Configuration/HW fitting for GNSS / GPS Receiver

Hello everyone,

We are working on a GNSS / GPS receiver application. Therefore, the LimeSDR’s configuration has to be best fit for this type of application. In our initial experiments we used a setup including Laica antennas, an external LNA and certainly the LimeSDR. Without the LNA we did not get a high enough RF levels, unfortunately.

The longer I work with the API the more question I am asking myself. Now, my aim would be to get rid of the external LNA and to understand the internals of the LMS7002M a bit more. Therefore a few questions to the more experienced ones in this Forum:

  • 1: Gains: The LMS7002M has three gain stages before the ADC. As already discussed here shall the level of the TIA gain have quite an impact. In our first tires we also recognised some intermodulation products. We figured out that we can get rid of them changing in LimeSuite under the tab RFE the “Decoupling Cap at output of RX mixer” to a higher value then the default 400pF. What are other consequences of this step?
    Back then we were not really aware of the relation between the gains. Therefore we didn’t pay attention to the individual levels but rather tried couple of constellations. Any hints what could be the right approach to get the optimal level for the three gain stages (LNA/TIA/PGA)? Did anyone manage to get it run without external LNA?

  • 2 Sampling Rate: Connected to the first question is this one. GPS is at 1.57542GHz and the signal of the L1 has a bandwidth of about 1MHz. Some oversampling was required, 8 Msps achieved a good enough signal" for our initial test. Perhaps some experimental values for this would be interesting, also about the digital-analogue oversampling ratio.

  • 3 The best software won’t fix an issue if the hardware is not yet optimised! With this I address the fact that the RF input ports are matched to mobile applications at 800-900 and 2000MHz or then wideband but with a high noise figure. As the LMS7002M-Data-Sheet-v2.8.0.pdf states are the three input ports - RXLNAL, RXLNAH and RXLNAW - slightly different but for a given frequency below the 2GHz range they all should be the “same”. Is this right?
    Therefore the only difference would be the matching network on the LimeSDR which is either matched to a too low or too high frequency for GPS applications.
    Did someone already do a modification to get the antenna matching network matched to the GPS range? In my opinion the best bet to increase the signal somewhat.

I would be glad to receive some hints about the configuration of the gains, sampling rates and the antenna matching network addressing GNSS applications.

Best regards, Reto