[time-nuts] overtone crystal question

[Glen English VK1XX]

Hi Bob

thanks for the insight.

OK so the Rm for an overtone crystal , measured at the fundamental might 
be a bad indicator of the overtone Rm.  I have found empirically, a 
loose relationship of Rm proportional to overtone number   from the 
fundamental. But loose- I mean +/- 50% which, as you point out, may be 
optimized for the overtone, which is why my numbers are so far out

cheers.


On 19/09/2019 10:48 AM, Bob kb8tq wrote:
> Hi
>
> Bottom line - if you are designing a filter, you need the real values for
> the Cm, Lm and C0. Guessing at them is likely to lead to trouble if it is
> a reasonably complex filter.
>
> Rm generally goes as the overtone. It can deviate quite a bit from that
> (as can the other parameters) depending on how the blank is shaped and
> plated. ( If I want a good 3rd overtone, it will be designed to work well there.
> It may be pretty bad on the fundamental ….).
>
> Bob
>
>> On Sep 18, 2019, at 2:35 PM, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
>>
>> I wonder if anyone can shed any light on this question, since this forum is loaded with those who REALLY understand crystals.
>>
>> I am modeling crystal filters (VHF)  in SPICE. There are some specific acoustic mode models for SPICE in some Post Doctorial papers, very interesting, they would be the best but rather painful to use.
>>
>> However I using simplified Rm, Lm, Cm, Cs, Cp, Ccase etc
>>
>> My question is, how does Rm vary with overtone number ?
>>
>> My assumptions are Lm stays the same, Cm reduces proportionally to the square of the overtone number.  Those assumptions are close enough and canon.
>>
>> I of course need the Rm number to acurately model loss.
>>
>> 73
>>
>> glen english
>>
>> VK1XX
>>
>>
>>
>>