In the book, instead of having a band- or lowpass-filter before mixing, it's coming right after mixing, which makes sense, considering there's gonna be a high-frequency component, which has to be filtered out.

But the lowpass-filter is called RF Image Rejection Filter, which irritates me, since that would be a band-pass instead and not something done after, but before mixing.

Curiously, afterwards there's an IF Band Selection Filter, which is meant to filter images?

enter image description here

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    $\begingroup$ So how does the book explain the pre-IF image rejecton lowpass filter? I hope it has an explanation? $\endgroup$
    – Fat32
    Mar 17, 2017 at 0:23
  • $\begingroup$ the one book, which is totally clear, because everyone knows that book. I'm not saying it's the greatest – it is – but you should never underestimate the ignorance of us – I tell you, it's terrible, it's the worst. You have to point us to every single resource you're referring to. Sad. $\endgroup$ Mar 17, 2017 at 15:16
  • $\begingroup$ I actually don't know "the book" but now am curious to know. This is of course, as Marcus and Fat32 are hinting at, incorrect. The RF "Image Reject Filter" should be before the mixer just as you suggest. As shown both RF images will pass through the low pass filter shown. Further, even if there wasn't an inteferor present in an image, a 3 dB penalty will result due to the image noise combining with the band of interest. So what book is it? $\endgroup$ Mar 18, 2017 at 0:55
  • $\begingroup$ Thing is, the book desktopsdr.com doesn't explain it at all and it doesn't seem to make sense to me? $\endgroup$
    – Starhowl
    Mar 19, 2017 at 14:51

1 Answer 1


Today I received an answer from the authors of the book. It turns out that figure 1.7a was wrong, and they sent me the updated one.

The corrected diagram will be added in this Summer's update!

Figure 1.7a


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