# Quick Question: Max Separation 2 Rx Channels LimeSDR

Can I receive two channels simultaneously with one having center frequency 900 Mhz and one having center frequency 1800Mhz ?

I am trying to receive two channels simultaneously on LimeSDR, using OsmoSDR source in gnuradio. I have read various similar threads, but I am confused whether receiving two channels with frequency separation more than 61 Mhz is possible or not.

It is not, as both RX mixers share the same PLL. So the maximum separation between both RX frequencies is limited by sample rate (it is achieved in digital domain using NCO).

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@ccsh : could you precise two things about usage of NCOs ?

• Can NCOs be used to listen to 2 different frequencies within the range of 61MHz, or instead can NCO allows to listen to two frequencies outside that range? I guess itâ€™s the first, but Iâ€™d appreciate to get confirmation.

• More significantly, if the two channelsâ€™ frequencies are separated by 40MHz, will they actually be limited to roughly 20MHz of bandwidth each (i.e. left center frequency minus 10MHz to right center frequency plus 10MHz => 60MHz in total)?

Sorry if my questions are dumb, Iâ€™m trying to understand how the NCOs actually work. Thank for anybody help on that subject, to learn new things is always a pleasure!

While theoretical details may be found in LMS7002m datasheet (look for a section called â€śTDD/FDD MODE ENHANCMENT OPTIONâ€ť), Iâ€™m gonna describe how I see it as it comes to practice

Basically if you want to transmit two different signals via two different TX antennas at different frequencies, and receive them in the same fashion (i.e. if you want to use full MIMO configuration), youâ€™ll have to meet two requirements at the same time:

• both channels DSP (i.e. NCO) frequency offsets must be chosen within a range of -samp_rate/2 and samp_rate/2
• combined bandwidth of both of your signals on frequency axis must fall into a range of -samp_rate/2 and samp_rate/2,

where samp_rate is the maximum sampling rate that you can use in your MIMO application without loosing samples.

Note especially the last sentence, as in other threads people are often talking about maximum sampling rate of 61.44 MHz, which is theoretical maximum sample rate available in SISO configuration. If you want to use MIMO configuration, maximum theoretical sampling rate will drop to 30.72 MHz, and practical sampling rate which youâ€™ll be able to use without loosing samples will be even lower, as it depends on the USB controller efficiency in your host machine, complexity of your signal processing tasks etc. For example, when simply plotting time courses and spectrums of received signals in GNURadio Companion, I was able to set sampling rate of no more than 27.46 MHz on quite powerful machine (i7-7700K processor, motherboard ASUS Z170 Pro Gaming).

Because they say that a picture is worth a thousand words, I think that both already mentioned requirements will become easier to understand when looking at following graphical interpretation of maximum possible signals bandwidths, which may be transmitted when youâ€™ll set sampling rate to 25 MHz and RF frequency to 1 GHz:

As for the whole description above: https://www.youtube.com/watch?v=L_IOsLYVKkY&feature=youtu.be&t=43s

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