Simple Frequency Calibration for Weak Signal Work
John Andrews, W1TAG
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Accurate calibration of your receiver and sound card is necessary for good results with the weak signal modes in use at LF. Modern receivers with master oscillators that are synthesized to provide all conversion frequencies greatly simplify the process. This article describes a method of making the necessary measurements and adjustments to an ICOM R75 receiver and a sound card, using the ARGO software. No other test equipment is necessary. You will need ARGO beta 1, build 128 or later, as the earlier versions do not have the calibration option. The method is not specific to the ICOM R75, requiring only that you know how to adjust the frequency of the master oscillator in your receiver. These procedures require the ability to receive 100 kHz Loran-C transmissions. This should be pretty easy, as most of us wish those signals were not so loud! You must determine the pulse repetition rate for the Loran-C transmitter closest to you. A good source of this information is at www.navcen.uscg.gov/loran/LoranSelectChain.htm. The GRI (group repetition interval) is shown in abbreviated form for each site, with most sites having two rates. For example, my nearest site is on Nantucket Island (Mass.), and the GRI's are given as 9960 and 5930. The actual rates are 99600 and 59300 microseconds. The frequencies corresponding to those periods are 10.04016 Hz and 16.86341 Hz, respectively. Harmonics of those frequencies near 800 Hz are listed below:
These are the 75th through 84th harmonics of the 99600 rate, and the 45th through 50th harmonics of the 59300 rate. Make a similar list for the GRI(s) appropriate to your area. We must first calibrate the ARGO software to the sound card in use. This requires an audio tone of known frequency, which can be taken from a Loran-C transmission. I suggest doing the Loran work in the daytime, as the screen clutter will be less. Set the receiver to 100 kHz, AM mode, and start ARGO in the 3-second dot mode. You should see many lines on the ARGO screen, but pick one that is very distinct, and close to one of the frequencies you calculated for your closest Loran-C site. Here in MA, the 833.33333 line was the brightest on the screen, and there were few lines nearby. Center that line on the screen, and select the 60-second dot mode. After it stabilizes (be patient), read the measured frequency from the top of the screen, or from the mouse cursor. In my case, I got 833.27 Hz. Now click on Setup, and Calibration. Under "measured frequency," enter the calculated value (in my case 833.33333), and under "displayed frequency," enter the value you read from the screen (in my case, 833.27). When you click OK, the frequency of the Loran-C line should change to the correct frequency (in my case, 833.33 Hz). Now ARGO will report the correct audio frequency. You may want to repeat the check with some other bright lines. Note that because ARGO uses different sampling rates for the various dot speeds, you may see frequency variations between the screens. You can use ARGO’s “save settings” option to calibrate it for various resolutions. Next, we will use the nicely calibrated ARGO program to set the R75 frequency calibration. Be sure that the receiver is thoroughly warmed up, and in CW mode. The BFO offset should be set to 800 Hz. Remove the screws that hold the top cover. Remove the cover long enough to locate the crystal oscillator section on the right side. That board has a slug-tuned inductor marked L2, right next to the crystal. Find an insulated tuning tool that will fit the slug. Don't fudge this! Be sure you've got the right tool, as we don't want to crack the slug. If you have the oven controlled oscillator module, use the trimmer as described in the manual. Put the cover back on, and let the temperature stabilize again. Tune the R75 to a standard frequency HF station, such as WWV on 5, 10 or 15 MHz, or CHU on 3330 or 7335 kHz. Again, daytime signals may be the cleanest on the ARGO screen. Here in southern New England, the mid-day signal from CHU on 3330 is the best choice, as the propagation is via the D layer, which is very stable at that time of day. Keeping ARGO in the 60-second dot mode, with 800 Hz centered on the screen, note the frequency of the line being displayed. Lift the cover, and adjust L2 for a reading as close to 800.00 as possible. This will be a little time consuming, and you should periodically replace the cover to re-stabilize the temperature. Once you have gotten as close as you can, try some of the other HF standard-frequency stations for comparison. They should be close. You can also try WWVB on 60 kHz and Loran-C on 100 kHz (CW mode). Both should give lines very close to 800 Hz if you have done a good job. If you are using WOLF, you need to know the actual sampling rate of your sound card. Set the receiver to 100 kHz, AM mode, for Loran-C. Record 15-20 minutes of the racket, using 8000 Hz sampling, and either 16 or 8 bit resolution. Save that as a .WAV file, just as you would for WOLF. Recall the frequency of the Loran-C line you used for the ARGO calibration. Now run WOLF with the following parameters: WOLF -q filename.wav -f xxx.xxxxx -m 60, where xxx.xxxxx is the frequency of that line (in my case, 833.33333 Hz). The display should show a very consistent list of frequency deviations calculated for each minute of the recording. In my case, I got -0.200 Hz. If the readings jump around, try the frequency of another line, or re-do the recording. Your sound card sampling rate is: 8000(xxx.xxxxx/(xxx.xxxxx + f)). In my example, inserting 833.33333 Hz and -0.200 for f, I got 8001.92 Hz. As above, you might want to check this with other good lines previously seen on your ARGO screen. Finally, if you use WOLF, and want to double-check your results, set the receiver to 100 kHz, and CW mode. We are going to use the actual 100 kHz frequency for this test. Record a half-hour of the Loran-C audio, using your regular WOLF recording setup. Now, run WOLF with the following parameters: WOLF -q filename.wav -r (samp. rate) -m 60. This will give you a list of frequency deviations from 800 Hz. If you’ve done a good job, you should be under 100 milliHertz. You can add this value to 800 Hz, and get a “-f” parameter to use with WOLF. Since the receiver’s frequency error will be a little different in the Lowfer band, you may want to tweak this as you start receiving WOLF signals in that range. Beacons such as Bill Cantrell’s TEXAS are exactly on frequency, and make a good reference. This procedure should provide sufficient accuracy for the common weak signal modes. Questions or comments can be sent to me at w1tag@charter.net, or at the LWCA Message Board. |