BENCH TESTS FOR THE DIGITAL READOUT
Bench checks can be performed by connecting a jumper from U6 pin 11 to the audio input. This eliminates the need for a radio, or an antenna... That's how I did my bench tests.

Adjusting the CALIB trimpot through its range will allow the various display segments to be tested, and (obviously) the value of the displayed number should increase smoothly, as the trimpot is adjusted. ( counterclockwise = increase number ) The 360 "rollover" function should be tested, as well.... confirm that the display "switches" from 359 to 000, as the trimpot is advanced.


CALIBRATION 1-SHOT

The stability ( or lack of it ) for the CALIB 1-shot ( on the MAIN board ) became obvious when the digital readout was tested on the bench... the timing capacitor ( C16 ) that I used in my unit was a low - grade "Z5U - type" capacitor, and it exhibited enough temperature sensitivity to change the name of this instrument to "DOPPLER / THERMOMETER". If you are "upgrading a "basic" Doppler to include this digital readout, you will probably have to address the same issue... Replace C16 with a "respectable" capacitor, such as a mylar or poly capacitor... avoid ceramics.

The problem is aggravated if the CALIB 1-shot is adjusted for long duration pulses... I found that my unit behaved well if the CALIB duration was set near zero, ( CALIB trimpot = full CW ) but when I adjusted it to maximum duration, I could cause a change of 20 degrees in the digital readout by "warming up" capacitor C16 with nothing but my fingertip.... probably a temperature change of about 15 - 20 degrees Farenheit.


ZC DETECTOR INHIBIT

I also installed the following modifications, to eliminate a "nuisance" problem involving the CALIB 1-shot :

This mod is not really necessary, but I reccomend it... it allows the CALIB 1-shot to be adjusted across a range of greater that 360 degrees, by inhibiting the zero-crossing detector while a CALIB pulse is still in progress... The 555 chip doesn't behave well if it recieves a trigger pulse while it is still generating an output pulse, and this leads to "multiple" LED indications on the pelorus display, and "illegible" numbers on the digital display. ( multiple / different numbers, superimposed on each other )

The "action" of the CALIB 1-shot is MUCH smoother with this mod installed, and allows the FULL RANGE of the trimpot to be employed... previously, the CALIB range was limited ( by this problem ) to ( approx. ) 330 degrees.

I have incorporated this mod into the PC boards that I sell, but the artwork on this website has not ( yet ) been updated...


FILTER SENSITIVITY

No temperature tests were performed on the low pass filters.... they have the potential to "uncalibrate" the digital readout, just like the CALIB 1-shot. Most of the filter "sensitivity" should occur near the cutoff frequency, which ( in this design ) lies 50 percent above the signal frequency. This is the main reason to keep the clock frequency near 8 KHz ( at the input to the 74LS93 ) See the THEORY section called FILTERS for a more detailed explanation. I would expect SOME sensitivity, especially since I didn't pay ANY attention AT ALL to the choice of capacitors that I used in my unit.

The capacitors involved in the LPF ( if you choose to replace them ) are : C10, C11, C12, and C13.


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