LimeSDR TX and unknown features in RX

Good evening,

I am developing a LimeSDR based GUI in Qt/C++ to be used for spectroscopy applications. However, I’m still struggling to understand some streaming behaviours of the board and I’m completely stuck right now.

My current goal is to design and transmit RF pulses with arbitrary shapes, bandwidth and carrier frequencies. So far, I have implemented the code necessary to generate them, i.e, generate an array of data points, with carrier frequencies ranging from 25-50MHz, but I get some features in the RX that I can’t avoid. Now i’m trying to transmit that array and receive it again with the LimeSDR.

To test my TX application, I just manually select the TX1_1 and the RX1_L antennas and connect them both like the following:

However, I’m getting some issues. First of all, I always have some weird features relatively to the first acquired data points:

These are the data points that I plot from the rx_buffer. In this case, the tx_buffer is a test array with 50000
samples (I,Q)=(1000,0), so I would expect that, by setting RX_LO = TX_LO, that I would also get an RX with a similar appearance (plus some noise). Is this wrong?

The code that I am using for the this part is described below:

#define CHANNEL_A 0
/* lms_device_t* board is initialized before on specific menu */

/* These are fixed test settings just for this situation. The user can actually change them in the GUI on a normal use*/

if (LMS_EnableChannel(board, LMS_CH_TX, CHANNEL_A, true) != 0 || LMS_EnableChannel(board, LMS_CH_RX, CHANNEL_A, true) != 0)

if(LMS_SetAntenna(board, LMS_CH_TX, CHANNEL_A, 1) != 0)

if(LMS_SetAntenna(board, LMS_CH_RX, CHANNEL_A, 2) != 0)

if (LMS_SetSampleRate(board, 1e6, 32) != 0)

if(LMS_SetNormalizedGain(board,LMS_CH_RX, CHANNEL_A, 0.4)!=0)

if (LMS_SetLPFBW(board, LMS_CH_TX, CHANNEL_A, 5e6) != 0)

if (LMS_SetLPFBW(board, LMS_CH_RX, CHANNEL_A, 5e6) != 0)

if (LMS_SetGFIRLPF(board, LMS_CH_RX, CHANNEL_A, true, 3e6) != 0)

if(LMS_SetLOFrequency(board, LMS_CH_TX, CHANNEL_A, 40e6)!=0)

if(LMS_SetLOFrequency(board, LMS_CH_RX, CHANNEL_A, 40e6)!=0)

if (LMS_Calibrate(board, LMS_CH_TX, CHANNEL_A, 5e6,0) != 0)

if (LMS_Calibrate(board, LMS_CH_RX, CHANNEL_A, 5e6,0) != 0)

lms_stream_t tx_stream;
lms_stream_t rx_stream;
const int number_samples = 50000;
int16_t tx_buffer[number_samples * 2];
int16_t rx_buffer[number_samples * 2];

for(int i=0; i<number_samples; i++)
    tx_buffer[2*i] = 1000;
    tx_buffer[2*i+1] = 0;
} = CHANNEL_A; //channel number
tx_stream.fifoSize = 1024*1024; //fifo size in samples
tx_stream.throughputVsLatency = 1; 
tx_stream.isTx = true; //Selects Tx or Rx
tx_stream.dataFmt = lms_stream_t::LMS_FMT_I16; //16-bit integers

if (LMS_SetupStream(board, &tx_stream) != 0)
       Error(); = CHANNEL_A; //channel number
rx_stream.fifoSize = MSAMPLE; //fifo size in samples
rx_stream.throughputVsLatency = 1; 
rx_stream.isTx = false; //Selects Tx or Rx
rx_stream.dataFmt = lms_stream_t::LMS_FMT_I16; //16-bit integers

if (LMS_SetupStream(board, &rx_stream) != 0)

if(LMS_StartStream(&tx_stream) != 0)

if(LMS_StartStream(&rx_stream) != 0)

if(LMS_SendStream(&tx_stream, tx_buffer, 50000, &tx_metadata, 1000) == -1)

int samples_read = LMS_RecvStream(&rx_stream, rx_buffer, 50000, NULL, 1000);

cout << samples_read << endl;


  • Some code to plot rx_buffer[2i] and rx_buffer[2i + 1] *
    Stop and destroy stream code stolen from the examples.

I have tried thousands of diferent combinations of these settings and I always get the same type of features, which makes me wonder: Is that RX actually something, or is just a lot of noite? I have set the stream to high latency in order to avoid the L character. I’ve also tried to bypass the DC corrector, assuming that I just had slowly varying signals, but with no progress

One thing that I have noticed is that, if I reduce the interface_rate (not the oversampling factor), these features shift in time, so I believe they may be some sort of initial features that RX stores before TX samples arrive (??). If so, how can I correctly setup RX in order to receive the samples when I want? I can’t solve this problem using metadata (or at least I couldn’t).

Also, is this doable at so low LO frequencies? I understand that the LimeSDR TX/RX are not optimized for such low frequencies, but is it possible to hack the hardware in some sense in order to get some better results? I don’t want to ruin this beauty.

Can some one help me, by shedding some light into this problem? It would be of great use any kind of help necessary. I really don’t know what to do and I wouldn’t want to abandon this project right now. What can you recommend me? What am I doing wrong?


That is affected by the matching networks on the board, so I think for testing purposes you could use chip’s internal RF loopback instead of the cable to have better results.

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Thank you for your quick reply. Indeed I can confirm that this glitch only happens during the very first acquisition (independently of the lime configuration) and disappears after it, i.e, if I repeat the process again and again. However the IQ data are then kind of random in amplitude and frequency?! Does this happens because of matching networks? Will I get more stable outcomes if I use te RF loopback? I’m going to test it, thanks!

But there really is no way to solve this without ignoring some initial samples / the first acquisition?

Also, when I try the exact same code but with 12 bit integers, I get this:

. Apart from the fact that the glitch is there (this is a first aquisition when the GUI is launched), what I want to understand is why the following “signal” is almost inexistent. It doesn’t matter how many times I repeat the experiment, I will get an almost zero signal with the same code that gave me a signal with 16-bit integers.

I think what you were seeing was just noise and nothing else. Chip has only 12bit ADC, so when you select 16bit integers they are the same values just upscaled into 16bit range. 2047(12bit) == 32767(16bit). Also I believe for connecting RF output and input like you did with the cable would need to use some kind of attenuator or set the chip Tx/Rx gains to very low levels. In this case using chip’s internal RF loopback would give you more stable result and give more control for gains.
I suspect the glitch at the start comes from firmware so software will not be able to fix it, options would be to just discard some of the beginning samples, or start the stream early and let the garbage samples be overwritten.

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Just to update the situation: it is actually possible to do this! I just wrote some tx_buffer with values for a decaying exponential and have sucessfully (I’m 99% sure) transfer it to RX.

Why did it work this time? I don’t know exacly. I just changed the strategy a little bit and wrote the tx data to a file then I read it back to the tx_buffer in int16_t data points format. I’m actually surprise this did the trick. Now I just have to optimize the noise reduction. I can also guarantee that the number of initial data samples to be ignored is directly proportional to the interface rate between the host CPU and the board. And this number is constant for each interface rate. I think I will make a calibration fit and share it. Do you think this issue will be fixed in the future, thought? It is kind of annoying :stuck_out_tongue:


Could you log this on the issue tracker at:

Also, just add: what an incredibly cool application! Is there or will there be any write-up on the other hardware used and overall process/technique?