When using a Software Defined Radio as a band scope, one thing that can greatly assist in frequency coordination between the SDR and the ham station transceiver, is for the frequency calibration to yield the same exact read out when listening to the same signal. This article addresses that question for the configuration where the NooElec Ham It Up v1.0 - RF up-converter is used in front of the Newsky TV28T v2 DVB-T USB SDR. In this configuration there are two frequency calibrations that should be done. The NooElec up-converter contains a bypass switch to take the up-converter out of the receive path. In converter bypass mode, the SDR operates within the normal approximate 48Mhz to 860Mhz SDR tuning range. In preparation for the first adjustment below, if the SDRSharpe software has already been set up for use with the up converter, be sure to remove the check mark on the (frequency) Shift option. This will turn off the frequency shift value normally used with the up-converter (as indicated by the greyed out -99,999,250 shown below). The first frequency adjustment is to calibrate the SDR itself to some known, accurate frequency standard. Since most of us do not have in our tool kit a precision frequency reference standard, this step requires using whatever is readily available. In my case, our club s 442.150Mhz repeater works out well for this purpose. The repeater is a Kenwood TKR-850 having a frequency stability of 1.5 ppm (parts per million). Since there is only one adjustment for the entire SDR tuning range, the 442.150Mhz is a good mid range frequency for calibrating the SDR.
The procedure is to simply to zoom in on the frequency display, then click on the Configure button as shown below, and adjust the Frequency Correction (ppm) number so that the signal source is as close as possible to the correct frequency. In the case below the ppm setting of 65 came very close. You may or may not come this close as the ppm increments of the 28.8Mhz crystal on the SDR multiply out to be quite large as the SDR tunes up in frequency. Write down this number, since if inadvertently changed, it will affect all the calibration settings in the next step. The 28.8Mhz crystal used in the consumer quality SDR will show some frequency drift as it warms up, enough to make the pitch of a SSB signal off a bit, perhaps 0.05 to 0.100Mhz during warm up. But the usefulness as a band scope is not greatly impacted. The next frequency adjustment will be for the NooElec up-converter. At first, there did not appear to be a mechanism to do this, until the frequency shift window caught my eye again. The value entered is the frequency offset needed when using the NooElec up-converter, normally set to an initial value of 100,006,000. The problem is that when using only one offset value, accurate frequency calibration can be achieved on one ham band, but there will be some amount of frequency calibration error on the other ham other bands.
Another feature in the SDRSharp software, the Frequency Manager, permits storing multiple offset frequency shift values, one for each memorized frequency value. Using this feature to store unique offset values for each ham band achieves the goal of accurate frequency calibration on all ham bands. I regularly use the Frequency Manager to quickly switch across the 80, 40, 20, 17, 15, 12, and 10M bands. For improved calibration, all I needed to do is store the proper offset values in each of my saved ham band settings. To begin this process, return the up-converter bypass switch back to the off position and place the check mark in the Shift box in the SDRSharpe software as shown below, to return coverage to the HF ham bands. Note the stored values for each memory save in the Edit Entry Information window. The shift frequency value for the NooElec up converter is stored in every saved memory setting. Rather than using the same value of 100,006,000 for each band, this value can be altered, so that the frequency calibration is correct for each stored frequency. I store one frequency per ham band, which is the centerpoint of the respective SSB bands. Activating the unique shift frequencies, does require using Frequency Manager for switching ham bands, which is a simple click on the desired entry as shown below. Of course if the digital readout at the top of the screen is used to go to a new ham band, the stored shift frequency will not be applied. So, I set about tweaking and storing unique offset values, initially using the WWV 5, 10, 15 and 20Mhz frequencies to determine the value. Then using my ham station radio, I listened to QSOs on each ham band and begin applying additional offset frequency tweaks until the pitch of the SSB signals were identical on the SDR and the ham station radio, of course with the frequency readout on both receivers set to the exact same frequency.
It took a little time to do this, but now when I hear a station on the SDR that I wish to have a QSO with, I can simply dial in the exact same frequency on the ham station radio and upon hearing the same pitch on the SSB audio, I know I am on the exact frequency setting to begin transmitting.. After taking the time to get the correct offset for all bands, those settings are not something I care to lose. One thing I ve already noticed is that if you re-run the install.bat batch file (refer to Installing SDRSharpe Software article) to update your software to the latest version, all saved settings are lost. However, there is a quick solution for this. After saving some frequencies in the Frequency Manager, a quick look in the SDRSharp subdirectory shows where those values are stored. A new file appears after the first frequency save in the Frequency Manager, named frequencies.xml. Opening that file reveals the saved values, and the offset frequency value stored for each ham band setting. Due to variations in crystal oscillator frequencies, on both the NooElec up converter and the USB SDR dongle, the values stored below probably won t be the correct for your setup.
So, several things have been learned; first, how to calibrate the SDR frequency display to be accurate for each ham band; second, how and where those settings are stored and can be protected against inadvertent overlay. Given the text format of the frequencies.xml file, a text editor can be used to make major changes if need be. I ve done that for example, on all the band pass settings as my preferences have changed. Be careful if you choose to edit this file and always make a backup copy first. I have created another subdirectory called settings, as shown below where I now keep a back up copy of my SDRSharp settings. Notice that there is another file called SDRSharp.exe.Config. It also contains settings you may want to preserve in your SDRSharp software setup. However, I strongly recommend you not edit this file. Take a peek and you ll see why. Conclusion By eliminating the frequency mismatch between the SDR and the ham station transceiver, the usability of the SDR as a bandscope for quickly finding DX, contest and other interesting activity on the ham bands is much improved. Now if only a higher precision 28.8Mhz crystal or TXCO could be found for the SDR. Source for the NooElec Up Converter: http://www.nooelec.com/store/software-defined-radio/ham-it-up-v1-0-rf-upconverter-forsoftware-defined-radio.html -.USO_GPK86Jo Source for the TV28T v2 DVB-T USB Stick (R820T) w/ Antenna and Remote Control http://www.nooelec.com/store/software-defined-radio/sdr-receivers/tv28tv2.html -.USOwPK86Jo