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Hi,
I am interested in implementing an analog transmit beamformer by using three or four RF interfaces on an WARP board.
I have learned that since each RF interface generates a carrier signal by using its own local PLL, a random phase offset will always exist between any two RF interfaces. Thus, I must compensate those offset values before applying per-antenna beamforming weight.
I have also noticed a possible solution called phase sweeping. But I'm also wondering if I could simply use an oscilloscope with a certain kind of cable to directly measure the signal applied to two antennas (let's say one is connected to RFA and the other RFB), so that I can read the offset between RFA and RFB from the oscilloscope. Thanks for help in advance!
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Your description of the phase offset between radios is correct. Keep in mind this inter-radio phase will change to a new random offset every time a radio re-tunes (powers up, resets, or changes center frequency). You will need to re-run the calibration process every time this occurs. You could measure the phases directly with an oscilloscope, but the scope must have wide enough bandwidth to capture the RF signal (i.e. expensive scope). A common alternative is to use one radio as a receiver to monitor waveforms from each transmit radio, adjusting the phase to each until all are aligned, then repeating this after swapping Tx/Rx roles.
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murphpo wrote:
You could measure the phases directly with an oscilloscope, but the scope must have wide enough bandwidth to capture the RF signal (i.e. expensive scope).
Thanks for the clarification. I'd like to transmit through a channel centered around 2.4 GHz with 2MHz bandwidth. So does that mean the bandwidth of my oscilloscope has to be at least 2MHz, or 2402 MHz? If I am wrong, could you explain how I can figure out the required oscilloscope bandwidth for my case?
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Oscilloscope bandwidth is typically specified starting at DC (0Hz). You'll have to study the specs of your instruments to gauge whether they're suitable for measuring 2.4GHz signals.
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