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Calibrating the SDR Frequency Display to match your
Ham Station Radio Frequency Display on each Ham Band
D. C. Eddleman, KR4UB 1/23/2014 2:29 PM
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.
Calibrating the SDR Frequency Display to match your
Ham Station Radio Frequency Display on each Ham Band
D. C. Eddleman, KR4UB 1/23/2014 2:29 PM
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.
Calibrating the SDR Frequency Display to match your
Ham Station Radio Frequency Display on each Ham Band
D. C. Eddleman, KR4UB 1/23/2014 2:29 PM
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.
Calibrating the SDR Frequency Display to match your
Ham Station Radio Frequency Display on each Ham Band
D. C. Eddleman, KR4UB 1/23/2014 2:29 PM
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.
Calibrating the SDR Frequency Display to match your
Ham Station Radio Frequency Display on each Ham Band
D. C. Eddleman, KR4UB 1/23/2014 2:29 PM
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-for-
software-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 - .USO-
wPK86Jo