Antenna / RF Filtering

I’m using a LimeSDR board for development, it’s going well. Will the final LimeSDRs which we have ordered have filters on the antenna inputs? My board has 700-900MHz and 2-2.6GHz (I think). Ideally there would not be filters, or if there are then the frequency range supported by the Lime chip shouldn’t be compromised.

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There won’t be filters on the final boards and these should be pretty much as the one you have is.


Ah - so no hardware filters - that’s good (I think). Any chance of a bit of a circuit diagram (better - picture) showing me what I have to do to remove LNA_L (or any other filter) please? I can solder, just have aging eyes :slight_smile: .

Pretty sure that the only thing between the U.FL connectors and RFIC — which has RF inputs/outputs for different bands, which are selected via software — is a balun. Schematics:

Excellent - there’s no way whatsoever that I can work on these filters - 10x too small. I’ll see whether I can maybe send the board to Lime to have the components removed. My last big hardware project had a 12BY7A as the driver and 6.3v heaters :slight_smile: .

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@andrewback according to your newer answer, schematics for LimeSDR in GitHub are for the final LimeSDR board. But there are GSM/3G/LTE specific filters. Which is really bad, a lot of people want to use LimeSDR as general SDR, in radioamateur bands, etc. Not only in GSM/3G/LTE bands. You were marketing LimeSDR as general flexible SDR with range from 100 kHz to 3.8 GHz, but with these filters, it is not true at all. And physically removing those tiny components from LimeSDR board is beyond ability of most people.

So how it really will be? You contradict yourself in those 2 answers (“There won’t be filters on the final boards” VS “these are the schematics for the boards that people will be receiving and are not a special version done for GSM development”).


Hi Andrew, LimeSDR should be fine for the HF bands with the new matching networks. However, the LimeSDR gain (LNAL) shows a moderate drop as one goes below perhaps 600MHz. Is this decrease due to the antenna matching components, and is it significant at HF?

I have a research interest that requires wideband response extending into the lower (VHF) band and I wonder if there is a way to maintain the LimeSDR gain into the VHF region. It occurs that I might build my own balun, if I can access chip Rx pin. Or perhaps LNAW will prove useful. Thanks for suggestions.


Looking at my Crowd Supply board I see what suspiciously look like filters in the RX inputs - the small coils have (or appear to have) the same markings as the 1.2 board I was sent some months ago to prepare the software.

I’m not noticing much difference is any in signals in the 88 - 108MHz range between the two boards.

How can I confirm that there are in fact no filters present?

As stated previously, these are matching networks and not filters.

@Zack, could you add PDF schematics to the LimeSDR-USB GitHub repo, so that people without Altium can at least examine the schematics.

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Thanks - I’ll have to play with settings, at the moment I just can’t get signal levels as high as they should be - must be muppetry this side of the keyboard :slight_smile: .

@andrewback, @SimonG4ELI

PDF schematics added. Check it here for socket version and here for plug. The only difference between socket and plug versions is USB connector. Everything else is the same.

Is the RF range for the RX L really only 700 MHz-900MHz?

This would mean LimeSDR is not really usefull for HamRadio on HF :frowning:

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Hi Sabine, the frequencies stated against the ports on the schematics are indicative of where the optimum performance is for those matching networks. However, the LimeSDR operates across the full frequency range stated, albeit not at optimal performance across the entire range.

Plots of the sensitivity and noise figure vs. frequency are being generated and we will share those soon.

No , I am not agree with you . The input circuit is designed very bad and it almost can not use for HF range. LimeSDR team need to redesign the input circuit

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What does ‘optimal’ mean though, Andrew? Are we talking a few dB or a few tens of dB?

There is work under way to plot the sensitivity, which should make this clear. We also plan to provide guidance on how optimal performance can be achieved outside of where it currently exists.

This should also show a comparison between the version 1.2 and version 1.4 performance . A lot of people made buying decisions based on the wide frequency range stated in the marketing material and results published by early tester using the the version 1.2 boards .

This is what was offered as the broadband specification for the 1.4 version boards . Lime Micro need to confirm that they are meeting this specification target with the production boards across the full frequency range

Has anyone here actually measured the matching sections yet to determine what the loss curve is?
I see a lot of people getting angry and concerned and making wild speculation who don’t have one in hand yet.
I saw something that looked like a subjective report from someone but they didn’t have a comparison with their own gear and they didn’t actually sit down with a spectrum analyzer and run it through it’s paces either so the report is totally subjective and meaningless.
So, who here has run these through an analyzer and knows what they are talking about?
Share the results.


Hello Simon,

I just went through the v1.4 diagram. if you are using the RX1H or RX2H antenna port be sure to control properly the signal path. There is a RF switch on both ports that enable you to loop back the TX signal from the TX2 port to the receiver (RX1H and RX2H) That RF switch is the SPDT where antenna port may be selected or the TX2 port. If your loopback from the TX port is selected there should be no signal or very weak signal present on the RX port from the antenna.

The other ports RX1L, RX1W, RX2L, RX2W does not have the RF switch and the antenna is coupled to the LMS7002 chip through the Minicircuits transformer TC1-1-13MA.
According to the datasheet, the mentioned transformer losses are 1dB up to 1GHz, 2db up to 2GHz and 3db up to 3GHz.
Return loss and matching is really good on the lower frequencies, nothing to worry about.

The transformers are 1:1 and there is nothing to un-solder or remove from the v1.4
there are no filters on the antenna ports, nothing except the mentioned RF SPDT RF switch on the RX1H and RX2H ports.