Example for making power amplifier harmonic measurements
The example shown in Figure 2
is a set-up using two PXI-based RF digitizers for measuring the fundamental and harmonics of a power amplifier up to 13 GHz for high-speed testing.
The high pass filter is performing the carrier reject function that a Yig-Tuned Filter (YTF) would normally perform on a conventional spectrum analyzer. In this example, we show how the PXI-based RF digitizer’s local oscillators will tune to the set analysis frequency within 325 µs. Compare this with up to tens of milliseconds for a YIG-based system—it is orders of magnitude faster.
In this example, the Device Under Test (DUT) is a 1W (+30 dBm) power amplifier at 5.4 GHz. A power splitter is used to enable parallel fundamental and harmonic measurements. An attenuator is placed between the power splitter and the carrier-reject high pass filter. This prevents the reflected fundamental power (from the filter) from damaging the amplifier. At the 3036 input, the fundamental level has been reduced by 30 dB. This has reduced the required dynamic range of the digitizer, allowing for lower input attenuation. With known input levels (for the fundamental) of approximately-13 dBm, the input attenuation of the 3036 may be reduced to 0 dB.
In summary, to measure the fundamental and 2nd harmonics using the set-up shown here would require only 325 µs for setting center frequencies, plus signal capture and transfer time.
Measuring other power levels
The approximate expected power levels of the DUT should be known and the external attenuators and digitizer input attenuation set accordingly. One of three DC-18 GHz attenuators (3 dB, 6dB or 10 dB) should provide appropriate attenuation levels for most users’ needs. Depending upon the DUT, reverse power protection may not be required. However, if the input levels presented to the RF digitizer require a number of levels of attenuation, external in-line attenuators are a good choice to provide the required input attenuation, in addition to preventing potential damage to the DUT from reflected fundamental power.
For measuring other frequency ranges, the needed high-pass filter cut-off frequency will change depending upon the frequency range of the DUT.
Harmonic measurements made using a 13 GHz PXI-based RF digitizer may be improved using simple external filtering. In addition, you can reduce measurement time by using parallel RF digitizers to measure fundamental and harmonic signals simultaneously.
About the author
Peter Brown is a Senior Applications Engineer with Aeroflex supporting their PXI 3000 Series RF modular instruments. Peter has worked in diverse RF and electronics design roles over a 20 year career. Peter has a BEng from the University of Leeds and MSc from Brunel University.
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