CALIBRATION PROCEDURE FOR MINIMUM LF ERROR in the RX-320
By Bill Farmer W3CSW
This procedure pertains to the GNRX320 receiver control program used to control the (K0BRA LF modified) Ten-Tec RX-320 receiver. The Spectran DSP program is used as a visual monitor and audio frequency counter. Frequency errors on the order of 1 Hz can be obtained over the entire 60 kHz to 500 kHz range of the RX-320 using this procedure.
1. Initially tune the RX-320 to the NIST Amplitude Shift Keyed WWVB signal on 60 kHz, using the CW Mode and the 300 Hz BW filter.
2. In the GNRX320 Configuration Menu, set the CW Frequency (BFO off-set) to 1000 Hz (K0BRA use -1000) and set an initial value of –75 Hz as the Frequency Correction value.
3. Run
Spectran in the .5 Hz resolution mode with the display adjusted so that 1000 Hz is centered in the display.
4. With the WWVB signal being received, note the predominant audio frequency on the upper display of Spectran. An accurate digital frequency readout is provided in the upper left-hand corner.
5. With Windows opened for both programs, adjust the initial RX-320 Frequency Correction value of –75 up or down until the WWVB tone observed in Spectran indicates a tone frequency closest to 1000 Hz.
EXAMPLE: The initial GNRX320 off-set setting of –75 produced a Spectran indication of 998.7 Hz instead of the desired 1000 Hz. Accordingly, the Frequency Correction value of –75 was then changed to –74 which resulted in a Spectran value of 999.8 Hz. Indicating a frequency error at 60 kHz of only -.2 Hz.
6. When minimum error is achieved at 60 kHz, tune the RX-320 up in frequency to the lowest frequency AM broadcast station heard. (e.g., 570 kHz) Ensure that the RX-320 is still operating in the CW mode and using the 300 Hz BW filter.
Notes:
W3CSW (1) the tone heard and displayed in the Spectran readout. E.g., If the Spectran readout indicates 1000.7 Hz, then the error at 570 kHz is only +.7 Hz. This indicates that over the complete range of 60 kHz up to 570 kHz, the receiver frequency calibration will vary from only -.2 Hz at 60 kHz to only +.7 Hz at 570 kHz. Naturally, the frequency tolerance maintained by the AM broadcast station used will affect overall calibration, particularly towards the high end of the receiver.
K0BRA (2) The error can be further reduced by adjusting the reference crystal oscillator frequency in the RX320 to achieve a precise 1 kHz spectrogram line. This is adjusted by tuning the variable capacitor at the far front of the RF PC board on the top of the unit. A good test signal is WWV at 10 or 15 mHz.
K0BRA (3) The RX320 program by Clifton Turner
http://pages.prodigy.net/kf5oj/KF5OJ.htm can be used by alternately adjusting the BFO frequency (under options/software options) to obtain a 1 kHz spectrogram line on WWVB and adjusting the crystal oscillator to obtain a 1 kHz spectrogram line on WWV at 10 or 15 kHz.K0BRA (4) The spectrogram is reversed in the sense that frequencies above the center frequency appear below 1 kHz and those below appear above. To fix this set the BFO to -1000 rather the 1000.
K0BRA (5) On field operations the temperature can change the RX320 frequency. By tweaking the software offset at 60 kHz the error can be cranked out for LF spectrogram monitoring quickly and easily.