Actually, LSE's rpm data is = analog from=20 the get-go. One way to turn one-three pulses per rev to an rpm = output is to=20 have a multiplier and counter. This can be done with a phase-lock-loop=20 multiplier set to the proper division rate, i.e. if you want a readout = every=20 second, a one-pulse-per rev would have to be multiplied by 60 before = being input=20 to a counter. Then you are faced with the choice of closed-loop = bandwidth=20 that will have a fast enough response time to keep up with motor = acceleration,=20 but slow enough to filter the crankshaft angular-rate variations. You'd = be=20 amazed at how much the crank accelerates and decelerates during one = revolution!=20 I know; I've built several. I guess you might call that a digital = approach.=20 I flew with a fellow that was trying a PLL tach I made on his = 140,=20 that had a one second readout rate. At first he thought there was a = problem with=20 it as it sometimes jumped aroud tens of rpm, until I pointed out it was = smooth=20 until we had some turbulence. If you see stable rpm readout in=20 your digital display in turbulence, your tach's smoothing constant = or=20 time-base rate is quite large. A u-processor-based system will = count=20 time between pulses or the number of pulses in a fixed-length gate, then = calculate the rpm. That's still a multiplier. Any time you have a = harmonic=20 relation between the pulse rate into a counter and the gate rate, you = can have=20 gate opening and closing errors where pulses are picked up or=20 dropped.

A lot of tachs start with a = triggered=20 pulse, at crank rate, to give a fixed width and amplitude pulse, then = filter=20 that through a simple R-C smoothing net for display on an analog meter = of some=20 sort, either DVM or D'Arsonval movement. LSE's rpm is 100 uV / rev / = min, so if=20 you do not have a good closed signal system from the LSE to the meter, = common=20 mode noise injected into the wiring can cause errors. The 40-70 rpm = error you=20 mention can be from 4mV to 7mV common mode error in the ground return; = that's=20 0.004V to 0.007V. LSE's MAP is 10mV / inchHg. Again, 6mV, 0.006V, on the = ground=20 can give 0.6" MAP error + or -! Proper wiring and grounding practices, = as was=20 much discussed on LML several weeks ago, is critical with low-level = signals=20 of any kind. Whether digital or analog, each approach has its own = type of=20 errors.