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AWR1243: Question about Image Rejection and image band interference monitor

Feng
Expert 2985 points
Part Number: AWR1243

Hi,

According to AWR1243 datahseet, its receiver is shown as following,

The image rejection part would work as the theory shown in below (pic is from TI doc Using a complex-baseband architecture in FMCW radar systems www.ti.com/.../spyy007.pdf)

My question is: if the image rejection filtering is always performed, the image band of FFT will be filtered out. Then how can we monitor the interference in image band?

And actually, according to my experiment, one AWR1243 works as FMCW mode to detect objects, the other one works as single tone interference, I can see the interference in image band.

But how to explain the image rejection here, why does it not filter the interference in image band?

The image rejection filtering could be disabled, not performed always?

Regards,

Feng

  • 0
    TI__Guru*** 141544 points
    Hi,

    We just released a short video presentation on this topic.

    Let me identify the link and will send it to you

    thank you
    Cesar
  • 0 in reply to Cesar
    Expert 2985 points
    Hi Cesar,

    Any updates?

    Regards,
    Feng
  • 0
    TI__Guru 58035 points

    Hello Feng,

    The image rejection happens when the signal is actually in the desired band and its 'image' gets filtered. But if the signal itself comes in the negative frequency (ie the image band for the desired signal) the filtering would not help. 

    In the image below you can see the chirp in green, the desired signal in blue and the interfere in red. You can see that at some point in the chirp the interfere will cross to the image band and at that point the image band will show up strongly in the image band since it would not get filtered. 

    Regards,

    Vivek

  • 0 in reply to Vivek Dham
    Expert 2985 points

    Hi Vivek,

    I understand the background theory.

    I performed one interference experiment as following and with my question. Please help me with it!

    My experiment setup: one FMCW radar, one single frequency interference@77.2GHz and one moving target. Data was captured by TSW1400.

  • 0 in reply to Feng
    TI__Guru 58035 points
    Hello Feng,
    I am not sure I understand the question. When an intentional signal is present in the image band it cannot be filtered out. What gets filtered is the image signal of the valid band.

    regards,
    Vivek
  • 0 in reply to Vivek Dham
    TI__Guru 58035 points
    Hello Feng,
    Sorry for not being clear in my previous reply, let me explain in a little more detail:
    1) In the complex mode of operation there are 2 modes:
    -- Complex 1X -> The visible frequency range is 0 - Fs (Fs being the sampling rate) . In this mode The image band is filtered out as indicated by the block diagram above.
    -- Complex 2X -> The visible frequency range is -Fs/2 - +Fs/2. In this mode the image band is not filtered and is provided as is to the user. I think the plot you are showing above is in Complex 2X and hence you are seeing the image energy as well. Is that correct?

    Now regarding the interference detection, the image band monitor is present before the image rejection filter hence it can be used in both the complex 1x and 2x modes.

    Regards,
    Vivek