Later he even built a superheterodyne receiver. It used an FM transmitter section and a double superheterodyne receiver. One major disadvantage to the superheterodyne receiver is the problem of "image frequency". The diagram at right shows the block diagram of a typical single-conversion superheterodyne receiver. The Advantages section below describes how the superheterodyne receiver overcomes these problems. The superheterodyne receiver has several advantages over previous receiver designs. The typical shortwave radio receiver design is a superheterodyne receiver. In 1918, Edwin Armstrong used only triodes when he invented the superheterodyne receiver. The superheterodyne receiver offers superior sensitivity, frequency stability and selectivity. The transponder consists of five functional systems&colon; superheterodyne receiver, decoder, modulator, transmitter, and power supply. Armstrong later used Fessenden's heterodyne principle in his superheterodyne receiver "(below)". By the mid 1930s, it was replaced by the superheterodyne receiver patented by Edwin Armstrong. There he developed the superheterodyne receiver to detect the frequency of enemy aircraft ignition systems. He patented the superregenerative circuit in 1922, and the superheterodyne receiver in 1918. The TR-1 is a superheterodyne receiver made with four n-p-n germanium transistors and one diode.