The G303 receiver series features an excellent frequency accuracy and stability for a receiver of its class. It is however possible to improve this accuracy yet further, by individual calibration. This procedure is recommended for experienced users only. The receiver calibration is accomplished by inserting a reference frequency parameter in the WRG3.INI file which resides in the MyDocuments\WINRADIO\G303 directory. The reference frequency parameter consists of two lines of the following format: [ClockCalibration] receiver_serial_number=reference_frequency The receiver serial number can be obtained for example from the About box in the G303 application. The reference frequency is the actual frequency of the internal reference oscillator in Hz. This is normally 20 MHz, i.e. 20000000 Hz. Each receiver is already factory calibrated, so a fundamental correction to the nominal 20 MHz reference frequency already exists and is stored in the receiver's internal memory. This correction can be overriden by the new parameter in the WRG3.INI file. To determine the true offset from a perfect tuning, firstly use 20000000 (i.e. the nominal reference frequency in Hz) as the reference_frequency parameter. Say your receiver serial number is 02L27011: [ClockCalibration] 02L27011=20000000 Save the WRG3.INI file with this value, then start the G303 application. The frequency error will be now much worse because the new parameter overrides the original factory calibration. Then tune the receiver to a known frequency standard. A high-accuracy signal generator can be used, or one of the WWV Time and Frequency Standard stations. Observe the peak with the spectrum scope and listen to the beat frequency in the CW mode with a minimum IF bandwidth. Then note down the frequency difference. Typically, this will be a negative number (for example, -652 Hz at 10 MHz). Then scale the frequency difference to 20 MHz. For example, if the frequency difference is -652 Hz at 10 MHz, it will be -1304 Hz at the 20 MHz reference frequency. Then subtract the frequency difference from 20000000. In our example, the resulting reference_frequency will be 20000000-(-1304)=20001304. The entire reference frequency parameter in the WRG3.INI file will be then as follows: [ClockCalibration] 02L27011=20001304 Save the WRG3.INI file, then restart the G303 application and observe the difference.
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