section - Bitsavers

PHASE LOCKED LOOP APPLICATIONSFLUTTER METER USING 5618+18V,~-.JR·::::1 ....----,HEADTAPE MOTIONFigureS-SOPHASE LOCKED LOOP FLUID FLOWMETERWhenever a phase locked loop is locked, its VCO signal is90 0 out of phase with the input signal (unless the inputsignal is off frequency and very weak). It has been foundthat the loop can lock to itself if its VCO signal is phaseshifted 90 0 and then applied to the input. The loop willthen run at whatever frequency develops a 90 0 phase shiftin the phase shift network. An interesting application ofthis technique was submitted by Richard E. Halbach ofMilwaukee, Wisconsin. Mr. Halbach used the time delay ofa mass (bolus) of fluid moving through a length of tubingas a phase shift mechanism so that the PLL frequencybecame a function of the flow rate. Figure 8-81 showsthis technique in block diagram form (a) and implementedusing a 5608 (b)."Simply stated, Nuclear Magnetic Resonance (NMR) isthat precession of nuclei, in this case hydrogen, in thepresence of a magnetic field. Its precession can be detectedby an appropriate receiver coil assembly. A coil up-streamfrom the receiver coil when activated changes the nuclearmagnetization of a bolus (or mass) of fluid in such a waythat this change can be detected in the receiver as the bolussubsequently passes through it ... transport delay from thetag coil to the receiver, an inverse function of flow velocity,can be used to introduce a 90 0 phase lag into the externalfeed back loop of the PLL, which when matched with the90 0 internal lag of the PLL, allows the loop to lock. TheVCO frequency is then a direct function of velocity offlow."[Note: A 565 can be used as well as the 560 at this lowfrequency. Frequency can be read out at the low pass filter(using a low frequency integrating voltmeter) or bymeasuring the period of the output waveform (flow is theninversely related to the period). If the VCO is run at acarefully chosen higher frequency and then divided downprior to the phase detector and tag coil, the frequency ofthe VCO as read by a counter can be adjusted to indicatein flow units directly.]PHASE LOCKED LOOP FLUID FLOW METERPHASE LOCKED LOOP FLUID FLOW METER+18V200MFD1V (P-P) INPUT FROM...-----+:-It---o RECEIVER DETECTOR+18V16C1100MFDNON-POLARC2270MFDNON-POLARPLL 560BFLUID FLOWC1 WAS CHOSEN FOR STABLE LOOP OPERATIONR1C2 DETERMINES OPERATING FREQUENCY RANGE OF 1-3HzQ1 Q2 Q3 FORM DIFFERENTIAL OUTPUT AND DRIVE FOR TAG CIRCUITRYFigure S-S1aFigure S-Slb75