17. Terminating Impedance
This is the required impedance to be seen on input and load side of the
filter to maintain a good characteristic response.
Terminating impedance is typically specified as a series resistance with
a parallel capacitance that should also include the stray capacitance of the
circuit board.
18. Power Handling
Power handling is usually specified as the maximum input power. In design, it
is closely related to the factors determining in- band inter-modulation
performance. Given the bandwidth, insertion loss and spurious response
requirements, the power handling capability of a filter can be estimated.
19. Vibration-induced Sidebands
Vibration-induced sidebands may appear on a crystal filter output signal when
the filter is subjected to mechanical vibration.
Vibration produces acceleration forces on the crystal resonators, causing
their resonance frequencies to change slightly -- typically a few parts per
billion for one G acceleration. For
sinusoidal vibration, the resonance frequency is modulated at the frequency of
vibration, and the peak deviation is determined by the acceleration sensitivity
of the crystal resonator and the amplitude of vibration.
Viewed on a spectrum analyzer, the filter output will have sidebands
offset from the carrier by the frequency of vibration.
For most filters, the vibration-induced sidebands are quite small and of
no concern. However, narrowband spectrum cleanup filters may require special
attention. Vibration-induced
sidebands are minimized by minimizing resonator acceleration sensitivity and by
control of mechanical resonance within the filter structure.
20. Settling Time and Rise Time
Settling time is the time it takes for the output signal to settle within a
specified overshoot percentage after the input has been subjected to a step
response, pulse, impulse, or ramp.
Rise time is often defined as the time required for the output of a filter to
move from 10% to 90% of its steady state value on the initial rise.
While the exact value of rise time can readily be calculated or
determined from filter handbooks, the following rule of thumb relating rise time
to bandwidth provides an useful estimates.
Tr = 0.35 / fc
Where Tr is the rise time in seconds and
fc is the 3 dB cut off frequency in hertz.
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