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Page last updated: 01/06/2012 |
Resultant hardware design:
The final design is detailed below in two parts, though built as a single assembly.
Audio control and switching schematic
Circuit description
IC101a forms a Wien audio oscillator used for PIP tone and Morse code Identification, the output frequency of which is switched and set to either 800Hz or1600Hz, switching being affected by IC102a and IC102b. The output of the PIP tone generator is buffered and amplified by IC101b, the gain of which is either 0db or 6db switching being affected by IC102c, keying being affected by IC102d. Please note the values of R121 and R122 must be carefully selected as they set the gain of IC101a, if the gain is set too low the oscillator will fail to start, but if the gain is set too high the output will be a square wave and as such full of harmonics. This issue will be corrected in the future, but currently careful selection of these two components has worked fine with no perceived long term stability problems.
Audio from the receiver is fed into IC101c a
differential amplifier providing an initial 6db gain. The Rx audio is then split
taking two separate paths with a further 6db of amplification being provided by
IC101d and IC103d. The amplified Rx audio from IC101d switched by IC104d and
combined with the output of IC102d is then further buffered by IC103a and IC103b
to provide a differential output for the transmitter. The amplified Rx audio
from IC103d then onto IC105 which decodes 1750Hz tone burst signals, IC104a
inverts the output of the tone burst decoder to provide positive going logic.
Power for the Repeater Controller (DC between 7.5v and 35v)
is regulated by IC106 providing 5vdc, a further 2.5vdc supply is provided by
IC103c and Q101 also acts as a virtual ground for all audio signals. In essence
we are implementing a virtual split plus and minus 2.5vdc supply for the
operational amplifiers, and a plus 5vdc supply for the switches and
microcontroller.
Process control and I/O schematic
Circuit description
IC1 a PIC16F84AP microcontroller provides all process control for the Repeater Controller, audio switching being directly controlled, whereas logic to and from both the receiver and transmitter are isolated by opto isolators OK1 through OK5.
R1, R2, C3, D1 provide reset a reset signal to the microcontroller, holding the microcontroller in a reset state until all supplies have stabilised.
Setting up the repeater controller
With the microcontroller unplugged make the following adjustment.
Tie pins 8, 10 and 11 of the microcontroller socket to 0v (pin 5), and pin 9 to +5v (pin 14) check audio tone on the transmitter output.
Adjust VR103 for tone level of -10dbm into 600R (735mV).
Tie pins 8 and 11 of the microcontroller socket to 0v (pin 5), and pins 10 and 11 to +5v (pin 14), check audio tone on the transmitter output.
Adjust VR102 for tone frequency of 1600Hz.
Tie pins 8 of the microcontroller socket to 0v (pin 14), and pins 9, 10 and 11 to +5v (pin 14), check audio tone on the transmitter output drops by 6db.
Tie pins 8, 9, 10 and 11 of the microcontroller socket to 0v (pin 14), check there is no tone on the transmitter output.
Tie pins 9,10 and 11 of the microcontroller socket to 0v (pin 14), and pin 8 to +5v (pin 14), inject a 1KHz tone at -10dbm into the receiver input and check the tone is present at the transmitter output at -10dbm.
Inject a 1750Hz tone at -10dbm into the receiver input.
Adjust VR104 for +5v on pin 2 of the microcontroller socket.
Remove the 1750Hz tone from the receiver input and check that pin 2 of the microcontroller socket drops to 0v.
Receiver - set the audio output level to -10dbm into 600R (735mV) at full system level.
Transmitter - set to full system level with an audio input of -10dbm (735mV).
Connect both the transmitter and receiver to the Repeater Controller inject a signal into the receiver modulated at full system level, and check the transmitter modulation is also at full system level. Fine adjustment may be necessary to the transmitters line input level control to ensure that the transmitter modulation is exactly a full system level whilst the receiver is active at full system level.
Plug the microcontroller back into its socket, and full system functionality should now be checked.