Strange behavior of Mini and GNU Radio below 30 MHz



Being a completely noob in SDR I have nevertheless embarked on writing my own transciever for the Mini using Gnu Radio Companion (GRC). Things work as expected above 30 MHz. TX output as per Marty’s measurements, even a couple of dB more and sensitivity OK.

But, below 30 MHz things go strange. I understand the idea (I hope) to use the NCO and do an up-converting. I use the formula: FNCO = 30 MHz - Foperate, and set the RX and TX frequencies to 30 MHz.

However, irrespectively of setting of the NCO frequency and RX/TX frequency, I can set the TX and/or RX frequency to any frequency, at least down to 3.5 MHz and the Mini tunes to the set frequency ,but RX sensitivity and TX power both decreases rapidly. At 24 MHz TX is down by 16 dB and RX sensitivity is down by 30 dB compared to 30 MHz and at 3.5 MHz TX is just above the specgtrum anlyzer’s noise floor.It seems to me the NCO setting in the GRC Lime source and sink do not work, but by some strange way the TX and RX synthesizers in the LMS7002 are simply tuned way below 30 MHz and their power decrease accounting for the decrease in power/sensitivity.

Please anybody who have tried tuning below 30 MHz using GRC with succes: What am I doing wrong?

best 73

Jorgen, OZ7TA

P.s: You may wonder why I do not simply use SDR Console or SDRAngel. I could easily do that, but then I would not learn anything.


I did not use GRC blocks but algorithm for frequencies bellow 30MHz must be identical to other programming languages solutions:

  • RX part uses upconverter,
  • TX part downconverter
  • 30MHz is local oscillator freq.
  • NCO is calculate-program separately for RX and-or TX (30MHz-NCO=desired freq)
  • LPF on RX should be increased how it received freq. decreases (TX LPF also) - 40MHz for 20m band
  • minimum sample rate for entire HF band is from formula 30MHz-16*fs=0.1MHz (16 is max decimation factor, 30MHz local oscillator): 1.86875‬MS/s
  • RF gains depends on band condition, antenna, preamplifier…
  • LimeSDR-USB with modifications can go down to 0.1MHz, Mini is limited down to 10Mhz (modification will include removal metal shielding, possible loss of warranty?)
  • RX and TX GFIRs are optional but highly recommended
    In any case, after your program is finished load Limesuite-GUI and check which parameters are written correctly and which are wrong.

73 de yt7pwr



Thank you for the response. I must admit I really am a noob in this business. I had put in a far too narrow analog LP filter in both RX and TX and had not fully understood the concept of the NCO and up/down conversion.

Now I have a narrow GFIR filter (500 kHz) and a wide analog filter (60 MHz) in both RX and TX. I am running with 2 MHz sample frequency in both RX and TX to cover most of the HF band.

So now it works even below 10 MHz. Running with an RX and TX NCO at 29 MHz, at 3.5 MHz the RX sensitivity is around -100 dBm and a TX power of -13 dBm without modifying the Mini.

Next task is to find out how to read and write to the GPIO pins using GNU Radio. When that is done the rest is purely filtering and amplification.

vy 73

Jorgen, OZ7TA