Spectrum Lab provides two methods of frequency calibration which can be utilized to set the frequency scale accurately on all bands. The first applies to calibration of the sound card by injecting a known audio frequency and comparing it to the frequency measured by SL. Figure 1 shows the Audio I/O screen for entering these values in the "Correct Frequency" and "Displayed Frequency" boxes on the right. After entering click on the "Calibrate Input SR" and the sound card's Sample Rate will be determined and entered in the "Sound Rate Calibration Table". I generally use a nominal SR of 11025 Hz and in the actual value is 11099.8922552 Hz which is now applied to the SL measurement. At this point Spectrum Lab is reading audio frequencies accurately. I used the standard tones of 500 and 600 Hz provided on the WWV carrier when listening in AM mode.
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| 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.
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| Figure 2. Spectrum Lab Radio Frequency Offset |
The offset can be determined in general by measuring the error for several know frequencies and applying a linear regression. This is possible because the frequency my TS-440 and as far as I know all modern transceivers is controlled by a single crystal. I used the standard frequencies of 5, 10, 15 and 20 MHz from WWV in Colorado to generate the curve shown in Figure 3. The linear regression for this data is:
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.
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| Figure 3. Offset Graph and Table |
Use of WWV for frequency calibration can be tricky due to Doppler effects caused by motion of the ionosphere. I made my measurements on a day when ionospheric conditions were settled and at a time when the night-to-day movement should have been completed. I confirmed this by monitoring the Doppler shift for several hours.
