Unstable Phase Differences in HackRF Synchronized Transmission

What outcome were you hoping for?

I have a saved signal source that I want to transmit using the hackrf_transfer command for synchronized transmission. I'm using a one-master, two-client setup and following the steps mentioned in #1386. I expected no phase difference between the two channels.

Goal: To observe overlapping signals (from two clients) on the oscilloscope, ensuring stable phase and voltage.

GNU flowgraph used to save the signal file:

What outcome actually happened?

However, I observe different phase differences each time I execute the hackrf_transfer commands.

My setup:

Commands I used (executed each command in a different terminal window):

hackrf_transfer -t sine.raw -f 500e6 -s 2e6 -a 1 -x 20 -d 0000000000000000f75461dc2c983ac3 -H 1 hackrf_transfer -t sine.raw -f 500e6 -s 2e6 -a 1 -x 20 -d 0000000000000000f75461dc282f83c3 -H 1 hackrf_transfer -t sine.raw -f 500e6 -s 2e6 -a 1 - x 20 -d 0000000000000000f75461dc2a3261c3

What operating systems are you seeing the problem on?

Ubuntu 24.04.1 LTS (Codename: Noble)

What is the output of hackrf_info?

hackrf_info version: git-71bc3940 libhackrf version: git-71bc3940 (0.9) Found HackRF Index: 0 Serial number: 0000000000000000f75461dc282f83c3 Board ID Number: 4 (HackRF One) Firmware Version: 2023.01.1 (API:1.07) Part ID Number: 0xa000cb3c 0x00664f68 Hardware Revision: r9 Hardware appears to have been manufactured by Great Scott Gadgets. Hardware supported by installed firmware: HackRF One

Found HackRF Index: 1 Serial number: 0000000000000000f75461dc2c983ac3 Board ID Number: 4 (HackRF One) Firmware Version: 2023.01.1 (API:1.07) Part ID Number: 0xa000cb3c 0x006f4764 Hardware Revision: r9 Hardware appears to have been manufactured by Great Scott Gadgets. Hardware supported by installed firmware: HackRF One

Found HackRF Index: 2 Serial number: 0000000000000000f75461dc2a3261c3 Board ID Number: 4 (HackRF One) Firmware Version: 2023.01.1 (API:1.07) Part ID Number: 0xa000cb3c 0x004b4f5b Hardware Revision: r9 Hardware appears to have been manufactured by Great Scott Gadgets. Hardware supported by installed firmware: HackRF One

Are you using any third-party software?

GNU Radio 3.10.9.2

Are you using any third-party hardware?

N/A