…About Grid-Dip Oscillators
The grid-dip oscillator – also known as a GDO or “Dipper” – was a fixture in the well-equipped ham shack of the mid-20th century, fading from popularity only as the use of vacuum tubes faded as well (it was the grid current of a vacuum tube that “dipped” when you tuned it to the resonant frequency of an antenna or other tuned circuit). CQ introduced the Dipper to the amateur community with a pair of articles in 1947. The first, in our March issue that year, is reproduced below. The second (to follow) has a greater significance in the history of CQ. – W2VU
The idea of using specially constructed oscillators for a wide variety of test purposes has long been understood and put to use by most engineering laboratories. Although some mention of the “grid-dip” oscillator has appeared in amateur literature, the lack of general usage is sufficient evidence that the subject is open to review and clarification. It is the purpose of this article to provide constructional ideas and design information on a typical oscillator suitable for amateur use. Further, to attempt to bring out some of the reasons why a good griddip oscillator is a necessity in the modern amateur station. In subsequent mention of this type of oscillator, we will refer to it merely as “GDO.”
The function of a GDO is certainly not complicated. For illustrative purposes we can compare it to the well-known absorption frequency meter. When the L/C circuit of such a meter is resonated to the same frequency as the oscillator or amplifier being checked, an indication of the frequency of the latter may be had by observing the kick in plate current as the absorption device is tuned past the operating frequency. This technique is well-known and requires no elaboration. It is significant that the absorption meter is entirely useless unless the element under test provides the necessary r-f energy. Thus an absorption meter would be of no earthly use to a man who has a new transmitter, the amplifier of which refuses to hit resonance. He would have no way of knowing whether the frequency of the tank circuit was too high or too low. A GDO would tell him this in thirty seconds without even turning on the transmitter!
A GDO is merely a specially designed, calibrated oscillator utilizing a sensitive meter (in the circuits in grid and/or plate are tuned to the same frequency as the circuit to be checked. Providing their own driving source, the use of such instruments for antenna and transmission line measurements alone will bring direct and useful return to their builders. Additionally, the frequency of any circuit within the range of the instrument can be determined quickly and with good accuracy.
