Doc Ewen: The Horn, HI, and Other Events in US Radio Astronomy

Slide 9: Detecting the Interstellar Hydrogen Line, 1951

The "switch frequency" technique is a method for detecting weak lines like the hydrogen line. A switch is introduced in the receiver to cause the receiver to tune rapidly (30 Hz) back and forth between two frequencies displaced by 75 KHz (in the case of hydrogen). The two frequencies are then tuned together to move slowly from lower to higher frequencies, beginning at a frequency well below the rest frequency of hydrogen. A synchronous detector in the video circuit assigns a positive output to one frequency and negative to the other. As the two displaced frequencies pass over a line, the output will be positive when the line is at the first receiving frequency and negative when it passes through the second. This (+) followed by (-) feature is referred to as the "S-Curve".

Normally, the switch frequency is applied to the first local oscillator in a superheterodyne mode. For several technical reasons, I elected to switch the second local oscillator in a double conversion superheterodyne. That had its own set of problems, but they were manageable.

Slide 10