Radio Frequency (RF) & Wireless Design

Since I have started making Intermediate Frequency (IF) filters and FM Discriminators, I think that there is a need for a discussion on the basics of what these things are and where they are used.

In RF receivers and RF transmitters the frequencies, these frequencies that are used in radio communications are beyond the ability for humans (or animals) to hear. So in order to get higher frequencies down in the audible frequency range that we can hear, we must convert thru a process called heterodyning.

Heterodyning is the process of mixing two frequencies that will give both of the original frequencies and also the sum (f1 + f2) as well as the difference (f1-f2) in the mixer output. Normally for down-converting of frequencies we only need the difference and all other frequencies are rejected by filtering.

https://en.wikipedia.org/wiki/Intermediate_frequency

In the heterodyning process, there is normally two conversions that take place to convert VHF band (30 MHz to 300 MHz) down to audible (60 Hz to 16 KHz). One of these mixer output frequencies is a HF frequency of either 10.7 MHz or 21.4 MHz. These are used as a standard so that manufacturers only have to make a few types. This is used in the first conversion frequency.

In the second conversion frequency, 455 KHz is typically the standard second mixer output frequency for the last conversion into the audible frequency range.

Local Oscillators (LO) to the mixers use specific crystal frequencies and usually Voltage Controlled Oscillators (VCO) or better Voltage Controlled Crystal Oscillators (VCXO) in order to create the desired mixer frequency outputs.

As part of the conversion process, there is a need to clean out undesired frequencies and this is where the Intermediate Frequency (IF) bandpass filters come into play. The bandpass is used to just allow the center of the center frequency such as 10.7 MHz, but also a tolerance that has a +/- KHz window for the roll off of the peak. If the Q of the frequency is sharp (high Q), then the window can be small, but if the Q of the frequency is dull (low Q) then the window needs to be larger.