What are the common sources of error and uncertainty in RF transmitter calibration?

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RF transmitter calibration is a vital process for ensuring the accuracy and performance of radio frequency (RF) devices and systems. However, calibrating an RF transmitter is not a simple task, as there are many sources of error and uncertainty that can affect the results. In this article, we will explore some of the common sources of error and uncertainty in RF transmitter calibration, and how to minimize them.

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John Sposato
Yasin Miar
Barry Hack

1 Measurement setup

The measurement setup is the physical arrangement of the equipment and components used to calibrate the RF transmitter. It includes the signal generator, the power meter, the cables, the connectors, the adapters, and the load. Any imperfection or mismatch in the measurement setup can introduce errors and uncertainty in the calibration. For example, the cables can have losses, the connectors can have reflections, the adapters can have phase shifts, and the load can have impedance variations. To reduce these effects, it is important to use high-quality components, to match the impedance of the devices, to avoid long or coiled cables, and to calibrate the measurement setup before and after the transmitter calibration.

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Barry Hack
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Remember that transmitter calibration is not just about power. You also need to consider frequency error and modulation. In digital systems the quality of the transmitted modulation can be significant and compromise the radio link budget. If transmit power is good and receiver sensitivity is good it does not imply good reception (even with a perfect channel without interference).

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Marcos Hernández V.

Ingeniero Electrónico
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To obtain measurements that are closer to accuracy, a reliable instrument with good adapters and efficient connections is needed. Measuring equipment should always be recalibrated at the measurement site, as the environmental factors of the location can alter precise measurement.

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2 Environmental factors

The environmental factors are the external conditions that can affect the RF transmitter and the measurement setup. They include the temperature, the humidity, the pressure, the vibration, and the electromagnetic interference. Any change or variation in these factors can cause errors and uncertainty in the calibration. For example, the temperature can affect the stability and drift of the devices, the humidity can affect the dielectric properties of the components, the pressure can affect the air density and refractive index, the vibration can affect the mechanical alignment and contact, and the electromagnetic interference can affect the signal quality and integrity. To minimize these effects, it is essential to perform the calibration in a controlled and shielded environment, to monitor and record the environmental factors, and to apply appropriate corrections if needed.

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3 Signal quality

The signal quality is the characteristic of the signal generated by the RF transmitter and measured by the power meter. It includes the frequency, the power, the modulation, the distortion, and the noise. Any deviation or degradation in the signal quality can lead to errors and uncertainty in the calibration. For example, the frequency can have drift or jitter, the power can have fluctuations or spikes, the modulation can have errors or impairments, the distortion can have harmonics or intermodulation, and the noise can have random or systematic components. To improve the signal quality, it is advisable to use a stable and accurate signal generator, to adjust the power level and modulation parameters according to the transmitter specifications, to filter out any unwanted signals or noise, and to verify the signal quality with a spectrum analyzer or an oscilloscope.

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4 Calibration procedure

The calibration procedure is the method and steps used to perform the calibration of the RF transmitter. It includes the selection of the calibration standards, the application of the calibration factors, the estimation of the measurement uncertainty, and the documentation of the calibration results. Any mistake or omission in the calibration procedure can result in errors and uncertainty in the calibration. For example, the calibration standards can have uncertainties or drifts, the calibration factors can have errors or biases, the measurement uncertainty can have incomplete or inaccurate sources, and the documentation can have errors or inconsistencies. To ensure a reliable calibration procedure, it is recommended to follow a recognized calibration standard or guideline, to use traceable and certified calibration standards, to apply appropriate calibration factors and corrections, to estimate the measurement uncertainty using a valid method, and to document the calibration results clearly and completely.

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Jeffry Perrin

Retired
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I agree completely with John Sposato’s comment here. Here is a common story. My company had a new product design in a factory in Thailand. The customer rejected the first container because the WiFi calibration was really bad. Engineering samples were great. I was 30 years doing RF. I went to the factory. There I found loose connectors, bad cables, poor fixture registration, no SMA torque wrench (many SMA connectors failed due over torque). The factory test engineers did not have any experience above 1 GHz. 2.4 GHz and 5 GHz will kick your a$$ on ALL the mistakes you can get away with at low frequencies and low power.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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John Sposato
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The two most common source of errors are lack of an experienced RF engineer and proper design and analysis tools. That said, having an RF engineer with the proper experience can partially compensate for not having the most advanced analysis tools needed for the job at hand. There is no substitution for experience in an RF engineer and the wisdom and confidence to do the research or to reach out to another engineer to consult. There’s no shame in not knowing everything. Don’t BS your employer. I see many engineers who want to be RF guys, but they lack the academics, experience and most of all an RF mentor. RF is often called “black magic” and it is an unforgiving discipline. Know your stuff first. You can’t fake science.

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Yasin Miar
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Algorithms implemented by Python or other languages must be well designed to minimize the error as well as optimize the timing.

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What are the common sources of error and uncertainty in RF transmitter calibration?

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3

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1 Measurement setup

2 Environmental factors

3 Signal quality

4 Calibration procedure

5 Here’s what else to consider

RF Engineering

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