Using PXI RF digitisers for concurrent measurement of harmonic levels
30 January 2013
With the proliferation of spectrum allocations for different wireless communications standards – particularly with the disparate bands now being allocated for LTE and LTE-Advanced – the likelihood is increasing that second or third harmonics from a device will impact on the performance of equipment in another band.
Consequently, it is more important than ever to characterise the harmonic performance of devices and to ensure that it meets specification, whether or not the band is licensed.
With common wireless communications bands ranging up to 6 GHz, the second harmonic frequency may be up to 12 GHz in the microwave frequency bands – commonly used for Doppler motion-sensing radar and satellite broadcasting channels.
Spectrum analysis or signal analysis are the normal methods for measuring harmonic levels. In a production environment these parameters may only be measured on a sample basis, so it is undesirable to tie up expensive test equipment permanently in the production line, although it is also necessary to be able to perform the test quickly without swapping equipment in and out of the line. Using a modular PXI system incorporating an RF digitiser module is an ideal solution to this dilemma.
It was with this application in mind that Aeroflex launched the 3036 digitizer module as an addition to its popular PXI 3000 Series of RF modular instruments. Designed for use in vector signal analysis of complex RF signals, the 3036 has a frequency range extending to 13 GHz, and provides a wide instantaneous digitized -1 dB bandwidth of up to 90 MHz, fast frequency switching in less than 325 µs, high linearity and low noise. This makes it ideally suited for use in high-speed low-cost ATE systems to test devices and RF components for wideband RF and microwave communications. System footprint is also a critical requirement in this application, and with a 3U footprint the 3036 is more economical in its space requirement than comparable equipment.
The 3036, shown in Figure 1, operates over the band 250 kHz -13 GHz with 13-bit ADC resolution. It can be used with an Aeroflex 3010 Series synthesizer module to provide precision conversion of RF signals into digital IF or I and Q sample data. This combination of modules has a compact size of 4 slots (3 slots for the 3036 and 1 slot for the 3010) to minimise overall test solution footprint. To speed throughput, a lower frequency digitizer such as the 6 GHz 3035C, can be incorporated into the system as shown in Figure 2. This permits simultaneous measurement of fundamental and harmonic performance, and in this way the manufacturer can make full use of existing PXI equipment inventory to speed up the test cycle.
The RF digitiser benefits from efficient, lean design as well as from the modularity of PXI instrumentation to deliver a more economical solution than other modular or general purpose rack and stack instruments, with cost savings in the order of 40% being achieved without compromising critical performance needs. Using PXI enables faster measurement speed, smaller sise and greater flexibility for easier system integration and future system evolution.
The PXI Studio software application that is supplied as standard with the 3036 can provide vector signal analysis of complex modulated signals. PXI Studio performs spectrum and time domain analysis of sample data for general purpose RF component testing and alignment of radio communications transceivers – ideally suited to the needs of RF test systems for manufacturing and design verification. Optional measurement personalities for PXI Studio enable measurements for a range of digital and analogue modulation types, as well as radio standards including LTE (FDD and TDD); UMTS / HSUPA; TD-SCDMA; CDMA2000 1xRTT and EV-DO; GSM / EDGE; WLAN; WiMAX; Bluetooth and ZigBee.
Figure 1: The 3U wide 3036 digitizer combines with a 3010 Series synthesizer to provide vector signal analysis of complex modulated signals
Figure 2: Test setup for measuring fundamental and second harmonic performance of communications power amplifier (PA)
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