Advertisement
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
with amplitude above 0.11 V peak and C9 recharges via R23. The LED dis-
play remains locked by disabling U11's strobe input whenever the Doppler
tone is too low for an accurate bearing. This is done by pulling pin 1 of U11
low via D5 when Q2 is turned on.
Audio overload indicator D3 tells you that audio clipping of the Doppler tone
is occurring. Clipping results if the signal level from the digital filter is too high
and can produce an erroneous bearing indication. The output of U1D goes
low whenever the output of the digital filter drops below 0.6 V as the ampli-
tude of the Doppler tone approaches the 0V supply rail. C7 discharges via
D1 and causes the output of U3C to go high, turning on Q1 via R16 and illu-
minating audio overload LED D3. We elected not to lock the LED display on
audio overload; doing so, however, only requires connecting a diode be-
tween the collector of Q1 and pin 1 of U11, similar to the low level lock out
function.
Phase Correction
If the audio output of the Doppler RDF FM receiver is incorrectly phased, S3,
phase invert, can fix that. (If phasing is incorrect, LED direction indications
are 180 degrees opposite that of the actual signal source.) Moving S3 to the
opposite position corrects the problem by letting U2C sense the trailing edge.
This is particularly useful when switching between different receivers. S2 dis-
ables the 8 kHz clock to disable the antenna spinning. This helps when
you're trying to listen to the received signal. Presence of the Doppler tone in
the received audio makes it difficult to understand what is being said, espe-
cially with weak signals.
Power Supply
Power is delivered via on/off switch S1. D6 provides supply voltage reverse
polarity protection by limiting the reverse voltage to 0.7 V. U10 provides a
regulated 5 VDC to all digital ICs. C29 through C33 are bypass filters. U10's
5 VDC output is dropped 2.5 V by resistive divider R43 and R45. Non-
inverting voltage follower U3B buffers the 2.5 V source to provide a virtual
ground reference for all analog filters and the digital filter. Using a virtual
ground 2.5 V above circuit ground allows op amps to process analog signals
without the need of a negative power supply voltage. Analog voltages swing
from near 0 V to near +5 V with the virtual ground level right in the middle,
2.5 V.
DDF1 • 10
Hide quick links:
Advertisement
