Instruction manual
DDF1 9
the digital filter. Even if the clock oscillator frequency drifts, the Doppler tone
drifts accordingly, but the center frequency of the digital filter follows it pre-
cisely because the same clock runs it. Excessive drift in the 8 kHz clock
should be avoided, however, because the analog high and low pass filters
that precede the digital filter have fixed passband centers of 500 Hz. A drift
of +250 Hz on the 8 kHz clock corresponds to +62.5 Hz (250/4) drift in the
Doppler tone produced. This value is acceptable because of the relatively
low Q of the analog bandpass filter.
Digital filter Q is calculated by dividing the filter's center frequency by its
bandwidth (Q=f/BW) or 500 Hz/4 Hz=125. It's very difficult to realize such a
high Q filter with active or passive analog filters and even more difficult to
maintain a precise center frequency. The slightest change in temperature or
component tolerance would easily de-Q or detune such filters from the de-
sired 500 Hz Doppler tone frequency. The digital filter makes the high Q pos-
sible and does so without the need for precision tolerance components. By
varying damping pot R19, the response time of the digital filter is changed.
This digital filter damping helps average rapid Doppler tone changes caused
by multipath reflected signals, noise or high audio peaks associated with
speech. A digitized representation of the Doppler tone is provided at the digi-
tal filter output. A two pole Sallen Key low pass filter built around U2B filters
out the digital steps in the waveform providing a near sinusoidal output corre-
sponding to the Doppler shift illustrated in Figure 1B
The zero crossings of this signal are detected by U2C and indicate exactly
when the Doppler effect is zero. The output of U2C is used to trigger U6, a
555 timer configured as a monostable multivibrator. When U6 is triggered
it’s output, pin 3, goes high for the time determined by the setting of the
“CALIBRATE” control, R36. When U6’s output returns to a low U11’s output
is latched and the LED in the display corresponding to the direction of trans-
mission is illuminated. Calibration of the actual direction of transmission and
the direction indicating display is accomplished by changing the delay be-
tween the Doppler tone zero crossing and the time U11 is latched. C23, R36
and R37 determine this delay by adjusting the time u6’s output is high after it
is triggered by U2C.
Low Signal Level and Audio Overload Indicators
Two useful modifications included in this design are the low signal level lock-
out and audio overload indicators. U2D continuously monitors the amplitude
of the Doppler tone at the input to the zero crossing detector. U2D’s output
goes low whenever the Doppler tone amplitude drops below 0.11 V peak.
This is done by referencing the negative input of U2D to 2.39 V, 0.11 V be-
low the nominal 2.5 VDC reference level output of U2B by means of voltage
divider, R21 and R22. U2D's output remains high when the Doppler tone is
present and above 0.11 V peak. C9 discharges via D2 whenever U2D goes
low, causing U3's output (pin 7) to go high, turning on Q2 via R24 and illumi-
nating low signal level LED, D4. D4 remains on until the Doppler tone returns