If you happened to get one of those $7 SDR radio dongles from ebay and need to put it to work... here's one thing I tried...

Tune in with the R820T... (Using HDSDR)


This is both radios running on one channel.  They are out of tune enough that the channels don't even overlap!

Now I switch them into beacon mode and tune to 925.000 MHz...


Now I tune them up by adjusting the EZRADIOPRO_OSC_CAP_VALUE to get this...


Through the magic of special effects, I can overlay them...


Below are the before and after pics of normal transmissions (non-beacon mode)...



Settings used...


Here are the actual results I measured (warming up and final calibration)...

Ground = +7.3 khz @ OV 204 (cold)
Ground = +6.5 khz @ OV 204 (warm)
Ground = +6.1 khz @ OV 204 (hot)
Ground = +0.1 khz @ OV 208 (warm)

Air = -6.5 khz @ OV 204 (cold)
Air = -6.2 khz @ OV 204 (warm)
Air = -7.1 khz @ OV 204 (warmer)
Air = +0.6 khz @ OV 199 (warmerer)

All the pictures in this post are to the same scale.  The FSK signals (wider pictures) are on a different frequency than the beacon signals.

Make any predictions on expected range improvement below...  

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  • @ Hans you can get it here http://www.ebay.de/sch/i.html?_sacat=0&_nkw=rtl2832u&_frs=1

  • Where did you find that $7 SDR Spectrum Analyzer?

  • Moderator

    @Hugues don't forget when you have finished tuning your radio you can use the SDR to put air traffic overlays on mission planner. When might we get some LoRa modules? We have been testing them down here for other purposes and very easily get 17km range ground to ground out of 100mW (that is line of sight though not over anything) http://www.instructables.com/id/Introducing-LoRa-/

    With one module on a kite acting as a repeater I bet it could cover the entire outback challenge area easily. 

  • Developer

    Great work.  So once this offset is discovered is it pretty stable or does it change with temperature or some other changes in the enviroinment?II wonder if we could create a calibration routine where one radio outputs a signal at a known frequency and the other radio tries to match it.  Perhaps that's the same as the rssi suggestion above.

  • Admin


    SDR: Software Defined Radio

    HDSDR: Free SDR program for Windows

    R820: USB Radio dongle



    Software-defined radio
    Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in hardware (e.g. mixers, filt…
  • MR60

    What is SDR ? R820? HDSDR ? This is Chinese for me. And what is this parameter OCS-CAP-VALUE you modify? In which sw is this?

  • thats excellent Jake. Will this software also work with the 433 MHz version. If not, are you willing to make one ? 

  • That's really awesome !

    Few years back, I purchased a pair of 3DR telemetry radios from the store, infact the store name was not 3DR at that time.

    Those radios were 100mw 915MHz and gave good working range upto 13km in air @300mt alt and 13dbi Yagi antenna at ground.

    Some other pair of radios did not give, I am able to understand that now that may be by luck my pair of 3DR radios

    might have been perfectly tuned.


    @Jake, two days back only I got a similar SDR dongle from ebay, which software you are using for the PC ?


  • Re: Live autolearn tuning...  I thought that it might be handy to have one radio transmit while you get RSSI from the other, and in that way tune one to the other.

    In fact, RSSI does seem to work with the radios, even in their beacon state.  So some experimentation may be in order.  With the right power level and distance between the two it might work well.

    But, my thinking is that the auto frequency correction will throw things off.  You'd probably end up just getting a plateau.  Though you could still probably use the edges to find the center.

    I would just add an extra register or permanent setting, but...  I really don't need the responsibility of making flight firmware.  My tune firmware lets you easily match up your radios and find the right OSC value, then we can all just load a copy of the nice, stable SiK v1.9... without a bunch of my kludges.

    All of the AT commands I added (except ATJ) WILL disrupt data transfer (usually).

    Now that it's out there we can find out together if it's worth bothering with.  Next, I'll probably port the idea to OpenLRSng if it turns out to have any merit/interest.

    I did steal several handfuls of lines from their beacon code to make things work.

    Which reminds me, GPL compliance is in the works.  It's just that at the moment I have easily three times as much commented out code as working code.  It wouldn't be very readable.

  • Excellent Jake, very nice!

    I'll tune my spectrum analyzer onto my radios in a couple of days here and see how bad they behave (I have a large selection of 3DR, HK and HM-TRP modules on my own hardware).

    Suggestion, how about making a "hidden" ATSxx register that would be used as the calibration factor ?

    Then it would be easy to change with a terminal program

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