RF Power Amplifier Repair-Routine debugging process of RF power amplifier

Time：2021-01-21                1506

For high-power power amplifiers above 10W, because the large-signal simulation model does not achieve a high accuracy, the working state of the tube in high-power is very uncertain, and the importance of debugging is compared with other RF devices. occupies a greater proportion.
The steps of RF power amplifier debugging generally consist of the following steps:
STEP 1 Determine if the chip is damaged
This step is mainly to determine whether the basic state of the board and the tube is normal. First, use a multimeter to measure whether the resistance of the power amplifier grid to ground and drain to ground is normal to determine whether the tube is damaged; then check the state of the circuit board to see if there is any false Welding, short circuit, whether the grounding is the best when assembling the power amplifier tube, etc.
STEP 2 Power-on measurement
Power on, measure whether the drain voltage and gate voltage are added, adjust the values of the drain voltage and gate voltage, and observe whether the quiescent current is normal (is there too large or too small or abnormal jumps).
When powering on, pay attention to the power-on sequence of the tube (GaN power amplifier tube first gate and then drain, and the power-off sequence is opposite; LDMOS power amplifier tube has no special requirements), and the output terminal and input terminal should be connected to the instrument or matched load when powering on. .
STEP 3 Small Signal Measurement
After calibrating the vector network, connect the input end of the power amplifier to the vector network port 1, the output end to the attenuator, and then connect to port 2 (set the attenuator compensation), power on the power amplifier, and observe whether S11 and S21 meet the design requirements. Generally, The power amplifier requires S11 to be lower than -10dB in the working frequency band. Since the vector network can display the Smith chart, theoretical calculation and actual debugging can be used to make the input standing wave of the power amplifier meet the requirements. S21 is about the design gain value, and it is guaranteed to be relatively flat within the band. Generally, the highest point of the gain is required to fall in the middle of the working frequency band. When looking at S21, the display frequency range must be widened as much as possible. If the S21 curve has abnormal peaks, it may cause the power amplifier to self-excite. During this process, the circuit needs to be fine-tuned (copper foil, cut and patched microstrip, replacement of component values, etc.) until it meets the requirements.
STEP 4 Large signal measurement
After connecting the attenuator at the output of the power amplifier, connect it to the spectrum analyzer, and set the correct compensation value (only compensate the line attenuation at the rear end of the power amplifier and the attenuation of the attenuator).
Set the initial input power of the signal source to a very small value (about -20dBm), gradually increase it, and observe the increase of the output power and the increase of the current until the saturation point (usually the 1dB compression point). Observe the spectrum analyzer, record each frequency spectrum and drain current, collect data, and calculate indicators such as saturated output power, gain, harmonics, and efficiency.
STEP 5 Linearity measurement
The signal source gives the debugging signal, the spectrum analyzer is adjusted to the IQ constellation diagram analysis mode, and the linearity index of the power amplifier is observed according to the relevant indicators.
If the linearity index is insufficient, try to adjust the matching capacitor and adjust the quiescent current of the power amplifier.
STEP 6 Multi-board consistency verification
Assemble the same board to ensure the same performance under the same PCB board parameters
STEP 7 High and low temperature test
Perform high and low temperature tests on the power amplifier board. It is mainly to observe whether the deterioration of its working performance and whether there will be self-excitation and other failures in the case of high and low temperature.
When the performance of the power amplifier changes greatly in the high and low temperature test, it is necessary to adjust the temperature compensation circuit; for the fault phenomenon in the high and low temperature test, try to reproduce and solve it in the normal temperature environment as much as possible.
Replenish:
1. In addition to the conventional indicators, it is also necessary for the power amplifier board to measure its open-short circuit protection performance, temperature compensation and over-temperature protection performance, continuous transmission capability, power output capability under mismatched load conditions, etc.
2. Capacitance debugging rod debugging, radio frequency cable spot measurement, etc. will better help improve the efficiency of locating problems and debugging, and usually need to prepare some responsive materials
3. After the power amplifier board is debugged, it does not mean that you can sit back and relax. After adding it to the joint debugging of the whole system, problems such as transient response may also occur. At this time, specific problems need to be analyzed in detail.
 
Common problems during debugging
Problem 1: No current in drain
Solution:
1. Check the power cord connection;
2. Welding problem of feeder circuit;
3. Filtering problem;
4. Whether the power amplifier tube has been damaged due to improper operation.
Problem 2: Excessive drain current
Solution:
1. Is the circuit stable?
2. Check whether there is a short circuit (pay attention to tin slag);
3. Whether the board is damaged;
4. Whether the power amplifier tube has been damaged due to improper operation.
Problem 3: S-parameters are not ideal
Solution:
1. Since the two circuits are not completely balanced, perform phase compensation. The debugging capacitor rod can be used to conduct debugging on the two circuits respectively. The capacitor rod itself has two feet, and the contact area with the circuit is different from the contact area of the actual capacitor, etc., there will be a certain capacitance value deviation, so use the capacitance value near the debugging value for fine-tuning during actual welding;
2. The matching is not enough. Because the best input and output impedance of the power amplifier tube has an imaginary part, and the balun structure can only transform the real part, it is necessary to cancel the imaginary part to a certain extent.
Problem 4: Inefficiency
Solution:
1. Small signal re-commissioning
2. Pay attention to whether the heat dissipation is reliable. If the heat dissipation is not enough, high-power signal transmission cannot be carried out continuously for a long time.
Question 5: The power amplifier is self-excited
Solution:
1. If the grounding is not good, you can increase the grounding area or connect with the shell;
2. The supplied DC power supply is not filtered enough, which makes it not clean enough. At this time, additional filtering measures are required;
3. The radio frequency signal is coupled to the DC voltage, and a magnetic ring can be placed on the low frequency line, and the low frequency line and the radio frequency line should be kept as far away as possible or change their direction;
4. The mismatch between the amplifiers is serious at some frequency points, resulting in the back and forth reflection of the radio frequency signal. In this case, the circuit needs to be readjusted or the isolation between the matches should be appropriately increased.
5. Poor heat dissipation, as the temperature increases, the internal performance of the power amplifier tube deteriorates, resulting in instability. In this case, it is necessary to strengthen the heat dissipation
6. The self-excitation of the power amplifier caused by parasitic parameters, such as the self-excitation caused by the introduction of parasitic parameters after the cover of the whole machine is covered. In this case, it is necessary to reconsider the form of the input and output circuits to avoid serious deformation of the characteristics after the introduction of parasitic parameters.
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