EMRFD Message Archive 6433

Message Date From Subject
6433 2011-06-30 06:29:18 peepholenz Crystals and Crystal Filters
Explanation desired:
When measuring high Q xtals the common advice is to wait at least milliseconds if not tens of milliseconds at each frequency point - otherwise the high Q won't let the measurements "settle".
Sounds eminently plausible and certainly you get rather distorted shapes if you look at xtal response with the likes of N2PK's wonderful VNA.
So if you now make up an SSB bandwidth (say 2800Hz) filter utilizing several such high Q xtals the highest pass frequency takes only about 360 microseconds for a full cycle- nowhere near the sort of 'wait' to measure a single xtal.The very sharp filter edges (maybe 60-90db in a few 10s of Hz) imply that the xtals high Q has not been 'degraded' by the filter topology.
So why/how does the filter "work" ? as it surely does.
6434 2011-06-30 11:20:14 Wes Re: Crystals and Crystal Filters
Hi Peter, and group,

Good questions all. Filter theory is a very interesting subject that has obvious impact on the equipment that we build.

The major feature that you may be missing is the difference between loaded and unloaded Q. The unloaded value is what is measured when characterizing a crystal for use in a filter. The circuit that we use for characterization is some sort of a filter that has a bandwidth that is as narrow as possible, dictated by the intrinsic losses of the crystal itself. This defines Qu, unloaded Q. A good crystal at 5 MHz might have Qu of 200,000. So the filter that is used for the evaluation of Qu will have a bandwidth of 25 Hz.

Let's now use the same crystal in a SSB bandwidth filter of say 2.5 kHz bandwidth. With the larger BW, the loaded Q is now 2000. This is the loaded Q and is reduced by a factor of 100 from the unloaded value. As such, we would expect to sweep it 100 times faster.

Remember that a so called "modern" filter, which includes virtually all of our crystal filters, is a circuit with many resonators. The resonators all share some of the energy applied to the filter. But each crystal is essentially loaded as if it were a single crystal loaded for the filter bandwidth. The term "modern" applies here to the so called modern filter theory that came along in the 1950s or thereabouts (I'm told) to replace image parameter filters.

Don't categorically condemn the N2PK VNA. You can control the settling or dwell time for each frequency that you set up for that instrument. This is the time it sits
6437 2011-06-30 19:10:43 peepholenz Re: Crystals and Crystal Filters
Thanks Wes
That makes sense. I certainly was not critisizing the N2PK VNA - it is the most useful box of tricks I have ever had.Particularly satisfying that I actually had to build it and in spite of that it works!
My only problem with it is that it tends to show up too many instances of my electronics ignorance.
6438 2011-07-01 10:30:08 Wes Re: Crystals and Crystal Filters
Hi Peter,

Yea, my VNA had the same effect here!

73, Wes