[time-nuts] IFCS 2020 tutorial - low noise electronics for time/frequency metrology

[Bob kb8tq]

Hi

The gotcha is that all X7R tells you is that the temperature dependence of the part
falls inside a certain set of limits. Back many decades ago there were very few formulas
out there to make this or that spec part. Pretty much everybody made a part type with
the same material. 

Since the material was the same, you could predict things like RF performance or microphonics
off of the temperature characteristic. The generalization that this or that sort of part would work
as a bypass at this or that frequency held across multiple manufacturers. 

As far back as the 80’s this started to change. We would find that this manufacturer produced
an X7R part that simply stopped being a capacitor in the HF region . Just how they
managed that … who knows. Needless to say, sister company or not, off the AVL with them. 

Bottom line is that some (most) X7R’s do quite well as RF bypasses up into at least the VHF range. 
Some are horrid for microphonic, others are quite good. Testing is your friend in this case. Since
the material (on a specific part) does not change very often, you can check parts out and then
have some confidence in your design. 

Bob



> On Jul 22, 2020, at 4:43 AM, Magnus Danielson <magnus at rubidium.se> wrote:
> 
> 
> On 2020-07-22 03:39, Richard (Rick) Karlquist wrote:
>> 
>> 
>> On 7/21/2020 4:27 PM, Magnus Danielson wrote:
>> 
>>> 
>>> In capacitors, two things make them microphonic, voltage over the
>>> capacitor and the high-dielectric constant as this makes it more
>>> sensitive to mechanical stress, and this also tends to make them very
>>> sentistive to thermal stress. So X7R is terrible for instance.
>>> 
>> 
>> The LT3042/LT3045 data sheets should be of interest in this regard.
>> ADI calls out a particular Murata capacitor that supposedly has low
>> microphonics.  However, I believe its tempco is X7R, so I can't
>> reconcile that with what you said.  It does lose less capacitance
>> at higher voltages than any other capacitor with a value of 10 uF
>> that I have seen, so from what you are saying, this should give
>> it lower microphonics.
> I was noting a general tendence, but there can be exceptions from the
> rule naturally.
>> 
>> Unfortunately, this technology appears to have been orphaned by
>> Murata, receiving the dreaded NRND notation.  It appears that
>> it will be available in distribution for the immediate future
>> at least.
> 
> Another problem, it can be hard to depend on too exotic components, as
> they can be end of life before your design is, if you are too dependent
> on them, large redesign or end of life remains the options.
> 
> Cheers,
> Magnus
> 
> 
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