Why superheterodyne receiver or SA use mostly 3 different IF frequency(Why not2)

[AttacK]

[ Specified attachment is not available ] [ Specified attachment is not available ]Hi everyone,

Firstly this is my first message in forum.

I've been researching SA for a while and I've found that they mostly use 3 IF. The 1st IF is higher than the input signal, 2nd IF is a lower frequency than the input signal. 3rd IF also has the lowest frequency.  After 3rd IF, demodulation is performed. The reason for turning the signal up once and down once is to get rid of the image signals, I understand why this makes a lot of sense. But why is the lowest frequency IF frequency not preferred directly, but it is lowered to the main band in 2 stages.

Why we need mid-IF frequency. In this example below, why we need 322.5 MHz?

Thanks in advance

[Another Kevin]

Wikipedia, which knows everything, has a decent answer: https://en.wikipedia.org/wiki/Superheterodyne_receiver#Multiple_conversion

Essentially, it has to do with filters.  In front of each mixer, you need a filter that will reject signals that are twice the IF frequency away from the signal to be detected.  If the final IF were 455 KHz (a common one for radio receivers) but the initial IF were 5 GHz, you'd need a filter that could pass a 5 GHz signal from one that's 5.000455 or 4.999555 - a tall order!  When radio hams are doing receivers, it's pretty common to have a downconverter from the band of interest to 28-29.7 MHz or 144-148 MHz (so that a station receiver can be used to work with it). The station receiver is likely to have 45MHz, 9 MHz and 455 kHz IF frequencies, with progressively narrower filters in front of the mixers.

Modern digital stuff means that you don't need quite as long a conversion chain, because the final filtering happens in DSP.  In that case, the final IF needs to be within the bandwidth of the A/D system.

[David Hess]

A high second IF is needed to provide sufficient image rejection while a much lower third IF provides most of the selectivity.