English: Block diagram of a simple reflex radio receiver, an unusual type of radio receiver invented by Marius Latour and William Preiss in 1917 that was used to a limited extent in the 1920s. The distinguishing feature of the reflex circuit is that the amplifier is used to amplify both the radio frequency (RF) and audio frequency (AF) signals at the same time. It functions as a tuned radio frequency (TRF) receiver. The RF signal from the antenna, after passing through the bandpass filter, a tuned circuit which selects the frequency of the station desired, passes through an amplifier consisting of one or more vacuum tubes or transistors, then passes through the high pass filter to the demodulator, which extracts the information bearing modulation (audio) signal from the radio carrier wave. This is fed back and passes through the same amplifier again, which does the audio amplification, passes through the lowpass filter which blocks the RF signal, and is applied to the speaker. Thus the amplifying devices do "double duty", reducing the number of active devices needed. The economical reflex circuit was used in inexpensive single vacuum tube receivers in the early 1920s, but feedback problems limited its usefulness.