USING SPECTRAL STITCHING TO MEASURE A PERIODICALLY MODULATED SIGNAL
Publication Date: 2017-May-04
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A mixer-based receiver typically down-converts a radio-frequency (RF) signal to a predefined lower frequency signal known as an intermediate frequency (IF) signal. Down-converting the RF signal to the IF signal allows the use of circuitry that can operate at lower frequencies and also provides a predefined IF bandwidth that can be used to accommodate a certain range of RF signals coupled into the mixer-based receiver. The use of such a mixer-based receiver has proved quite successful in a wide variety of applications. However, certain types of RF signals can present unique challenges to a conventional mixer-based receiver. More particularly, a conventional mixer-based receiver having a fixed, predefined IF bandwidth may be unable to accommodate a periodically modulated RF signal having certain characteristics such as a minimum frequency contained in the RF signal exceeding a frequency range of a down-converting stage of the mixer-based receiver, and a difference between a maximum and a minimum frequency contained in the RF signal exceeding the IF bandwidth of the mixer-based receiver.
One practical application where such a problem may be encountered is in a measurement setup where a microwave signal analyzer is used to measure an error-vector-magnitude (EVM) of IQ quadrature signals provided by a modulation signal generator to a device-under-test (DUT), the IQ quadrature signals containing low-frequency (LF) energy in a range from DC to about 100 MHz. As is typical of many microwave signal analyzers, the minimum operating frequency of a down-converter mixer in a receiver of the microwave signal analyzer can be about 10 MHz, and the IF bandwidth of the receiver can be about 40 MHz.
Due to the 10 MHz minimum operating frequency limitation of the mixer, IQ quadrature signal components below 10MHz have to be acquired in the microwave signal analyzer by bypassing the down-converter mixer. This can be carried out by using controls to route the IQ quadrature signals to an analog-to-digital converter (ADC) and capturing data over a low-end bandwidth of about 40 MHz. After capturing this data, the controls are used to switch in the down-converter mixer into the IQ quadrature signal path and an LO oscillator can be adjusted in incremental steps to set various LO frequencies for capturing measurement data associated with the rest of the 100 MHz bandwidth. As can be understood, the 40 MHz IF bandwidth limitation in the receiver necessitates the use of at least two different LO frequencies for capturing the remaining 60 MHz worth of measurement data in the IQ quadrature signals.
The 60 MHz worth of measurement data (obtained via routing the IQ signals through the down-converter mixer and using two different LO frequencies) has to be then combined with the 40 MHz measurement data previously obtained by bypassing the down-converter mixer, so as obtain measurement dat...