RX1 Adjustment

[5B4AIY]

Good Morning, Martin,

Perhaps I can help. You are referring to the crystal oscillator frequency compensation adjustment. The master oscillator is a 20MHz crystal, and it has a manufacturing tolerance such that the frequency of oscillation may not be exactly 20MHz. By performing this adjustment you can ensure that your oscillator is exactly on frequency, and that the actual frequencies generated by the DDS chip are exactly as shown on the display.

To perform this adjustment, from the power OFF state, press and hold the VFO tuning knob to operate its push switch, and turn the power on. The transceiver will enter the compensation mode. The software will set the output of the DDS chip to 1MHz. However, because of the manufacturing tolerance of the crystal, the output frequency generated may not be exactly 1MHz. Measure the frequency with an accurate frequency counter, and adjust C20 until the frequency is 1MHz. This is best measured at J2 on the DDS & Control Board.

It is not absolutely necessary to use a frequency counter, this adjustment can also be performed whilst the receiver is operating in its normal mode. Short wave broadcast station's carriers are normally generated from a high precision master oscillator, and thus if you tune the receiver to the precise carrier frequency - on the SW band these carriers are spaced every 5kHz - and then adjust R20 for an exact zero-beat note at the audio output, you have essentially compensated the master oscillator.

You can also use a standard frequency transmission carrier, these are found at 2.5MHz, 5MHz, and 10MHz as well as other frequencies, and you can then zero-beat with any of these to compensate the master oscillator.

I hope this answers your question.

73, Adrian, 5B4AIY