In the 1920s, commercial IF filters looked very similar to 1920s audio interstage coupling transformers, had similar construction and were wired up in an almost identical manner, and so they were referred to as "IF transformers". By the mid-1930s superheterodynes were using much higher intermediate frequencies, (typically around 440–470 kHz), with tuned coils similar in construction to the aerial and oscillator coils. The name "IF transformer" was retained and is still used today. Modern receivers typically use a mixture of ceramic resonator or SAW (surface-acoustic wave) resonators as well as traditional tuned-inductor IF transformers.