LimeSDR for 125KHz signal

Hi all,
I’m trying to read and replay a 125KHz signal using the LimeSDR and GnuRadio. For this purpose, I built a low frequency loop antenna on my own. I checked that the antenna works using a signal generator and a spectrum analyser.
The problem is that when I plug the antenna into the LimeSDR I am not able to read anything except noise (I plug it in the RX1_L path).
Is there someone who has ever used the LimeSDR for this kind of signal and can help me understand why is not working properly?
Thank you in advance.

My experience is that even with modification you can forget all bellow 500 kHz.
Preamp like LNA4HF can help a bit only above 1 MHz.


I agree. Under 1 Mhz, the response drops. BUT, with a bandpass filter & LNA (Low Noise Amplifier) like something from the Minicircuits ZHL line, or as an example, make a lowball offer “Will not stay on base, parts are missing. No way to know if it works. Willing to chance it” in the offer. I would say about $60 would be my offer.
Feel free to message me as I am a STL builder, too.


Thank you for the answers guys.
I am a bit disappointed that the LimeSDR is not working for the promising frequencies as it is. I should have investigated deeply before buying it (I bought it for many purposes, but the main one is the LF stuff). I’m moving to other hardware.

If you already have the Lime, it will be the best choice for VLF in the SDR world. Yes, it has a downturn in sensitivity. Heck, it almost covers 4Ghz.You will definitely need an antenna that is somewhat worthy of the signal that you are trying to receive. A 1/4 wavelength will be a couple thousand feet long, as a guess.
When working VLF, you need specialized equipment. Add about $100 outboard tot he lime (PC not included) & you will have a fine VLF transciever.


must admit that my best companion ever for RFiD on 125 kHz
was an old Collins 51S1 RX :smiley: and than in PC through sound blaster audio…
Today with Lime SDR one may source the signal before 3rd mixer in range of 2-3 MHz.
Nice number of old fashion communication receivers are just waiting on the shelf
to be used like preselector or converter for modern SDR toys.

Despite what Lime says, the LimeSDR in not useable at 125 KHz for either receive or transmit. You will need an upconverter and downconverter to make it work.

@AA7QQ, I am also interested in the VLF/LF range, but have no experience. When you mentioned an outboard for the lime, do you mean something like this:
And then, do you know any good resource to explain how to configure the software (say, GQRX…) to know what frequency it is observing with one of these attached?

I have read some of VLF. has a VLF forum, It will be good to read up on interfacing. I have no direct experience.


I have a Ham-It-Up from NooElec and it works very well for upconversion. It should work equally well for downconversion but I haven’t actually tried it. It can’t do both at the same time, so you will need 2 for RX/TX.The frequency shift is always 125 MHz, so if the software can’t do the conversion, you can just do it yourself.

P.S. As I recall, the local oscillator on the Ham-It-Up isn’t that precise so there is an upgrade that improves it. For VLF, I would definitely get the upgrade. It doesn’t cost very much.

Dear all, we have to develop an application in the 125Khz range, RFID, but I am reading there are some difficulties, is it due to LimeSDR sensitivity in this range, any more comments? should we necessary employ an upconverter? is there was any hw upgradare in the lastes years? thank you!

Sensitivity at 125kHz isn’t going to be great and you may want to use an upconverter.