An intermediate frequency is created by mixing an incoming radio signal with a local oscillator signal. This downconversion allows the receiver to amplify and filter the signal at a fixed, manageable frequency rather than trying to tune high-gain amplifiers across a wide band. It is the core principle behind the superheterodyne receivers used in almost all wireless communications.
The IF stage is oscillating, check the bypass capacitors around the mixer tube.
Related terms
Superheterodyne
Describing a receiver architecture that converts incoming radio frequencies to a fixed intermediate frequency for easier filtering and amplification.
Mixer
A non-linear circuit that combines two different input frequencies to generate new output frequencies.
Downconversion
The process of translating a high-frequency radio signal to a lower intermediate or baseband frequency.
Upconversion
The process of shifting a low-frequency signal to a higher frequency band.