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 workar ound. The image reject filter is actually rejecting the image noise from t he LNA before it goes to the mixer. Image Noise rejection....duh. The nois e from the LNA at both the on frequency and the image frequency combine tog ether in the mixer. It does squirrely things. It can actually look like i t gives you less noise , for a while and then greater noise. The noise flo or just walks all over the place depending on how the two noises add togeth er. Use the image filter after the LNA...for the noise rejection, not nece ssarily for the image signal rejection. Fortunately, in my case I had a sin gle frequency receiver and had board space underneath. I used a piece of s mall diameter coax and cut it to the correct length to make a notching filt er 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 rejec t filter.
- posted
4 years ago