Instruction manual

11
8. ADAPTING TO OTHER RADIOS
The Audio Panel is designed to accept single ended (one side grounded) audio from receivers
and provide transmit audio output at a level of 250 mV peak. Adapter cables are used to custom
tailor these signal levels to various radios used. Appendix 1 contains the schematics for the
adapter cables designed for the initial configuration of BEARS 1. The following describes the
considerations for adapting other radios to the Audio Panel.
Receive Audio
In most cases, adaptation of receive audio to the audio panel is simply a matter of connecting a
cable with a 1/8 inch phone plug on one end to the receivers external speaker jack and the other
end with an RCA plug connected to the audio panel.
In some cases, however, radios that are designed for high power audio output for driving a PA
system speaker cannot have one of the external speaker leads grounded. The radios of this type
typically have a DC voltage on the speaker leads of about 6 Volts. The circuit below is used in
these applications.
Transmit (microphone) Audio
The microphone output of the Audio Panel is at a relatively high level to accommodate legacy
radio equipment such as the Motorola Spectra and the E.F. Johnson Challenger. These radios
were designed for carbon microphones with very high output. When the panel is used with
more modern radios designed for dynamic or electret microphones, the output must be divided
down using a resistive divider. The resistive divider and any required coupling capacitor are
housed in the 6 pin DIN plug of the adapter cable. The Icom 706 required the use of a
transformer to eliminate a ground loop causing hum in the transmitted audio. Each radio must
be evaluated to design the adapter cable. The following steps must be taken.
1. Determine if there is voltage present on the microphone lead. If so, a coupling capacitor
must be used.
2. A resistive divider may now be designed to drop 100mV from the Audio Panel to the
voltage level required for 3 kHz deviation (60% mod for AM). The leg of the divider to
ground is 220 ohms. The figure below shows a simple way to determine the value of the
unknown resistor RX.
3. Key up the transceiver and adjust the variable resistor (RX) for a reading of 3 kHz deviation
on the Boonton 8210 (60% mod for an AM transceiver such as CB).
4. Measure the value of RX and construct the attenuator within the 6 pin DIN plug.