Intermediate Frequency Transformer (IFT)

In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception.

Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an intermediate frequency for amplification before final detection is done.

The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency.

According to Wikipedia, perhaps the most commonly used intermediate frequencies for broadcast receivers are around 455 kHz for AM receivers and 10.7 MHz for FM receivers, some other frequencies can be used in special purpose receivers, though.

IF transformers or IF amplifier transformers are simply tunable inductors, usually with an integral fixed capacitor, and are typically used inside cheaper transistor radios. Mostly they are used as synchronously tuned filters because each stage is coupled by an active device.

In other words, an IF, or Intermediate Frequency, Transformer (IFT) is a tuned air core transformer used in just about all analog Superheterodyne receivers in past and current AM and FM designs.

Intermediate Frequencies (IF) have been standardized on the broadcast bands with 455 kHz used for AM and 10.7 MHz used for FM.

Most Superheterodyne receivers have two or more IFTs to increase signal gain and selectivity (selectivity is the ability of a radio receiver to focus on one broadcast while rejecting others that are close in frequency as the desired one).

Superheterodyne Radios take an incoming broadcast and convert it to an intermediate frequency using a process called Heterodyning.

Heterodyning is used as it is more efficient and cost effective to design a radio frequency (RF) amplifier for a small window of frequencies than to design one efficient across and entire broadcast band.

For the noobs, essentially there are two coils in an IFT usually (but not always) one above the other.

The IFT is tuned to the intermediate frequency (IF) of the radio which is the difference between the tuned signal being received and the local oscillator (LO) in the radio which usually runs at a higher frequency.

So if you are receiving a signal at 1500 kHz, the local oscillator could be running at 1955 kHz. The difference here is 455 kHz and this is what the IFTs will be tuned to. And to some extent, correct tuning of the IFTs will affect the audio signal quality in subtle ways. 

Put another way, in a traditional AM radio where the received signal is in the range 540 kHz to 1650 kHz, the local oscillator (which's in fact a variable frequency oscillator) signal is always a constant 455 kHz higher or 995 kHz to 2105 kHz.

Rather unsurprisingly, vintage medium wave (MW) AM radios often had double tuned IFTs. These old school IFTs do not have taps and therefore are high impedance devices that are intended for vacuum tube (valve) radios.

So, IFTs used in valve radios usually have tuned primary and secondary circuits whereas transistor radios usually have a single tuned circuit.

Therefore, an IFT for use with transistors will have one adjustment (the primary) and an IFT for use with valves will have two adjustments (the primary and the secondary).

More to come soon (hopefully) ◌⇲

(thanks to https://www.electronics-tutorials.com)