Why Am I Seeing Images With Large Spans Using the NI PXIe-5663?Hardware: Modular Instruments>>RF Measurement Devices>>PXI-5660, Modular Instruments>>RF Measurement Devices>>PXI-5661, Modular Instruments>>RF Measurement Devices>>PXIe-5663
Problem: I have configured my acquisition to use a span that is much larger than the instantaneous bandwidth of the NI PXIe-5663. This setup requires that the driver pull several 50 MHz captures and combine them to fill the graph. When doing this at certain center frequencies, I see one or two images. Why am I seeing these images?  Solution: The reason is that the NI PXIe-5663 does not have inherent hardware protection against RF image frequencies. To understand why this effect occurs, we need to understand how the RF signals are captured. When we capture signals between 330 MHz to <3.0 GHz, we use high-side LO injection by default. This configuration means that we set our local oscillator (LO) equal to RF + IF. For signals between 3.0 GHz to 6.6 GHz, we use low-side LO injection. This setup means that our LO is set to RF - IF. For more information, refer to the NI RF Vector Signal Analyzers Help (see link below or go to Start » Programs » National Instruments » NI-RFSA » Documentation if you have the NI-RFSA instrument driver installed). Inside the help file, navigate to Devices » NI 5663 RF Vector Signal Analyzer » NI 5601 RF Downconverter. The help file explains how the downconversion and digitizing process captures two instantaneous 50 MHz bands. These two captures are the 50 MHz surrounding centers at LO + IF and LO - IF. This result is shown in the following figure for high-side LO injection.  The issue is caused by the fact that to capture such a large span we have to change the LO and recapture. Described below are a few scenarios to explain the results that are being seen. For example, assume that our signal is at 2.64 GHz. When we capture with a span of 50 MHz, the LO uses high-side injection (<3GHz). This configuration sets the LO to 2.64 GHz + IF(187.5 MHz) = 2.8275 GHz. As mentioned before, this setup equates to a downconversion that acquires two 50 MHz bands. One capture is 50 MHz around the point of interest at 2.64 MHz ,and the other is 2.8275 GHz + 187.5 MHz = 3.015 GHz. Assuming we are simply acquiring a single tone that is directly connected, we should see no signal at 3.015 GHz. If we have a signal at 3.015 GHz, it is added to the image during the downconversion/digitizing process, as shown in the following figure. 
 The case of high-side LO injection is simple. As you start from the much lower frequency of 2.165 GHz, your LO is starting at 2.165 GHz + 187.5 MHz = 2.3525 GHz. As you shift your RF and LO to capture each new band, you eventually move to the RF frequency of 2.265 GHz. At this frequency, you have an LO of 2.4525 GHz. Considering what we learned previously, we know that this LO will capture signals at both LO - IF = 2.265 GHz and LO + IF = 2.64 GHz. As you can see, this setup captures 50 MHz around our center frequency of 2.64 GHz, which includes any signals surrounding 2.64 GHz. So, inside the 2.265 GHz band of data, we see an image of our signals surrounding 2.64 GHz. For the other image, consider the effects of low-side LO injection. By default (this can be changed using the niRFSA Property Node), when you shift your LO above 3.0 GHz, you use low-side injection, which adjusts your LO to RF-IF. Thus, as we shift our RF by 50 MHz from the start frequency, we eventually hit 3.015 GHz., which sets our LO to 3.015 GHz - 187.5 MHz = 2.8275 GHz. As stated before, this configuration captures signals surrounding both 3.015 GHz and 2.64 GHz., which accounts for the second image, which appears in the band surrounding 3.015 GHz. In conclusion, this effect is unique to specific frequencies. You must calculate for your specific center frequency and span to see if they will return signal images. One workaround and possible solution is to know that these images will exist and account for them when using larger spans. Another workaround is to set the LO to use high-side or low-side injection so that you will only have one possible image. This scenario makes it easy to find and account for this image. Related Links: Product Manual: NI RF Vector Signal Analyzer Help (July 2008) Attachments:
Report Date: 04/10/2009 Last Updated: 12/29/2009 Document ID: 4W9BJ7DE |
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