In case you belong to the impatient fraction and want to check the truth behind the claimed 15 seconds immediately,
then klick the link leading you directly to the
bullet "Let's measure" and follow the instructions. Don't forget to start your stopwatch!
After having performed the measurement, I hope you agree that the speed of the "DAB-Method" for calibration,
presented here, is unbeatable.
Sometimes it is useful to have a receiver tuned exactly (what ever that means) to the frequency commanded. Cheap RTL-SDR dongles (as I am using some) usually don't fulfil this requirement. They have frequency errors up to about 100ppm, sometimes even more. Ppm means "parts per million". For instance, if you commanded your receiver (100ppm accuracy) a frequency of 500MHz, it might in reality be tuned 50kHz off the nominal frequency you requested, because 1ppm corresponds to 500Hz.
However, there are also other signal sources of interest. Consider airband. The channels have nowadays a separation of 8.33 kHz. At 130MHz, an inaccuracy of 100ppm is 13kHz error, meaning you are inevitably tuned to a wrong channel (if you are lucky enough to live in one of those countries where airband monitoring is not a criminal offence.... For this reason, no spectrum example here).
The rtl_test program belongs to the rtl_sdr suite providing some small but very useful programs
for basic applications of the RTL-SDR hardware (QIRX uses their rtl_tcp program as the server for I/Q data).
I learned about the necessary -p option of rtl_test in this reddit discussion.
With its -p option it does the job in about 10 to 15 minutes. In my case, the windows version did not output any ppm value, but the Linux version worked. As discussed in the mentioned Reddit link, the program is using the "high-resolution timer" of the PC as the frequency reference. In my case, the output stabilized after about 12 minutes. The following two screenshots show the output after 6 and 15 minutes, respectively. The program was started in Linux with ./rtl_test -p (this line unfortunately cut off in the left picture).
Despite the large scatter of the "current PPM" values, these changes seem to average out after some minutes.
This program might be found on https://cognito.me.uk/computers/kalibrate-rtl-windows-build-32-bit/.
Its working principle is to look for GSM base stations and compare their well-known exact frequencies with the ones of the RTL-SDR hardware.
The interesting point in this software is the use of the near 1GHz GSM frequencies. It would be interesting to compare with
other methods obtained by using other reference frequencies.
Unfortunately, in my case the program did not output any ppm value. Although - with several tries - it found in its necessary first run up to three GSM base stations, the second run using any one of these stations resulted in no output. However, the author reports succesful measurements on the mentioned website. The program needs its time searching for GSM base stations, which might take several minutes. After that - according to the author - the ppm errors of the RTL-SDR hardware should be output within seconds.
A DAB receiver - from its working principle - must be able to find rapidly - compared with the other described methods -
the central frequency of the multiplex it is to be tuned to. The difference in this respect between an "ordinary" WFM receiver
and a DAB receiver is the very high accuracy of the determination of this center frequency in DAB. While the accuracy of a normal
WFM receiver needs only to be in the kHz region, a DAB receiver must be able to perform this task at least a factor of 100 better.
This is due to the digital nature of the broadcast signal.
Unsurprisingly, the accurate recognition of the transmitters' frequency is one of the more difficult tasks in any DAB receiver, and an important part in the synchronization procedure.
We want to use the frequency of a DAB transmitter as the reference frequency for our measurment. Thus, we first must be sure that its accuracy is sufficient for our purpose.
DAB Transmitter Accuracy: The requirements in the DAB Standard with respect to the frequency accuracy of a DAB transmitter are surprisingly low. ETSI EN 302 077-1 V1.1.1 (2005-01) states in 18.104.22.168:
A deviation of 100 Hz at a frequency of 200MHz corresponds to a deviation of 0.5ppm, an astonishingly high value for a transmitter in a Single Frequency Network (SFN).
The centre frequency of the RF signal shall not deviate more than 10 % of the relevant carrier spacing from its nominal value. This results in the following allowance for frequency deviation:
- transmission mode I < 100 Hz;
Single-frequency networks (SFN), in particular, place very stringent requirements on the frequency accuracy of a DAB transmitter of less than 10–9.
A frequency deviation of 10–9 at 200MHz equals to 0.2Hz or 200mHz, a factor of 500(!) better than required in the standard.
Anyway, I assume here that for our purpose the accuracy of the transmitter as a frequency reference is more than sufficient.
What you need:
Of course! We could ask ourselves if we now can forget about expensive (ok, not so much any more) TCXO receivers, usually having a frequency accuracy of around 1ppm. TCXO means "Thermally Controlled Xtal Oscillator" and usually denotes a quartz oscillator kept at a constant temperature, achieved by some controlled heating. The answer is "yes, but", the "but" translating to "only at the temperature of the measurement", being in reality a "No, we can't".
You seem to belong to the curious, because you are still here! You could ask, if it is possible in some way to check or even improve our measurement. The answer is a clear "Yes, of course we can!".
Let's take a brake, before we continue with part II of our little excursion.