Measuring phase shift from cable length

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carson.teale
Posts: 2
Joined: Tue May 12, 2020 5:01 pm

Measuring phase shift from cable length

Post by carson.teale »

Hi,
I'm trying to measure the phase shift that a carrier acquires when propagating through a length of cable. I have the bladeRF x40. I simply connect a 1 m sma cable from the TX port to the RX port. I'm using a Matlab script to configure the bladeRF using cli commands. My Tx samples are just ones, so that I just transmit a tone at the carrier frequency at 900 MHz. Then I expect to be able to obtain the phase acquired from propagation through the cable just by atan(imag(rx_sig)/real(rx_sig)). But when I try this, the measured phase is not consistent. It seems to be random between different measurements. Is there some fundamental reason why this won't work? Or am I just not implementing it incorrectly? I don't get why the phase measurement is not repeatable.
I should be able to use this phase measurement along with the permittivity of the cable I'm using, to back out the length of the cable (modulo the wavelength).
encreed
Posts: 1
Joined: Fri Oct 22, 2021 4:12 am

Re: Measuring phase shift from cable length

Post by encreed »

The sign does to be sure move at the speed of light for that medium, which is typically some type of plastic protecting the cajole.

Most strong plastics have a dielectric steady of somewhat more than 2, so their speed of light will in general be around the 0.65c territory. Frothed dielectrics have a lower dielectric consistent, so support quicker flags.

To a decent first estimate, you can utilize the equation

length = wavelength(in link) * (stage shift in degrees/360),

where the wavelength(in link) = 0.65c/recurrence = 0.65wavelength(in air)

Assuming you need more precision than that rough 0.65 figure, then, at that point, look into the information sheet for the speed of signs in your particular sort of link.

As the impedance of the link relies upon the sqrt(ratio) of L and C per unit length, and the speed relies upon the sqrt(product) of L and C, the speed and the impedance are not straightforwardly related.
tracyberge
Posts: 1
Joined: Tue Sep 05, 2023 5:33 am

Re: Measuring phase shift from cable length

Post by tracyberge »

There are a few factors that could contribute to the inconsistency in your phase measurements when using the bladeRF x40 and attempting to measure the phase shift through a cable. Here are some possible reasons and suggestions to consider: Donkey Kong

1. Cable Variability: The cable you are using may have inherent variability in its electrical properties, such as impedance, capacitance, and attenuation. These variations can introduce inconsistencies in the phase shift measurement. It's important to ensure that the cable you are using is of high quality and has consistent specifications.

2. Interference and Noise: External interference from other devices or sources, as well as noise in the RF environment, can affect the accuracy of your measurements. Make sure you are conducting your measurements in an environment with low RF interference and minimize sources of noise.

3. Calibration: It's crucial to calibrate your bladeRF system to ensure accurate measurements. Proper calibration involves calibrating the Tx and Rx paths, compensating for any phase imbalances, and accounting for any system-specific characteristics. The bladeRF documentation or support resources should provide guidance on how to perform calibration for your specific device.

4. Signal Averaging: Taking multiple measurements and averaging the results can help mitigate the effects of noise and variability, leading to more consistent phase measurements. Consider implementing signal averaging in your Matlab script to obtain more reliable results.

5. Timing and Synchronization: Ensure that the timing and synchronization between the transmitter (Tx) and receiver (Rx) are properly aligned. Any timing offset or synchronization issues can introduce errors in the phase measurements. Double-check the synchronization mechanisms you are using and verify that they are functioning correctly.

6. Measurement Resolution: Ensure that you are using a sufficiently high-resolution sampling rate and capturing enough samples to accurately capture the phase shift. Inadequate resolution or insufficient sample size can lead to inaccurate measurements.

7. Consideration of System Limitations: Keep in mind the inherent limitations of the bladeRF x40 device itself. It may have certain performance characteristics or limitations that could affect the accuracy and repeatability of your measurements. Review the device's documentation and specifications to understand its capabilities and any potential limitations.

It's also worth mentioning that utilizing the phase measurement and cable properties to back out the cable length may require more complex calculations and consideration of additional factors, such as the velocity factor of the cable and signal propagation characteristics.

If you continue to experience difficulties with achieving consistent and accurate phase measurements, it could be beneficial to reach out to the bladeRF support community or seek guidance from experts familiar with the specific device and measurement setup.
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