On Fri, 9 Apr 1999 18:38:25 -0600 adastra <foster@ADASTRAN.COM> writes:
>I am constructing a PIC based system to display speed in MPH as a
>function
>of the Doppler frequency from an X band radar antenna.

>The conversion factor is 31.398 Hz per MPH. I plan to measure the
>period
>over a few cycles and, either by implementing an inverse math
>function, or
>some clever lookup scheme, derive the BCD data for the display.
>Resolution
>needed is 1 MPH.

This seems too obvious, but if you count how many cycles occur in 31.8492
ms (1/31.398), the count will be directly in mph.  Probably you want to
count for a multiple of this period, then divide the result by the number
of periods counted.  This will reduce the +- 1 mph uncertainty that would
otherwise be present.

If you synchronize the start of counting to a cycle of the output, then
the count will jitter less.  At these low frequencies it may also be
practical to use a combined frequency/period measurement.  But for now
just to get something on the display, count for 31.84 ms and display.

To display in BCD, for small numbers (<255), it's practical to use
repeated subtraction of 10, 100, etc.  Or count in BCD in the first
place, though that makes averaging, division, etc. more complicated.

>
>I would appreciate any insights, shortcuts or alternatives to this
>plan, but
>even more so, some ideas on dealing with the following complication:


>   (The background signal is also of a
>somewhat smaller amplitude than the signal of interest.)

That's probably the key to it, if the signal of interest is larger than
the background it will dominate the counting.  It will be very hard to
extract anything useful if the interference is comparable in level and
frequency to the desired signal.

I think that many implementations use an analog PLL to lock on to the
signal of interest, then count with a digital counter.  If the output of
the PLL is changing rapidly, it is likely to be noise and should be
ignored.  (About the simplest way would be to look at the counts and only
display after 2 or 3 close to the same count have occurred)  Once locked,
the PLL could also serve as a tracking filter, though that may not be
practical since the signal is of such short duration.

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