the “Select” button the microcontroller uses the 1pps as a frequency counter gate. The 27MHz
reference value is divided by 4 and routed to the Timer1 input of the microcontroller, where it is
counted for 4 seconds to obtain a frequency count. This value is stored in the EEPROM in the
configuration parameter editable in menu item “8.5 Ref frq”.
The firmware calculates compensation adjustments to correct for the frequency error and ensure
an output frequency precisely on target (typically with sub Hz accuracy). Naturally changes in
ambient temperature will also cause the crystal oscillator frequency to change somewhat.
Using the GPS to calibrate the reference oscillator, system oscillator also automatically parses the
serial data stream and sets the real-time clock and the Maidenhead Locator square (computed
from latitude and longitude); these are used by the Beacon mode when using the radio as a
WSPR beacon.
The system is capable of calibration of 27, 25 and 12MHz reference oscillators.
NOTE: If using the OCXO option in the QCX+ and QCX-mini models, make sure you set
menu 8.5 manually to 25,000,000 before using the GPS calibration function.
NOTE: If building the kit for 160m operation, use a 12MHz reference crystal and set menu
8.5 manually to 12,000,000 before anything else, including using the GPS calibration
function.
When you press “Select” the calibration will go quickly through several phases, resulting in a
measurement of the reference frequency. Calibration takes 6 seconds, once the GPS module has
satellite lock. The bottom line of the display will show the status as follows:
•
Waiting
Y
ou have just pressed select; the QCX is waiting for the GPS receiver
to achieve a valid satellite lock, and the presense of the 1pps signal
•
6
The start of the countdown to zero, in seconds; followed by 5 and 4
•
3 27,003,623
Countdowns 3, 2, 1 are three successively closer measurements of the
reference frequency
•
0 27,003,620 OK!
Final calibration value, which is saved in EEPROM and can be seen in
menu 6.5
When the calibration is complete, you can unplug the GPS or proceed to calibrate the system
reference frequency etc.
8.12 Cal sys osc
Press Select!
If you have connected a GPS receiver module such as the QRP Labs QLG1 GPS receiver
with a one pulse per second (1pps) output signal, then when you press
the “Select” button the microcontroller uses the 1pps as a frequency counter gate. It counts the
20MHz system clock internally for 4 seconds to obtain a frequency count. This value is stored in
the EEPROM in the configuration parameter editable in menu item “8.6 System frq”.
Refer to the previous section for a description of how the display will update during the calibration
process.
QCX operating manual, firmware 1.07, manual edit 0.03
37
