The power amplifier (PA) is the core component of the transmitter circuit, especially for broadband transmission, where it has a significant impact on non-constant envelope modulation methods. It can be said that the PA is the decisive component for both the performance indicators and operational duration of a transmitter using non-constant envelope modulation.
During the debugging and testing of a PA, a vector network analyzer (VNA) is generally used for impedance matching, gain adjustment, and power and efficiency tuning, while a signal generator and spectrum analyzer are used for linearity adjustment. These two sets of instruments are used alternately to achieve the optimal balance between the power, linearity, and efficiency of the PA.
Why Use Two Sets of Instruments for Testing a PA, and What Data Needs to Be Tested from Selection to Output?
- Impedance Matching
The PA is an active device, and to achieve high power output, the impedance must be minimized according to the formula(P=U2/R). The larger the transistor, the smaller the impedance. For maximum power transfer, impedance matching is essential. The VNA is the instrument used for source-pull and load-pull testing, completing the input and output matching, and thereby the initial PA matching design.
- Frequency and Power Scanning
Once the input and output matching are done, the working frequency and output power of the PA need to be tested. The VNA can perform frequency and power scanning, completing the bandwidth and power range testing of the PA.
- Linearity of the PA
For broadband PAs, memory effects are present, and these memory effects are a key factor limiting the effectiveness of digital predistortion (DPD). The memory effects in a PA arise due to AM-PM distortion. For DPD to achieve effective linearization, an accurate AM-PM distortion model is essential. The VNA can test the AM-PM distortion of the PA, providing accurate data for the DPD model.
AM-PM distortion is a more accurate measure of the linearity of a PA. However, RF engineers often cannot directly visualize PA distortion.
Typically, intermodulation distortion (IMD) is used to represent PA distortion, which is a common metric for engineers designing PA modules.
Looking at the Taylor series expansion of a two-tone signal, regardless of whether the signal passes through a nonlinear device, intermodulation signals will be generated. This explains why IMD remains unchanged even when the PA operates in back-off.
Typically, IMD is tested using a signal generator and spectrum analyzer. The relationship between IMD, output power, and OIP3 is given by the following formula:
IM3 (dBm) = 3Pin (dBm) – 2IIP3 (dBm) + G (dB)
= 3Pout (dBm) – 2IIP3 (dBm) –2G (dB)
= 3Pout (dBm) – 2OIP3 (dBm)
Although IMD can accurately measure the distortion of a PA, for engineers working on complete PA systems, Error Vector Magnitude (EVM) or Adjacent Channel Power Ratio (ACPR) more intuitively reflects the final performance requirements of the PA.
From the perspective of a communication system, system engineers are more concerned with signal distortion rather than PA distortion. By determining the input and output signal metrics, the specifications required of the PA can also be determined. Therefore, EVM and ACPR are more commonly used in communication systems to measure PA distortion. The amount of distortion allowed for the PA can be calculated using the cumulative EVM formula.
Before testing, it is also necessary to measure the Peak-to-Average Power Ratio (PAPR) of the broadband signal. Some waveforms undergo crest factor reduction at the baseband end, while others leave crest factor reduction to the PA. The difference between baseband crest factor reduction and PA crest factor reduction lies in whether the PA introduces new distortion, which is crucial for PA design.
- Harmonics
In addition to the linearity metrics mentioned above, attention must also be given to PA harmonics, as these affect the design and selection of filters that follow the PA.
- Efficiency
While linearity is important, efficiency is equally critical. While focusing on linearity, it is also important to ensure that the efficiency meets design requirements, achieving a balance between linearity and efficiency.
