Spectrum Laboratory ("SpecLab") is a freeware program written by Wolf Buscher, DL4YHF. It does a wide variety of things useful to the low frequency
experimenter. You can download the latest version of SpecLab at DL4YHF's Web Site . Since the program supports a wide variety of standard sound card
sampling rates, it offers a one-stop location to measure the actual sampling rates of your (cheap or expensive) sound card.
Programs used for weak-signal, low data rate communication at LF generally require accurate and stable incoming signals. This includes the sampling rate of
the sound card used to do the A/D conversion. Standard sound card rates applicable to radio work include 5512.5, 8000, 11025, 22050, 44100 and 48000 Hz.
Of these, the three lower rates are most frequently used, as the incoming audio frequencies are generally below 2.5 kHz. ARGO has a sampling rate calibration
screen allowing you to enter measured and actual frequencies. But WOLF and SpecLab both require you to set the correct sampling rate, generally 8000 Hz for
WOLF and 11025 Hz for SpecLab's Digimode Terminal.
To measure the sampling rate, you need an accurate audio frequency source. I have a divider chain on my 10 MHz frequency standard, and can use a 1 kHz
output. But most of us don't have access to such things. The Loran-C transmissions on 100 kHz do offer a wide spectrum of stable audio frequencies when
received in AM mode on a low-frequency receiver. If you set your receiver to 100 kHz and put it in AM mode, you can see a forest of spectral lines using
ARGO, SPECTRAN, or SpecLab. The Loran-C system is divided into "chains" of individual stations that key their signals at the same data rate. If you are
unfamiliar with which chain is closest to you in North America, visit the U.S.C.G. Loran Site.
Since many LF experimenters use receiver BFO frequencies of 800 Hz, here is a list of Loran lines near 800 Hz:
| Canada East | 784.14840 | 792.58010 | 801.01180 | 809.44351 | 817.87521 |
| Canada West | 784.64107 | 792.98831 | 801.33556 | 809.68280 | 818.03005 |
| Great Lakes | 791.52731 | 797.10145 | 802.67559 | 808.24972 | 813.82386 |
| Gulf of Alaska | 785.17588 | 791.45729 | 797.73869 | 804.02010 | 810.30151 |
| Newfoundland | 784.04402 | 790.92160 | 797.79917 | 804.67675 | 811.55433 |
| North Central US | 790.10856 | 796.13993 | 802.17129 | 808.20265 | 814.23402 |
| Northeast US | 788.15261 | 793.17269 | 798.19277 | 803.21285 | 808.23293 |
| North Pacific | 790.79079 | 795.79580 | 800.80080 | 805.80581 | 810.81081 |
| South Central US | 790.84287 | 796.04579 | 801.24870 | 806.45161 | 811.65453 |
| Western US | 789.73843 | 794.76861 | 799.79879 | 804.82897 | 809.85915 |
Start with ARGO. If the lines that are supposed to be active in your area are not bright, or if there is confusion about which line is which, you may want to calculate the frequency of some lines outside of the above range. Lines will occur at multiples of (50,000)/(GRI) Hz. For example, the Northeast US chain has a bright line at my QTH at 833.33333 Hz. That is the 166th harmonic of (50,000)/(9960). If you find a bright Loran line on your Argo display, do the math to find the exact frequency. Spectrum Lab's phase meter will have a much easier time if you can find a fairly bright line without too many nearby partners. Note: These measurements are best done in the daytime, when a minimum number of lines are visible by skywave, and the propagation is stable.
Armed with the frequency of a Loran line that's coming from your receiver, close out any sound card applications and start SpecLab. You will not be concerned with the "waterfall" or other spectral displays for the moment. Select "View/Windows" and "Time Domain Scope." On the Time Domain Scope, click "Presets" and "Phase Meters" and "Phase Meter for 800 Hz." This will automatically set SpecLab to an 11025 Hz sampling rate. If you are trying to calibrate at some other rate, go to the SpecLab main screen, click "Options" and "Audio Settings." Under "Audio Processing" in the middle of the screen, choose the correct nominal sample rate, and click "Apply" to engage it. Then close that window and go back to the Time Domain Scope.
Click "Vertical" and the "turn off" button for Channel 2. Then click "Aquisition + Trigger," select 9216 for Decimation Factor and click "Apply." You should now have a forest of red and green dots on the screen. Under "L.O. Freq1(cal'd)" enter the frequency of the Loran line that you calculated from above, and click the "Apply" button. With luck, the green dots, which represent phase, will start to organize themselves. Give it a few minutes.
If the sampling rate is fairly close, you will see parallel "railroad tracks" running at some angle up or down. If the rate is way off, you will have to focus on the green dots as they appear on the right side of the screen. It should quickly become obvious whether they are running up or down. Here are the rules of the game for the motion of the dots: If the dots are running up the screen, you need to increase the sampling rate; if they are running down the screen, you need to decrease the sampling rate. To adjust the sampling rate, go to the SpecLab main screen, click "Options" and "Audio Settings." In the middle of the screen, the current calibrated value of the sampling rate will be shown in blue. If you edit that value and click "Apply," you should shortly see the change on the right of the Phase Meter screen. Your goal is to level the line out, so that it neither runs up or down the screen, but stays horizontal. Have fun!
To speed up the process, you can make some measurements and do the math. The right side of the Phase Meter screen is calibrated in degrees, and the bottom in seconds. Look at the lines and see how many seconds it takes to move a certain number of degrees. For example, suppose that you have a lot of railroad tracks on the screen, and it is taking 15 seconds for the line to go from -180 to +180 degrees (for a total of 360 degrees). That means that the indicated frequency is 1/15 or 0.06667 Hz off frequency. If you are looking at a loran line at 833.33333 Hz, and the sampling rate is 11025 Hz, then the sampling rate is off by (11025/833.33333)*0.06667 = 0.882 Hz. By the rule above, if the lines are running up, you should increase the sample rate by 0.882 Hz. Do that, and then watch what happens for a few minutes. You should gradually be able to home in on a sampling rate that gives a fairly flat line. If you overshoot, it will be obvious, as the line will start running in the opposite direction.
A tip: The sampling rates may be proportional to each other. If you nail the actual value of the 11025 Hz rate, you should be able to scale it down to 8000 and 5512 Hz proportionally. That could save a lot of time if you want to calibrate all of the commonly used rates.
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