Hi all, I'm working on a project that involves modeling an incoming signal's phase as a stochastic process, and I'm seeing a weird phase artifact on the E320. It looks like a slow periodic phase perturbation - my best guess is something pulling a PLL, because it always returns back to a settled state. It occurs with any external clock, but not when using the internal clock. I either need to find a way to correct the behavior, or to understand the root cause so I can confidently consider a different USRP that won't exhibit this behavior.
I confirmed the same behavior on 3 different E320 radios, first using an external OCXO (3Vpp bipolar sinewave) and then using a benchtop function generator to create 10MHz square or sinewave clocks. In all cases with external clock, the phase artifact can be observed. I am using only UHD utilities, two radios, and simple offline processing of the samples: (1) Cable radio A (transmitter, an E320) to radio B (receiver, any USRP) with 30dB inline attenuation. Determine appropriate gains on both radios to ensure the receiver will receive a robust, unsaturated signal level. (2) Radio A uses UHD's tx_waveforms utility to send a 150kHz sine wave with 400MHz carrier frequency and 500kHz sampling rate, where reference clock can be internal (no problem) or external (problem). (3) Radio B uses UHD's rx_samples_to_file utility to capture 10 seconds of data at the same frequency and sampling rate, always using internal clock. This is repeated for various clock options on the transmitter, everything else held constant. In a theoretically ideal system, the unwrapped phase of the received baseband sinewave should be a line, but in reality it'll wander due to imperfect clocks, noise, and other systems effects. I want to see the wander, so my processing is: (1) Compute the unwrapped phase over the 10 seconds of the captured I/Q samples. (2) Compute the best-fit linear trend line of the unwrapped phase, and subtract it (3) Plot the unwrapped phase residuals Here are some images showing internal clock, external OCXO, and external function generator square wave: < https://imageshack.com/a/u1YW7/1 >. For all three cases I'm showing the unwrapped phase residuals over the full 10 seconds, and then plot zoomed into two seconds to show more detail. You can clearly see the periodic phase issues on both the external clock cases, but not the internal clock case. Is this a known issue? Any speculation on what might cause this effect when using an external clock? I can't figure out what the internal TXCO does that is distinct here-- they both feed into the same ADF4002 PLL. The internal clock runs at 20MHz, but I was able to try an external clock at that rate (required a 2-line patch to UHD) and it didn't make a difference. The only other USRP I have on hand is an N320, and this issue does not seem to happen on that radio model when I use the same OCXO. Thank you, -- David Raeman Synoptic Engineering
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