Hans said:
>I do not know whats inside the filter but if you are lucky
>it is a ladder filter with no ferrite transformers that degrade
>the filters big signal performance. Sherwood filters are
>of course OK but is this firm still in business?
I did a lot of work on IMD in crystal filters back in the mid 1970's. The
ferrite transformer business I found interesting, since I was never able to find
a filter where the ferrite transformer was the problem - and this was working
with a number of filter manufacturers. I read about it in Ham Radio magazine,
but that was all. We would measure a filter, and then remove the transformer and
provide resistive matching: invariably the answers were the same once we had the
same voltage at the first crystal. There are a number of causes for the IMD. The
most important is that the level of signal overstresses the crystal material, so
that Hooke's law no longer applies (stress/strain is constant) Secondary effects
come from the final finishing of the crystal, and surface configuration - one
theory for that is there is non-homogenous surface layer which has different
characteristics to the main body of the quartz. Thomson-CSF in Paris were the
people who produced the best filters in those days: we discounted all those
manufacturers who said that IMD didn't happen in filters - and there were a few
of those!
One interesting thing was the allowable in band signal level. I saw a demo where
a filter had been made with glass encapsulated crystals in vacuum, and at about
+20dBm input in the passband, in a dark room, you could see a blue glow around
some of the crystals! Run at that level for very long, there was fairly major
metal dispersion - I guess a sort of plasma etch. The voltage was so high
ebcause of the very high Q - I suspect the filter was designed to ahve that high
working Q for the demo -it was done at a research place.
Sherwood are still in business - they have a web site.
Tom, what signal generator, wanted signal input level, wanted signal SNR and rx
bandwidth do you use to measure blocking 1 kHz away?
73
Peter G3RZP
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