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I am building a two-way communication setup based on software defined radios.

Setup: The Tx needs to send a sinusoid to the Rx, and the Rx - after some processing - sends its own sinusoid in return. It is basically a ranging problem.

Problem: What I want is to keep the Tx LO running during this time (without any transmission) so that its phase can be compared with the phase of the incoming signal from the Rx later.

Equipment: Currently I am using HackRF SDRs but I can get USPRs if needed. If a specific hardware is required for this purpose, I would be glad to hear about that as well.

EDIT: I just thought of a simple way to pose the question again. Does a typical LO (in HackRF for example but in other devices as well) maintain phase continuity when switching from Tx to Rx mode?

QMC
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  • Are you sure the SDR hardware is capable of this? – Joren Vaes Aug 04 '17 at 06:25
  • No, I am not sure. My initial thought was that it could be an automatic feature or could be done by setting relevant register bits in the microcontroller. – QMC Aug 04 '17 at 06:30

1 Answers1

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This is readily achieved, by either

a) using a design which utilizes the same oscillator and oscillator frequency for upconverting signals as it uses for downconverting, such the that the local oscillator does not need to change frequency when switching modes. This will work best with an IQ design where the target frequency falls at 0 Hz on the baseband side, or at least would +/- any error.

b) using entirely distinct transmit and receive paths, such that the transmit LO never changes frequency, and perhaps that the receive LO is phase locked to the same source and you are able to receive some siphoned off transmit energy to perform a reference phase comparison.

However, your question is a bit incomplete on the surrounding details likely necessary to obtain the actual benefit of such steps.

Chris Stratton
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  • 1. So do you know any SDR/device that implements option a (or less preferably option b) above? 2. What surrounding details do you think are required? – QMC Aug 07 '17 at 23:44
  • You indicate that you are using SDR's to do this. If so, and your LO is digital, all you have to do is keep generating it, however you are doing so right now. This kind of detail is what is missing from your question. Is your radio a direct-conversion or a superhet? How much of an SDR is it? is it all digital until the antenna, or is the output of your transmitter a modulating signal, such as a complex baseband symbol stream, then is mixed up to RF in the analog domain? – Jotorious Aug 10 '17 at 13:02
  • Additionally, "does a typical LO maintain continuous when switching between Rx/Tx?" Full duplex radios generally have independent local oscillators for the Rx and Tx path. But assuming they don't, are you changing frequency when you switch from Rx/Tx and back? More importantly, if there aren't 2 LOs, you obviously can't compare them. But if there are 2 LO's how do you get the phase of the transmit LO, without using your receiver? – Jotorious Aug 10 '17 at 13:10
  • I'm not saying this can't be done, but don't forget that PA's, LNA's, antenna matching networks, antenna feed lines and antennas will all create additional phase shift between the two ends of the link making comparing the phase difficult at best. These phase shifts should be relatively constant at a given frequency and given temperature so if you are using a CW carrier for the ranging what you are looking for might be possible. – Dean Franks Oct 10 '17 at 23:39
  • Comparing the phase will also only give you range within a single wavelength, you will have to use time of flight to determine the integer number of wavelengths you have to add to this range. Multipath will also likely be an issue. – Dean Franks Oct 10 '17 at 23:39