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Abstract
The temperature shift of a narrow band crystal IF filter follows in general the "Beckman" curve for the par- ticular cut of the quartz crystal. On the basis of this, if a quartz crystal oscillator, using the same cut of the crystal is incorporated in the feedback loop by the means of a mixer, the VCO will shift its frequency in such a way as to compensate for the shift of the center frequency of the filter.
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MOTOROLA Technical Developments Volume a October 1988
CRYSTAL IF FILTER COMPENSATION CIRCUIT
by Ronald H. Chapman
The temperature shift of a narrow band crystal IF filter follows in general the "Beckman" curve for the par- ticular cut of the quartz crystal. On the basis of this, if a quartz crystal oscillator, using the same cut of the crystal is incorporated in the feedback loop by the means of a mixer, the VCO will shift its frequency in such a way as to compensate for the shift of the center frequency of the filter.
The shape of the response of a crystal filter doesn't change much because all of the crystals move in the same direction with temperature. Therefore it is mainly the center frequency shift which needs to be compen- sated for.
The second injection oscillator is a VCO that is a part of a phase locked loop. The output of the VCO is mixed with an un-temperature compensated crystal oscillator whose frequency is close but not the same as the desired IF frequency. In the block diagram shown the frequency is offset by 100 kHz. The difference frequen- cy is selected by the low pas filter and applied to the normal feedback dividers.
Since the feedback frequency from the dividers is forced to be equal to the reference frequency, as the offset oscillator frequency changes with temperature, the VCO must also change by the same number of cycles to keep the input to the dividers constant, The value of the feedback divider is chosen to correspond to the differ...
