ZXF103Q16 データシートの表示（PDF） - Zetex => Diodes

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ZXF103Q16

VARIABLE Q FILTER

Zetex => Diodes 

ZXF103

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.

R4

Q ∝ R3

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;

Fo

Q =−3dBBandwidth

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.

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 2 - JULY 2002

6

6

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