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Aligning Resonant Detector Circuits

IP.com Disclosure Number: IPCOM000075759D
Original Publication Date: 1971-Nov-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 3 page(s) / 65K

Publishing Venue

IBM

Related People

Weythman, GA: AUTHOR

Abstract

Some detector circuits have two adjustments performed during an alignment procedure, one of which controls the center frequency of the detector thus controlling the center of the accept-band. The other adjustment controls the width of the detector accept-band. Such a detector may have a logical 0-level output if the detector is accepting a particular frequency and a logical 1-level output if a frequency is being rejected by the detector.

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Aligning Resonant Detector Circuits

Some detector circuits have two adjustments performed during an alignment procedure, one of which controls the center frequency of the detector thus controlling the center of the accept-band. The other adjustment controls the width of the detector accept-band. Such a detector may have a logical 0-level output if the detector is accepting a particular frequency and a logical 1-level output if a frequency is being rejected by the detector.

Heretofore, it has been difficult to determine the exact self-resonant frequency of low-Q circuits which determine the center frequency. As an example, using an analog meter to determine the center frequency alignment is difficult, because of the relatively flat resonant response curve of a low-Q circuit. An oscilloscope and a signal generator have been used to determine the exact band of frequencies that the detector was aligned to accept. The signal generator is tuned to one of the points where the detector should just start detecting and then the band width is adjusted until this accept-reject region is sensed on the oscilloscope screen. The signal generator is then retuned to the other side of the band and the detector accept-reject point is checked. If the alignment of the detector has a skew, then the operator must alternately adjust the center frequency and the band width until the detector is correctly aligned. This process is time consuming and the accuracy is somewhat determined by ability of the operator to interpret the oscilloscope and frequency meter.

Waveform A, Fig. 1, defines a voltage vs. frequency of an LC resonant circuit. The center frequency of the detector accept-band is controlled by adjusting the resonant frequency of the circuit. Line B illustrates the action of the variable- detector threshold level. This adjustment controls the width of the accept-band of frequencies. The output from the detector is shown at C. Four dashed lines, marked F1, F2, F3, and F4, are drawn through the curve and detector output. Frequency F1 is the "low-reject" frequency. Frequency F2 is higher in frequency than F1 by the alignment tolerance of the detector, and is referred to as the "low- accept frequency". Frequencies F3 and F4 are selected to fall on either side of the high frequency accept-reject point of the detector. The difference between F1 and F2 and also the difference between F3 and F4 determine the accuracy of the final alignment.

The tester of Fig. 2 includes frequency generators F1-F4 (or a single switchable signal generator), and a switching section to select the correct frequency to be applied to the detector input. The logic level output of the...