LimeSDR RXOUT and clock sync

Hello everyone,

The LimeSDR looks very powerful and I’m interested to get one for my project.

I have basically two needs:

  1. A direct access to the analog I/Q signals coming from a 2.4GHz RF BLE signal emitter, before any re-sampling
  2. Synchronizing two RF receivers (LimeSDR boards in that case) with the same external clock source, in order to get the RX I/Q signals from both receivers perfectly in phase (assuming the distance between the emitter and both receivers is the same)

I’ll use the LimeSDR as receiver only.

I have seen that the LMS7002M chip allows access to I/Q signals before the ADCs, through RXOUTI and RXOUTQ pins, regarding the datasheet:

The analog receive signals can also be provided off-chip at the RXOUTI and RXOUTQ pins by closing the RXOUT switch. In this case it is possible to power down the on chip ADCs/TSP and use external parts, which can be very useful for more resource-demanding applications or where higher signal resolution is required.

So here’s my two questions:

  1. Are RXOUTI and RXOUTQ pins available on the LimeSDR board ?
  2. Will it be possible to synchronize two boards in order to get perfectly in-phase I/Q signals from two different board ?

Thanks a lot for your help!
Best regards,

Hi @acube,


LimeSDR-USB got reference clock synchronization circuitry. You may synchronize two boards from one external reference clock source or you may synchronize first LimeSDR-USB board from external clock source and the second one may be synchronized from the first board.

I do not know your application, but LMS7002M is 2x2 MIMO. Maybe one board is enough?

Hi @Zack,
Thanks a lot for your quick reply !

You’re right, for the beginning, one LimeSDR board could be enough to measure if there’s a phase-shift between the two receivers. It really depends on the sampling frequency of the IQ signals. The idea is to be as precise as possible to detect nanoseconds of phase-shift.

Assuming I will use the LimeSDR for my project:

  1. Will it be possible to get the I/Q signals on the GPIOs (or some external pins) ?
  2. If yes, what will be the sampling rate of the I/Q signals at that stage ?

Regarding the last question: I read in another post (here) one of your reply saying that the ADC works at 80MHz :

“You have 4 ADCs running at 80MHz in parallel; this is 4 Samples * 80 MHz = 320 MSps (mega samples per second);”

  • If I get it correctly, that means the I and Q signals will be sampled at 80MHz by the IQADC. Is that correct ?
  • Cause in the LMS7002M datasheet, I read that the ADCs are running at 160MHz, am I missing something ?
  • Is there a way to increase the sampling rate ?
  • After the IQADC, is there any other re-sampling of the IQ signals ?

Thanks a lot for your help.

Hi @acube,

There are only 8 GPIOs on LimeSDR-USB board. But I doubt if this is enough for you. But yes, it is possible to route some LimeLight (digital interface) lines to these GPIOs in FPGA.

It may be the same as at digital interface between LMS7002M and FPGA.

ADCs may run at 160MHz. We engage decimation by two to cut sample rate by 2. Hence 80MHz.

You have to consider interface. Pins at the interface are capable to achieve ~150MHz frequency DDR mode. Hence you have to fiddle between ADC sampling rate, decimation, check interface frequency constraint to maximize bandwidth.

Check Figure 11 in the datasheet. As you may see, there is decimation block. Hence, the only resampling in LMS7002M may appear in TSP only.

It is possible to bypass decimation, but then you have to supply interface clock externally which must Fadc x 2 (ADC sampling clock frequency times 2) in MIMO mode.

@Zack you’re amazing, thank you for all you answers. Helped me a lot.

I don’t think I’ll get the precision I need if the IQ signals are sampled at 160MHz. I will definitively need the RXOUTI and RXOUTQ pins in order to perform my accurate phase-shift measurements.

I found the LMS7002 Evaluation Board on digikey (here) but it’s really out of my budget.

To your knowledge, is there a board with a LMS7002M on it, who provides these RXOUT pins and is cheaper? :slight_smile:

Thank you!

Hi @acube,

UNITE7 is designed for LMS7002M transceiver evaluation hence all the features are accessible and this is the only board which got a convenient access to the RXOUT pins we have.
There is LimeSDR-QPCIe board which features external 14 bit ADC connected to RXOUT pins, but sample rate is 160MHz too.