Intermediate frequency transformers have two types: single-tuned circuit and double-tuned circuit. The coupling mode can be inductive coupling or capacitive coupling. There are two tuning modes: inductive tuning and capacitive tuning. At present, transistor radios and black-and-white and color TVs use inductive tuning in large quantities. The magnetic core in the coil is adjusted to change the inductance, thereby adjusting the resonant frequency. The inductive tuning intermediate frequency in transistor radios often uses a single tuning mode, that is, only one capacitor is connected in parallel to the primary to form a tuning circuit, and the other circuit is not connected in parallel with the capacitor. For circuits with higher requirements, a double tuning mode is required, that is, capacitors are connected in parallel to the primary and secondary of the double tuning intermediate frequency, and both can be tuned. Some of the two circuits use capacitive coupling, while others use inductive coupling.
The structure of a small intermediate frequency, the entire component is installed in an aluminum shielding cover, with lead pins at the bottom of the cover and adjustment holes on the top. The coil is wound on an "I"-shaped magnetic core and fixed in a bakelite seat, and the coil connector is led out by the pins of the bakelite seat. The outer surface of the magnetic core coil is covered with a nylon liner, and the inner wall of the nylon frame has threads for screwing in the threaded magnetic cap. Adjusting the position of the magnetic cap means fine-tuning the inductance.
Most of the intermediate frequency transformers in radios are single-tuned, with a simple structure and small space. Since the input and output impedance of the transistor are low, in order to match the input and output impedance of the transistor, the primary has taps and has a secondary coupling coil with a small number of turns. The advantages of the double-tuned type are better selectivity and wider passband, and are mostly used in high-performance radios.
Two-stage intermediate frequency amplifiers are usually used in transistor radios, so three intermediate frequency amplifiers are required for coupling and transmission of front and rear stage signals. The intermediate frequency amplifiers in actual circuits are often represented by BZ1, BZ2, and BZ3. Their positions in the circuit cannot be arbitrarily changed during use.