BENCH TEST AND INITIAL CHECKOUT

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.

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.


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.

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.


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.

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.


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