I've mentioned before that crystal filters are not perfectly linear
passive devices and can in fact generate intermodulation products. The
reason is that a crystal's motional parameters are not constants, but
rather are a function of the applied drive, something called "drive
level dependency" in the trade.
I've just completed a quick IMD measurements for a 4-pole 21.4 MHz
monolithic crystal filter (ECS 21K7.5) and posted data at
http://www.cliftonlaboratories.com/Updates.htm under the 03 July
updates. This is a long update, in several stages, so read it all,
should you find the first part interesting. The ECS 21K7.5 is a $10 part
in single lots, so it's certainly not an expensive filter.
Monolithic crystal filters are known for IMD, and the test was run with
a rather high drive level, around +9 dBm. However, the data is quite
instructive and should convince anyone who doubted that crystal filters
could themselves be a source of IMD. (These measurements are quick and
dirty and I will refine them as I move towards a final filter design. )
Bringing the discussion back to the K3, it shows the importance of
getting the roofing filter correct, as intermodulation generated in it
will come in "down the pipe" in the DSP stage.
And, it's not just crystals that are non-linear passive devices.
Capacitors and inductors also have the same problem. In the case of a
capacitor, some dielectrics have a dielectric constant that is a
function of applied voltage. Ferrite and powdered iron (but to lesser
degree than ferrites) are also non-linear. Hence any component in the RF
path must be suspect and carefully evaluated for IMD performance. (I've
written about non-linear capacitance. See
http://www.cliftonlaboratories.com/capacitor_voltage_change.htm, but
have yet to make serious measurements on non-linear inductors.)
Jack K8ZOA
www.cliftonlaboratories.com
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