ZXF103Q16 Zetex Semiconductors, ZXF103Q16 Datasheet - Page 6

• Manufacturer Part Number: ZXF103Q16
• Description: VARIABLE Q FILTER
• Manufacturer: Zetex Semiconductors

Q
Q
Designing for a value of Q
As mentioned on the configuration pages, there is a
proportional relationship between the ratio of R4 and R3,
and Q.
These resistors define the gain of an inverting amplifier
that determines the peak value of gain and therefore the
Q of the filter, as Q is described as;
This value of required gain is quite critical. As the
maximum value of Q is approached, too much gain will
cause the filter to oscillate at the centre frequency Fo. A
small reduction of gain will cause the value of Q to fall
significantly. Therefore, for high values of Q factor or
tight tolerances of lower values of Q, the resistor ratio
must be trimmed.
ZXF103
3dBBandwidth
R4
R3
Fo
6
Frequency dependant effects must be accounted for in
determining the appropriate gain. As the frequency
increases, the effective circuit gain reduces. The required
gain is nominally two but at higher frequencies it will
need to be slightly greater than two in order to
compensate for loss of gain and internal phase shifts.
This is not really a problem for circuits where the desired
Fo remains constant, as the phase shifts are accounted
for permanently. For designs where Q is high and Fo is
to be ‘swept’, care must be taken that a gain appropriate
at the highest frequency does not cause oscillation at the
lowest.
Variation in Q increases from device to device, as the
value of Q increases, due to internal gain spreads.
ISSUE 1 - APRIL 2002