"ssylee"
I'm reviewing some notes and came across this question: "Why might one choose a low intermediate frequency (IF)? A high IF?"
When I'm reviewing the answer as mentioned in the following, I couldn't figure out why, nor find the internet resources that would help explain why this is the case. It would be great to hear some comments here. Thanks in advance.
"A lower IF allows one to achieve higher selectivity for a given filter quality factor and thereby achieve greater selectivity (ability to reject adjacent signals).
** Fact: "filter quality factor" = "Q" = ratio of a filter's bandwidth to its centre frequency in Hz.
So a filter with 10 kHz bandwidth operating at 1 MHz has a Q of 100. But a similar filter, also with Q of 100, operating at 100kHz would have a bandwidth of 1kHz - so it is 10 times more "selective" that the 1MHz version.
The term "selectivity" = essentially the same as bandwidth in relation to radio signals.
A higher IF shifts the image frequency farther away from the frequency of the desired signal.
** The "image frequency" is that frequency obtained by adding or subtracting the IF frequency to or from the RF one.
With a higher IF frequency, image frequencies are more distant from the RF ones and hence less troublesome to remove by filtering.
This makes it more practical to use a pre- selection filter or preselector (inserted in front of the mixer?s RF input) to suppress any signals that appear at the image frequency."
** A "pre-selection filter" refers to a fixed bandwidth filter operating across the RF range to be received. For this to work, any image frequencies must fall outside the range of the filter.
The majority of radios ( ie AM or FM broadcast receivers ) have a *variable* RF filter which is adjusted as you "tune" the radio to operate at the actual radio frequency to be received. This allows lower IF frequencies to be used where image frequencies may fall inside the RF range to be received.
..... Phil