Common indicators and test methods of RF PA

In wireless communication systems, the power amplifier PA in the radio frequency front-end is a very critical device. Its main function is to amplify low-power signals to obtain a certain amount of radio frequency output power. Because wireless signals are greatly attenuated in the air, in order to stabilize the quality of communication services, it is necessary to amplify the modulated signal to a large enough size and then transmit it from the antenna. It is the core of the wireless communication system and determines the communication system. It can be said that any wireless communication system is indispensable for its quality. We call it the crown jewel of RF front-end devices, which is actually not an exaggeration at all. This article will introduce common indicators and test methods of radio frequency PA.

Preparation Before preparing to test, it is necessary for us to prepare the equipment and components required for the test, such as the following list:

• dynamometer

• signal source

• Spectrum Analyzer

• filter

• coupler

• Attenuator

• DC power supply

• digital waveform generator

• Test or evaluation board

• Several chips

• Other devices or equipment

Before testing, we need to pay attention to the following points:

Before powering on, check whether the circuit is normal to avoid the risk of short circuit and impedance matching problems;

During the test connection process, be careful to select the correct attenuator to avoid damage to the equipment and test chip;

Before transmitting RF signals, make sure there is no problem with the RF connection;

Correctly set the compensation value of the instrument;

For test accuracy, the equipment used needs to be calibrated with the same equipment as a reference.

Below we will learn with you the testing principles and testing methods of several common indicators.

PAs in the maximum output power GSM mode usually work in the nonlinear region. When the output power Pout reaches the maximum, it does not increase with the increase of pin, but increases with the increase of vramp voltage. Usually, we need to test the maximum output power Pout max of this PA chip during mass production testing. Provide normal voltage to all pins, add the required RF signal, adjust vramp to the maximum, and measure the signal size at this time. In the power test in GSM mode, special attention should be paid to the fact that we use a 12.5% ​​duty cycle for power testing. There are usually two methods to achieve the required duty cycle. The first is to control it through the duty cycle of the Tx Enable pin level waveform. The second method is to control it through the waveform of the radio frequency signal, that is, the burst waveform.

Maximum output power

Added power efficiency PAE Added power efficiency PAE refers to the ratio of the difference between the output signal power and the input signal power and the DC power consumption. Through this indicator, we can see the loss of an amplifier during the power conversion process.

What needs special attention here is that the output power and input power need to be converted from dBm into mW or W before performing formula calculations.

The PAE can be found by bringing the measured voltage, current, input and output power into the above formula.

Pin and Pout Usually, this test is used for PAs that work in the linear region. At this time, the pout of the PA will increase as the number of pins increases. Theoretically, we can definitely make the pout of the PA reach what we want by adjusting the size of the pin. value, the pin at this time is the value we measured. Of course, we can extend an algorithm to measure through the program. We can give a smaller pin value before testing so that pout0 is smaller than the target pout. Because the pin and pout of PA are linear, we can again Increase the pin by pout-pout0 and then observe whether the target pout is reached. If the target is not reached, you can continue to increase or decrease the pin according to the measured value of the target pout and the actual pout. After repeated several times, the pin can generally be found quickly.

Everyone should note here that when adjusting the pin, it is best to adjust it from small to large.

For harmonic testing, generally only the 2nd and 3rd harmonics need to be tested. Harmonics after the 4th harmonic have a higher frequency and have a larger attenuation, so the power is smaller and generally do not need to be tested. A special device filter is needed here. The reason why the filter is used is because the output power of the PA is large. Even if an attenuator is added, the instrument itself will produce harmonics and affect the PA harmonic test. Therefore, , when testing harmonics, we need to use a high-pass filter to filter out the carrier before conducting harmonic testing. Refer to the environmental connection above. Finally, the corresponding harmonics are measured according to the corresponding frequency points, so that the measured harmonics are more accurate.

harmonic

ACLR and ACPR Adjacent channel leakage ratio (ACLR) and adjacent channel power ratio (ACPR) are actually the same thing. They are standards used to measure the amount of interference or power in adjacent frequency channels. They can be used to characterize non-standard channels. An important indicator of linearity.

ACPR

When testing ACPR, we generally need to use a bandwidth signal. Here we can use the modulation signal of the corresponding modulation method for testing. Generally, we can choose the four adjacent channels on the left and right for testing. Because it is the ratio of the adjacent channel and the current channel, we usually define its unit as dBc.

The full name of EVMEVM is Error Vector Magnitude, which can standardize the amplitude error and phase error of the modulated signal and is an important indicator of signal quality. As shown in the figure below, it can be obtained by the vector difference from the ideal signal:

EVM

When testing EVM, we also need to prepare the modulation signal. After adding the modulation signal to the chip under test, use a spectrum analyzer to select the correct modulation method and parameters for testing.

EVM test

It is worth noting that when testing EVM and ACLR, you must pay attention to the appropriate carrier input power. If the input power is too high, the linearity will become worse and the EVM will be too large.