A tough challenge for test engineers is explored in terms of test methods, pitfalls, and measurement errors.
For the test engineer, RF and microwave power amplifier testing imposes unique challenges. Although there are many tests that must be performed to verify compliance to required specifications, two common tests deserve special attention: linearity and compression.
Linearity Testing
RF power amplifier (RFPA) nonlinearities introduce third-order intermodulation distortion (IMD) to the amplifier output signal when driven by a multitone input signal. In the traditional two-tone test, IMD will show up as third-order products that occur at a frequency of 2f1 – f2 and 2f2 – f1 as shown in Figure 1.
A practical two-tone IMD test setup is shown in Figure 2. It consists of two signal generators and two isolators with the signals summed in a two-way combiner and then fed into the DUT. A spectrum analyzer performs the actual measurement.
The isolators are necessary to prevent signal energy from one source leaking through the power combiner into the other source. The presence of the signal from one generator at the output of a second generator will cause its output level to modulate and create IMD.
As an alternative, a vector signal generator will greatly simplify the test setup. Modern vector signal generators use dual arbitrary waveform generators (Arbs) to in-phase and quadrature-phase (I/Q) modulate the synthesizer output. Additionally, advanced vector signal generators use I/Q predistortion to generate two-tone signals with better than 70 dB of intermodulation suppression.
Prior to measuring the DUT, it is important to verify that the IMD created by the test system is 20 to 30 dB below the DUT specification. Since the test setup and spectrum analyzer-generated distortion components occur at the same frequencies as the distortion components we wish to measure, there is no way of knowing the phase relationship between the test system and DUT signals. The potential range of uncertainty is the following:
Uncertainty (in dB) =
20 log(1 • delta(dB)/20)
where: delta (dB) = the difference in decibels
between the test system and
the DUT-generated distortion
products
These errors are shown in Figure 3.
To measure the test-station IMD, remove the DUT and connect the two-tone source to your load and measure the IMD of the source over all required power levels and frequencies. Another simple test to verify you are getting good results is to increase the level of attenuation in the spectrum analyzer. If the intermods levels do not change, then the intermods created by the spectrum analyzer are not affecting the measurement.
Linearity Testing: Multitone Signals
Linear RFPAs used in communications systems require a multitone signal source to characterize their in-band distortion performance. A multitone input signal simulates real-world conditions where many wireless traffic channels are combined and amplified by the output power amplifier.
Creation of a multitone test signal is nontrivial. One approach would be to extend the block diagram of Figure 2 by adding additional signal sources. While this method will work, it may not be practical to tie up expensive signal sources for this one test. Another approach would be to purchase a dedicated multitone generator.
A third approach is to use an Arb-based vector signal generator's synthesizers that can create several hundred signals within the Arb bandwidth. Additionally, Arb-based synthesizers allow the phase of each CW signal to be preset or randomized to set the composite signal crest factor.
One drawback is the maximum power the vector signal generator can deliver while maintaining linear performance. The maximum available power typically is +15 dBm and must be reduced by the signal pk/avg ratio of the composite signal. The pk/avg ratio is equal to:
pk/avg = 10 log(NumberOfTones)
For example, a two-tone signal has a pk/avg ratio of 3 dB while a 16-tone signal provides a maximum pk/avg ratio of 12 dB.
If the available signal level is not sufficient, an external linear power amplifier must be used to increase the signal level into the DUT. In the past, this external amplifier had to have a 100-W or greater rating so it would not introduce intermodulation products into the test system. This no longer is the case.
Modern Arb-based synthesizers offer predistortion correction software that predistorts the I/Q modulation to cancel the intermodulation products created by the external power amplifier. With this software, the external amplifier can be much smaller and less expensive than required in the past.
Compression Point Testing
The output power of a power amplifier cannot increase indefinitely. In the extreme, there will be a point where an increase in input power does not produce a discernible increase in output power. The transfer characteristic for a typical power amplifier is shown in Figure 4.
Gain compression, or P1dB, is defined as the output power level where the gain has been reduced 1 dB from its small signal gain. It also is sometimes desirable to measure the saturated power capability of an RFPA. The saturation point (PSAT) is defined as the point where any increase in input power does not produce a corresponding increase in output power.
Network Analyzer Methods
Modern vector network analyzers (VNAs) have power sweep capability that allows fast and accurate compression measurements. The VNA can produce a plot as shown in Figure 4.
In addition, modern network analyzers allow for absolute power calibration. Power calibration requires the use of an external power meter to calibrate the input power sweep for absolute power. Test fixturing and cabling losses can be calibrated during power calibration.
Some VNAs have canned routines that automatically place a marker at the P1dB point and return the gain and input power. If the network analyzer does not have this feature, it is easily implemented in a user's automated test routine.
Care must be taken when using the network analyzer. It is easy to overdrive and possibly damage the amplifier under test. Pretesting of the amplifier should be performed to determine the expected gain so the input power sweep limits can be set accordingly.
This method is not suitable for pulsed amplifiers. Measuring pulsed amplifiers with a CW VNA can overheat and damage the amplifier.
Power Meter Techniques
A test to determine the compression point of an amplifier can be performed using the test setup shown in Figure 5. This setup can be used for CW or pulsed compression testing.
For radar and other pulsed power amplifier applications, the compression test must be performed using a pulsed signal source. The pulsed source will not overheat the DUT, and since the power transistors are not heated to the extent they would be using CW, the power amplifier can produce more power.
Either a pulsed or an average type of power head can be used. When using average power heads, the pulse modulation duty cycle must be known. Since the power meter measures average power, a correction must be applied to determine the peak power:
PowerCorrection = -10 log(PW/PRI)
where: PW = pulse width
PRI = pulse repetition interval
There are some advantages with using duty cycle to calculate the pulse power. The duty-cycle technique provides the lowest cost solution with average power meters and sensors being less expensive than peak and average power meters and sensors.
With some pulse modulated signals, the pulse may not be purely rectangular since there is an associated rise and fall time as well as overshoot and ringing on the signal. The combination of these effects creates an error in the calculated result.
In this case, a peak power meter is required. It has the capability to make a time-gated power measurement. With time gating, the power can be measured during the settled portion of the pulsed waveform. Also, the peak power meter will return the peak power, eliminating the need to correct for PW and PRI.
About the Author
Ed Crean is senior RF engineer at Symtx and has 22 years of experience in RF design, manufacturing, and management. Before joining Symtx, he specialized in high-power RF amplifier design and development at Nokia and Andrew Corp. Mr. Crean holds a B.S.E.E. from the University of Buffalo and an M.S.S.E. from Texas Tech University. Symtx, 4401 Freidrich Lane, Austin, TX 78744, 512-328-7799, e-mail: [email protected]