Short Range Wireless Figure 4. A DAC output. 200MHz to 5dBm RFIN.
Figure 5. A 5G test vector comparison between the SMW200A output and MxFE DAC output.
with a 192-tap FIR filter with double- precision floating-point values. Because the FIR filter takes four hex value codewords as register inputs, some precision is lost while converting to this format from the double values in MATLAB. The expected effects of the datatype conversion on the filter response are shown using a PFILT model and are compared to the MATLAB Filter Designer output.
Figure 6. A MATLAB-generated low-pass magnitude response and group delay compared to implemented filter response in an MxFE PFILT model.
Figure 7 shows the results of a 100MHz test tone-filtered and looped back to the DAC of MxFE. The harmonics generated by non-linearities in the ADC buffers have been filtered by the PFILT, bringing the spurious free dynamic range (SFDR) from 55.9dB to 81.9dB. The implemented filter shows a slower roll-off to 60dB attenuation than the simulated filter. The group delay was shown to remain flat in the pass band at (N-1)/2 = 95.5 samples for 192 taps. A multitone test vector was generated using the R&S SMW200A. This train of tones will conform to the shape of a filter over a broad range of frequencies. The power level of each tone was kept to approximately –40dBm to avoid intermodulation distortion. As such, the DAC output response with and without filtering is shown with a reference level of –40dBm.
Figures 10 and 11 show a comparison
between a 200MHz CW at –15dBm. The Continues on page 50
after the PFILT matches the output signal from the loopback path.
A 5G-NR test vector was also used to test the accuracy of indirect loopback by using a signal with a more complex spectrum. Figure 5 shows the power spectrum of the test vector from the Rohde & Schwarz SMW200A vector signal generator compared to the DAC output with loopback.
Results
Filter coefficients for the profiles shown in the results were generated using the MATLAB Filter Designer and retrieved using a Python script that captured trace data from the spectrum analyser.
Two plots were generated from each MATLAB Filter Designer output. The first output is the ideal filter profile that shows the digital filter response, which is possible
Figure 7. A comparison of filtered and nonfiltered DAC output.
Table 1. MATLAB Low-Pass FIR Specifications
Figure 8. A multitone test vector MxFE DAC output comparison with and without filtering. Compared to MATLAB- generated filter mask. Reference level of –40dBm.
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Components in Electronics
November 2023 49
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