Guest
Image reject filter (in the context of a typical single conversion superhet design).... have you ever tried to not use the image reject filter because the preselect filter would rightfully reject image frequencies coming into the unit? I have, and had to scramble at the last minute to find a workaround. The image reject filter is actually rejecting the image noise from the LNA before it goes to the mixer. Image Noise rejection....duh. The noise from the LNA at both the on frequency and the image frequency combine together in the mixer. It does squirrely things. It can actually look like it gives you less noise , for a while and then greater noise. The noise floor just walks all over the place depending on how the two noises add together. Use the image filter after the LNA...for the noise rejection, not necessarily for the image signal rejection. Fortunately, in my case I had a single frequency receiver and had board space underneath. I used a piece of small diameter coax and cut it to the correct length to make a notching filter at the image frequency. We produced many thousands of those boards with the coax on the bottom because I had a misconception about the image reject filter.