I have designed a simple TX/RX system in GRC with a single USRP N210 and CBX-40.

I am transmitting sinusoidal signals at various frequencies within the 2-6GHz range and detecting the amplitude and phase of the received signals at each frequency when connecting a coaxial wire from the Tx port to the RX port of the USRP.

I am doing the FFT at reception and "looking" at the correct sample of my IF (samp_rate/bin_size*N). I therefore detect the amplitude correctly.

However, I observe that the phase is only detected correctly (hence, it has always the same value because the cable connecting both ports has a fixed length), for certain frequencies. In my case, for 2GHz, 2.1GHz, 2.2GHz etc. (every 100MHz).

I would like to have a finer phase resolution and I don't know what of the HW components (could be the PLL) or SW settings could solve my issue. Could it be related to the "tuning policies" of the USRP?

Thank you.

  • 2
    $\begingroup$ This might be better asked in electronics.stackexchange.com. Phase will not be constant over frequency but will ramp up depending on your cable length. But I agree there could be some subtle details in the way the Tx and Rx tuning is done. For absolute phase measurement, usually you would compare a short internal reference cable to the external cable, perhaps with two RF switches. $\endgroup$
    – tomnexus
    Sep 29 at 12:47
  • $\begingroup$ Thank you for the advice. Will do! $\endgroup$
    – Fati
    Sep 30 at 9:47

2 Answers 2


(Ettus support here) You can't tune the RX and TX LOs of the CBX to defined relative phases; the fractional-N synthesizers can't do that.

I have my doubt that it actually works every 100 MHz – the hardware just can't synchronize phases (this is not just some misled application note – it's a hardware constraint of the LO synthesizer).

So, what you want to do is impossible.


Even if using the same oscillator, the measured received phase will vary depending on the ratio between the delay (due to transmission line, coax, connectors, and other propagation parasitics) between the two measurement reference points, and the period or wavelength of the signal whose delta phase is being measured. e.g. If the coax plus parasitics adds up to exactly one wavelength or period (1/f) in delay, then the phase might be zero. If an odd multiple of half a wavelength, then the phase might be 180 degrees. etc.

This assumes things such as the oscillator source be equidistant in time between the two measurement mechanisms, etc.


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