BENCH TEST AND INITIAL CHECKOUT
Apply power and run the BALANCE trimpot through its entire range... Audio should
initially be present at full strength in only one earpiece, and at reduced strength
in the opposite earpiece. As the BALANCE trimpot is adjusted through the middle of
its range, the audio should "switch" to the opposite side of the headphones.
Return the BALANCE trimpot to the switching point and adjust the DEADBAND control
through its entire range. Within the deadband, audio should be present in BOTH
earpieces, at full strength. At one end of the adjustment range for the DEADBAND
trimpot, the deadband should have zero, or nearly zero width. ( Use the BALANCE
trimpot to test the DEADBAND width ) At the other end of the DEADBAND adjustment
range, the DEADBAND width should equal a few turns of the BALANCE trimpot.
Use a cliplead to test each antenna switching output, by feeding each output back
to the audio input... one output should produce full audio in only ONE earpiece,
and the other output should produce full audio in the OPPOSITE earpiece.... in both
cases, the 500 Hz switching frequency should be audible in the headset.
If the identities are wrong, swap the wires for the antenna drive signals, or swap
the audio channel wiring to the headset jack. If another model of radio is later
used, it may be necessary to repeat this process. ( or add a toggle switch )
The deadband adjust trimpot can be set for any convenient level, usually around
30 to 50 degrees of headset rotation, to exceed the deadband. Changes of audio
level in the reciever ( adjusting the volume control on the reciever ) WILL AFFECT THE
DEADBAND WIDTH, because this also affects the size of the detected Doppler pulses
that drive the comparators... increasing the volume control setting will increase
the error signal amplitude, and ( therefore ) cause the deadband zone to become
NARROWER.
The "balance" trimpot is provided to achieve symmetry of the left / right switching
thresholds.... it is adjusted ( initially ) to provide mid - supply voltage at the output
of U1A, then "fine tuned" by experiment so that the deadband zone extends
equally on both sides of the "null tone" position.
It is probably advisable to use connectors with some form of positive indexing,
or a locking mechanism, to prevent the antennas from accidentally "rotating" out
of their proper ( vertical ) orientation. This can easily happen if one is climbing
through foilage or overgrowth, ( searching for a transmitter ) and it may not
become obvious until a lot of time has been wasted, chasing false leads caused
by "cockeyed" antennas.
The choice of headsets deserves some attention, as well. For myself, I chose a
set of Koss headphones, on sale at Radio Shack, for about U.S. $35. These had
"full cup" earpieces, ( to attenuate external noise ) and a sturdy metal headband
which could be drilled for antenna connectors, without any serious loss of
strength. The cup earpieces will also do a good job of "holding on" to my head, as
I move and hunt.
I later learned that these headphones were MONAURAL, and I had to modify them for
STEREO operation, by changing the coiled cord. Apparently, they were intended for use at
sporting events, such as automobile races, with scanner recievers. I also used a
coiled cord for the antenna switching signals, and RG-174 co-ax for the RF
signal. The coiled cord that I used for the switching signals did not have enough wires
to supply all the required signals to the ANTENNA board, so I relied on the shield of
the RF co-ax to provide the DC ground connection for the ANTENNA board.
To minimize the trouble of all these cords running to one headset, I decided to "merge"
the two coiled cords into one by "weaving" them together... successive turns
of the finished cord actually "alternated" between the two individual cords.
( successive turns = cord 1 / cord 2 / cord 1 / cord 2, etc. )
I also ran the RF co-ax cable down the center of both coiled cords, resulting in
a single "merged" cable, running to the headset. When finished, it was really quite
clean and convenient.
The initial bench checks can be performed without an antenna or a radio... feed
an audio source into the DETECTOR unit and connect a set of stereo headphones
to the output... I used the CD-ROM on my computer as an audio source, since it also
plays CD music disks, and has a mini-phone jack on the front panel, for headphones.
FINAL TEST / ADJUST
With a new TDOA unit, it is impossible to predict which antenna will cause
attenuation in which earpiece... it must be determined by experiment. This is true
because there are no "speaker phasing" standards for communications radios...
the audio signal ( after FM detection ) must pass through several audio amplifiers
before it reaches the Doppler detector unit... some of these audio amplifiers may
invert the audio signal phase, but others may not... The exact number of phase
"reversals" is unknown, and depends on the design of the particular radio.
THE HEADSET
The antennas can be permanently mounted on the headset, but the use of
antenna connectors offers some advantages... antenna connectors will allow the
use of a vareity of antennas, depending on signal strength... larger elements for
weak signals, smaller ones ( or none at all ) for strong signals. Connectors will
also allow the use of an external antenna array, if desired.