TX/RX frequency range documentation #1

alexwhittemore · 2016-05-03T21:00:18Z

This isn't necessarily an issue, but for lack of a better place to discuss, I'll ask here: the LimeSDR is advertised with a working frequency range from 100kHz to 3.8GHz, but the schematic notes on the RF matching networks don't match this. I'm assuming (speaking of RX) that "RXRF2_L" is "low frequency", "_H" is "high frequency," and "_W" is "wideband." I suppose the overlap is to allow for mechanical design flexibility (using a single connection and antenna for full coverage) at the price of receive signal strength? Beyond that, both L+H and W seem to cover only 700MHz to 2.6GHz. Given my poor RF design skills, I don't know just looking at the matching networks if that's shorthand, or if those matching networks actually roll off sharply at those bounds and I'm missing something else. So I guess, what's the deal?

ztamosevicius · 2016-05-04T07:47:52Z

This is just a RF matching question. It may be tuned to lower frequencies using different components.

alexwhittemore · 2016-05-04T18:04:58Z

I guess the point is, is it a question of poor performance, or is it "you must modify the networks yourself to use the board at 100kHz"?

2ftg · 2016-05-06T12:32:20Z

The RF matching components are size 0402.
So while doable with a steady hand and sharp eye, we are getting in to the microscope, micro tweezers and preheater territory.
While I might be ready to stack some caps on top of the existing 100nF one's for improved LF response, I'm unsure how many others would be ready to do so.

Some stuff based on reading the schematics and googling:
TC1-1-13MA+ balun transformer is used for 30 MHz-1.9 GHz branch, 700 MHz-900 MHz branch and on the 700 MHZ-2.6 GHz branch. Both TX and RX.
Datasheet shows that it is specified for 4.5 to 3000 MHz. https://www.minicircuits.com/pdfs/TC1-1-13MA+.pdf

TCM1-63AX+ balun transformer is used in the 2-2.6 GHz branches for both TX and RX.
Datasheet shows that it is specified for 10 to 6000 MHz. https://www.minicircuits.com/pdfs/TCM1-63AX+.pdf

The coupling caps in the 30 MHz-1.9 GHz branch are 100nF, which should be usable on HF, but might be too little at LF.
I think the HF branch should work well for most of the HF bands and the 700-900 MHz branch should also work reasonably on 2m/70cm.
If the LimeSDR folks say that it will cover 100kHz-3.8GHz I'd say it does it, but the performance might be degraded at the band edges, this is my guess as an amateur and general radio hobbyist.
Here's a screencapture of the RF section for those without Altium or windows to run Altium viewer. https://i.imgur.com/GhfW6vr.png
I don't do RF design professionally nor do I represent the LimeSDR project in anyway. So take what I say with a grain of salt.

alexwhittemore · 2016-05-06T17:39:17Z

Thanks for the analysis! Better info than I'd have come up with.

Unrelated to the RF side of things, if you ever find yourself having to do a rework on components like that at your bench at home with a pretty-good-but-not-pro soldering station, get some ChipQuick. The stuff is seriously magical.

heitorPB · 2019-06-25T02:32:39Z

Are there any real tests at very low frequencies so we can check it?

ztamosevicius · 2019-06-26T08:57:54Z

Check this:
https://myriadrf.org/news/optimising-limesdr-matching-hf/

Jelly478558074 · 2020-05-18T05:50:46Z

LimeSDR USB，What is the name of the transmitting antenna?

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TX/RX frequency range documentation #1

TX/RX frequency range documentation #1

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