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truly an elegant feature of the Doppler RDF design. Using the same clock os-
cillator to spin the antenna and clock the digital filter ensures the Doppler
tone produced by the spinning process is precisely the center frequency of
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 indicate exactly when the Doppler effect is zero. Zero crossings
are detected by U2C and used to fire a monostable multivibrator (U6) built
around a 555 timer. U6's output latches the red LED in the display corre-
sponding to the direction of transmission with respect to the green center
LED, D16. Calibration between the actual source of transmission and the red
direction indicating LED latched in the display is easily accomplished by
changing the delay between the Doppler tone zero crossing (firing of U6) and
the generation of the latch pulse to U11. C23, R36 and R37 determine this
delay. Increasing or decreasing the delay is achieved by increasing or de-
creasing the value of the calibrate potentiometer R36.
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
DDF1 • 9
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