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USRP B210 1PPS Trigger

From: Hasan Nayir
Subject: USRP B210 1PPS Trigger
Date: Thu, 19 Nov 2020 14:55:47 +0300


We are trying to transmit signals at the same time from USRP's connected to different PC. For this purpose, we are using 1PPS and 10MHz reference signal. We connected 1PPS and 10MHz coming from octoclock to USRP B210 ports. We have some issues that cant trigger transmissions at the same time with 1pps. In order to better understanding of 1PPS and 10MHz reference signals, we have done some tests. Test results given below.

Fig 1:  The first of these tests was done by checking the signals sent by the two USRPs identified in the same GNU Radio flowchart. In this test scenario, 1 PPS and 10 Mhz reference signals were connected to the USRP. We have seen that the phase differences remain constant throughout transmission so we think it uses 10 MHz properly. However, since they are not triggered at the same time, there is still a shift in the initial times of the transmitted signals. In Figure 1, the reason the signals seem to be sent with better synchronization because they use the same GNU Radio flowchart for two different USRPs. Because when we tried the same test by removing 1 PPS (even at 10 MHz, see: fig3), the signals were sent again at similar starting timings as shown in figure 1.

Fig 2: In this scenario, we take off 1 PPS signals from the USRPs and connected only the 10 MHz reference signal, and as a result of this test, although there was no synchronization in time, there was a constant phase difference between the carriers of the signal we sent.
Fig 3: In order to better understand the contribution of 10 MHz, we performed a test by removing both 1 PPS and 10 MHz reference signals from the USRPs this time. There was a non-constant phase shift between the carriers of the signals we sent to this test.

Fig 4, Fig 5: As can be seen from the figures, it seems that the USRP RF signals are not triggered by 1 PPS signal. ( yellow one 1 PPS, blue one transmitted RF signal with 1 Hz pulses)

As far as we understood from the tests, we observed that although we connected 1 PPS signal to USRP, we could not send a signal with 1 PPS. How can we trigger to transmit signals using 1PPS?

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