I’m using a LimeUSB in dual channel, at 55MS/s, central frequency 2450MHz, each channel shifted by +/- 23MHz to cover from 2400MHz to 2500MHz.
Sometimes, when I receive a powerful signal, I may see 0, 1 or more “mirrors” or “ghosts”.
I have a few questions:
- Where do the mirrors come from? Is that called aliasing or something else?
- Is there a way to remove or limit those mirrors (while still covering the full 2400MHz band)?
- Can you compute where they will be and their expected power, knowing where the real signal is?
- Why do I sometimes have 0 mirror, 1 or 2, despite always having a strong signal?
- For wider signals, the mirror sometimes corresponds to only a part of the signal. Is that to be expected?
Below the 1st channel is in black, the 2nd in red. The yellow/orange correspond to the min overtime to have an idea of the noise floor.
Here’s an example with no mirror, even if the signal around 2465MHz is quite strong:
Here’s with 1 mirror (the mirror at 2470 is symmetrical to 2430MHz around 2450MHz):
Here’s with 2 mirrors (real signal at 2442MHz, symmetrical mirror at 2458MHz, other quite powerful mirror at 2497MHz = 2442+55):
Here’s with a wider signal, ~20MHz. On the waterfall we can notice part of its mirror on the left: