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

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

Paul,

 

It isn't the wiring.

 

LSE produces two pulses per revolution, conditioned by a diode and resistor, that are supplied to the VM EPI 800.  The display is in increments of 10 RPM. 

 

The prop balancer optically detects the presence of an adequately sized piece of reflective tape on one blade (one pulse per revolution) and reports RPM in tenths. 

 

The balancer, while a little jumpy because of the display accuracy, and the VM match within the VM display accuracy of 10s of RPM.  This means the LSE "knows" the RPM, but cannot display it accurately thru LSE parts connected with the LSE harness.   The LSE display is a LSE supplied digital LCD Simpson "meter."  Perhaps the more likely explanation is that the meter needs calibration.  It should not be harmonics since the error is present in flight and on the ground thru varying RPMs and the error magnitude varies with the RPM, avionics on or not.