|Figure 1. Spectrum Lab Audio I/O Screen|
The second frequency adjustment is via the "Radio Frequency Offset" value found on the Spectrum(2) screen, Figure 2. This is the frequency to which the radio is tuned and adjusts the scale to read in units of the actual frequency being received. For example, on 30m, my rx dial is set to 10138.70 kHz to produce an audio tone of 1300 Hz for 10140 kHz. If the master oscillator were perfect then the frequencies measured by SL would also be perfect. However, even the best receivers are off by at least a few Hz (unless locked to GPS or an atomic standard) so a little extra correction is needed. This can be applied by adding or subtracting the appropriate value to the rx display. My rx on 30m reads low by 7 Hz and I add this to the offset value as 10138.707.
|Figure 2. Spectrum Lab Radio Frequency Offset|
F(offset), Hz = -0.2 + 0.672*F(rx), MHz,
where F(offset) is the offset needed to correct the received frequency and F(rx) is the receiver frequency. Ideally the intercept value should be zero but the value of -0.2 is easily attributable to measurement error. The offset values for each band are also printed on Figure 3 for quick reference.
|Figure 3. Offset Graph and Table|