A section of Chapter 4, DSP-10 User's Manual

Software Set of EZ-Kit Frequency - You can compensate for the EZ-Kit 24.576 MHz xtal oscillator errors in software. This allows this error to be reduced from a Hz or two to at most, 0.37 Hz. This is only of interest to stations using very good frequency references that want to remove the last few Hz of error. Staions using the internal crystal can ignore this.

The configuration file variable ezkit_dfreq is the observed error, in Hz, at 145 MHz. To set this variable, you need an exact frequency reference at 145 MHz, such as a frequency synthesized signal generator locked to a reference. Alternatively, if a 10 MHz reference is being used, you can divide this by two in a digital divider and receive the harmonic at 145 MHz.

PROCEDURE - Run LTI 1200, no windowing ("Non"), center freq 375 Hz, with the dial set to 144.999625 and look for the symmetry in the response. If the response is to the right by 1.5 Hz, enter to the .cfg file "ezkit_dfreq 1.5". This will shift the response to the left by about 1.5 Hz. It is not exact because of the DDS step size (0.37 Hz). Watch the waves in the waterfall and minimize these. Note that changes of less than 0.7 Hz may not change the display at all. Experiment with various values and center it in the best region. If, by chance you can get the error to zero, there is no windowing spectrum and the waterfall is a straight line down---nothing on the sides!

If things are going well, your screen should look like this!

BACKGROUND - Here is what you are doing. The second I-F of the DSP-10 is in the DSP at about 15 KHz. The conversion oscillator for this last I-F (BFO if you want to call it that) has its frequency set by the 48 KHz sample rate of the CODEC. If you check through the DSP software, you will find that everything keys to that sample rate. The 48 kHz is in turn divided down by the CODEC from the 24.576 crystal oscillator. This oscillator might be off frequency by a kHz or two.. When divided to the conversion oscillator frequency of 15 kHz, it still represents an error of a Hz or two. The correction in software adjusts the conversion oscillator frequency to remove the error. The frequency step size is limited by the 16-bit phase accumulation to 24,000/65536 or 0.37 Hz.

Table of Contents

Valid HTML 4.01 Transitional