QCX assembly Rev 1.08
117
To combat the radiation issue, the ATmega328P microcontroller is sited at the back of the board,
right under the LCD module. The tracks between the LCD module and the microcontroller are
therefore kept as short as possible, to minimise radiated noise.
To keep noise out of the supply, the 5V supply to the microcontroller and LCD module is filtered by
100uH inductor L6 and 220uF capacitor C47 (may be supplied as 470uF, even better!).
In combination,
these measures ensure that there are no “clicks” in the audio when tuning the
receiver; just a small “flutter” as I call it, which is a natural consequence of the sudden change in
frequency (Fourier rules).
Liquid Crystal Display module
The transceiver uses an HD44780-compatible LCD Module with 16 characters by 2 rows. The
LCD is operated in the 4-bit mode in order to minimise the I/O pins used. No data is read back
from the LCD which means the Read/Write pin can be grounded. In total only 6 I/O pins are used
for writing to the LCD.
The usual contrast adjustment trimmer potentiometer is R47 and must be set to obtain a readable
display.
The LCD backlight consumes about 30 or 35mA of current. The backlight could be connected
directly to the 5V supply but this would somewhat increase the power dissipation of the 7805
regulator. In order to avoid overheating the regulator, this backlight is powered instead directly
from the +12V rail via R48, a 270-ohm resistor. This resistor value was chosen so that over a
supply voltage range of 7 to 20V the backlight brightness still appears reasonable. If you want a
reduced brightness display, you could increase the value of R48.
Sidetone
In this transceiver, the sidetone is generated by Pulse Width Modulation using the ATmega328’s
Timer1 peripheral. The frequency is configurable in the software via the configuration menu, and
so is the volume. In order to control the volume, the microcontroller adjusts the duty cycle from
50% (maximum volume) down to under 1% (for minimum volume).
The sidetone is injected into the receiver path via a 3.3K resistor at the input to the CW filter. The
sidetone generated by the microcontroller is a squarewave, rich in harmonics. As the volume is
reduced, the duty cycle percentage drops and the amplitude of the 700Hz fundamental tone also
drops. There are many harmonics of course, and the CW filter does a great job of removing these,
so what is left in the earphones is a pleasant and pure 700Hz sinewave. This is why the sidetone
is injected at the CW filter INPUT.
During transmit, when the sidetone is operational, the mute switch Q7 is also closed
– but there is
enough leakage through the imperfections of this switch that the sidetone gets through anyway.
The 3.3K sidetone feed resistor R59 is chosen to pump enough signal through that it overcomes
the attenuation of the mute switch. Without the mute switch, R59 would be a much higher value.
Key paddle, rotary encoder switch and buttons
The microcontroller keeps an eye on all the buttons, key paddle inputs, and rotary encoder
switches. When button or switch closures occur as the operator activates a control, the
microprocessor responds immediately as required.
The onboard microswitch “key” is wired in parallel with one of the paddle inputs. The paddle
inputs, and the rotary encoder switch, are read using dedicated microcontroller I/O signals.
Summary of Contents for QCX 5W CW
Page 9: ...QCX assembly Rev 1 08 9...
Page 10: ...QCX assembly Rev 1 08 10...
Page 12: ...QCX assembly Rev 1 08 12...
Page 50: ...QCX assembly Rev 1 08 50...
Page 105: ...QCX assembly Rev 1 08 105...
