Depending on the bandwidth of the filter, typically, the natural impedance
of the filter will be anywhere in the range from 1.5k to 7k. Using ECS
10.7MHz crystal filters as an example, the 50kHz filter has an impedance of
around 6.5k, while the 25kHz filter has an impedance of around 3k.
Most crystal filter don't have a low impedance, but there are exceptions.
The 330 ohm impedance that you describe is typical of the Murata 10.7MHz
ceramic filters. An example would be the SFE10.7V, with a 13kHz bandwidth.
The skirt selectivity on these types of filters is very poor, so about the
only use that they really have is as a roofing filter at the 1st I.F. of a
dual-conversion receiver.

Pete

"MAc" wrote in message
...
I only found (app note), that filter should be 330ohm