Looking at the unmodified schematic (pages 5 and 6): https://github.com/myriadrf/LimeSDR-USB/blob/master/hardware/socket/1v4/LimeSDR-USB_1v4s.pdf
The matching networks, which are what I would call front end filters, are optimal (by default, no modification) for:
LMS7002M RF1
TX1_1: 30MHz to 1.9GHz
TX1_2: 2GHz to 2.6GHz
RX1_L: 700MHz to 900 MHz
RX1_H: 2 GHz to 2.6 GHz
RX1_W: 700MHz to 2.6 GHz
LMS7002M RF2
TX2_1: 30MHz to 1.9GHz
TX2_2: 2GHz to 2.6GHz
RX2_L: 700MHz to 900 MHz
RX2_H: 2 GHz to 2.6 GHz
RX2_W: 700MHz to 2.6 GHz
The tuner can technically go from 100 kHz to 3.8 GHz, but the amount of attenuation on each input will vary as a function of frequency. It would be really good if Lime, or anyone else with the required hardware, could provide frequency response graphs from 100kHz to 3.8GHz on each of the 6 RX inputs (RX1_L,RX1_H,RX1_W,RX2_L,RX2_H,RX2_W) as the input frequency is swept from 100kHz to 3.8GHz with a constant input level of say -45dBm.
Looking at the above information from the schematic I would assume that on an unmodified board RX1_W and RX2_W or RX1_H and RX2_H should provide the least attenuation for 2.6 GHz to 3.8GHz The filters look like first order (or possibly second order), so the roll-off should be gentle. If it is first order then it has ~6dB per octave roll off. If I assume that it is a third order filter (~18dB per octave roll off) and that the attenuation was 10 dB outside the pass band (@3.8GHz), Iāve no idea what it actually is, but a theoretical 12-bit ADC has a dynamic range of 74 dB, so even if the signal of interest was being attenuated by 10 dB you should still be able to see it, if you are not then you need to look at adding external pass/block filters and external gain. While making sure that all signals entering the LimeSDR are less than +12dBm, I would be aiming for 0dBm, or even -10dBm, to give plenty of headroom. But I do not think that is something that you need to worry about if you are just looking at 2.6 GHz to 3.8 GHz, because the roll off is so gentle.
Not an easy question for anyone else to answer, only you know how you plan on using your LimeSDR. For me leaving one input unmodified, which will filter out more noise outside the pass band makes sense at least for me.
Many thanx for your explanation mzs.
So I assume you modified RX1_W (I will probably do the same).
@zack please add this information from this thread to the crowdsupply page.
The information that the LimeSDR can reasonably be used from 100kHz to 3.8 GHz is simply not true.
That should be visible on the page where you actually can still buy the Hardware!
@zack so with the current HW it is not possible to use anything above 2.6GHz without massive attenuation into near nothingness?
I completely agree with you on this (actually I have asked for it a long time ago, but it was completely ignored.
In the same thread where it was promised that the final HW will have broader filters)
Hi there, I also received LimeSDR 1.4 recently, but had no time to test it. Yesterday I managed to connect it. using USB 3.0 port, good antenna and good pigtail. It looks like my board is defective too. Iāve tested it using latest Kali linux and gqrx, compiled LimeSuite from source. It looks like, it is deaf, I canāt listen even to FM broadcast. Iāve tested my RTL dongle and I have pretty much higher signal.
@Zack could you please advice on how to test the board to say exactly if it is defective or not? What software should I use? Should I install Ubuntu or something specific software?
The board is completely unmodified, no hardware or software modifications were made.
Iām a bit disappointed with the purchase since Iāve waited for it for so long and received weak hw for $300.
This is not a HF issue, Iām not able to receive even 103.5Mhz signal better than when Iām using RTL. Okay, I will test the board later today and will get back to you. What if the board is defective one? What should I do?
GQRX has a gain control settings that are hidden in the interface but can be exposed on the right side panel if you click on the right tabs. Many people have the LimeSDR and are able to not only adjust the gain settings for better reception, but will often be MUCH better than your RTL-SDR (I know, Iāve owned about everything). Please look for the gain settings panel on the right side interface and also make sure you have the right antenna selected when you do this - - nothing will work if youāre connected to the Highband antenna port and youāre selecting no antenna or something other than the Highband.
Thanks for the advice, yesterday tried to play around with the settings, gqrx etc etc. Looks like it works but wierd, Iām still not sure if the board is working properly. I noticed no change when I choose RX1_L or RX_W, same noice, same reception, prob. it is gqrx issue. Also it is strange, but I have a signal on that frequency I choose, no matter what frequency just a yellow line on the waterfall. Will try ubuntu also later, have no time to install, so using kali for now.
For instance what is the difference between