.
>>>>
>>>> Sent from my iPad
>>>
>>> Minicircuits would disagree with that and its their amplifier.
>>>
>>> Bruce
>>>>
>>>>
>>
>> Well, it's a bit inconsistent between gain, directivity and S parameters.
>>
>> at 5.4 MHz the ZFL-500LN has the following S parameters @ 15 V in the
> .s2p file (there's two units' measurements in the files)
>>
>> S11 -35.5, -32.1
>> S21  29.7,  29.5
>> S12 -47.5,- 47.2
>> S22 -39.5, -28.3
>>
>> On the other hand, the data sheet "typical data" says: at 5.4MHz 27.9 dB
> gain, 26.5 dB directivity, which Minicircuits defines as isolation-gain
> (implying isolation is 54.4)
>> http://www.minicircuits.com/pdfs/ZFL-500LN.pdf
>
> I think you are wrong there. From the Touchstone .s2p data you give above:
>
> Isolation (dB) = - |S12|=+47.5 dB.
>
> Gain (dB) = |S21|=+29.7 dB.

Yes, that's what I would think: roughly 29 dB gain, roughly 47 dB 
reverse isolation... BUT, that's nothing like what the data sheet says.



>
> Directivity, a term I can't say that I have seen much applied to amplifiers
> other than by Minicircuits,  is according to the Minicircuits definition,
> the difference between isolation and gain. But I don't see a note of
> whether its gain - isolation or isolation-gain.

I just assumed it's whichever has the positive sign, since the published 
numbers are positive: so isolation-gain..


   Assuming the latter, the
> directivity would be
>
> 47.5-28.7=18.8 dB.

>
> That doesn't agree with your result,  nor does it agree with the
> Minicircuits typical value of 26.5 dB.
I was using the data sheet numbers for gain/directivity and working 
backwards to a reverse isolation.



>
> But the Minicircuits typical gain in the data sheet is 27.94 dB.
>
> I learned from a Minicrcuits sales rep that the "typical" data in their
> data sheets is the result of one device taken randomly from the production
> line. It is not any sort of average. I find that very odd, and so did the
> sales rep, but I was told that was true.

I'm not surprised at all.  And it's obvious that the data sheet number 
doesn't happen to match either of the two devices they provide as 
S-parameter data.


>
> I think it is time to confirm these numbers with Minicircuits,  but I would
> not rely on "typical" data if you want a guaranteed isolation.
>
>> Maybe there's some sort of assumption about source/load impedance (e.g.
> not necessarily perfect 50 ohms)
>
> I doubt that is so. S-parameters by definition assume that all other ports
> are terminated in devices which reflect no power at the system impedance.
> In the case of most devices that's 50 Ohm loads, but in the case of mixed
> waveguide/coaxial devices waveguide loads would be used.

Yes about the S parameters.. I was wondering if the "gain" and 
"isolation" numbers made some other assumptions..


>
> Minicircuits do a range of high isolation amplifiers.  I assume that they
> guarantee figures for those although I have not checked.


The ZFL-500LN is one of the high isolation amps (I searched for 
isolation>40dB to get the list.



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