Electrical – Maintaining phase continuity of LO when switching from Tx to Rx mode

oscillatorsoftware defined radio

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?

Best Answer

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.

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