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High Pass Filter
"A low cost piece of test equipment for the radio amateur."
Shown below is the Heath HD-1250 with a 3 to 6 MHz coil.
The GDO above is the B&W mode 600 from the 1950's. The curved shape on top was designed to avoid contact with components when probing into circuits.
Above - Paco dip meter
This site provides information about both dip meters and Grid Dip Oscillator (GDO)
Some of the functions of the dip meter have been replaced by the antenna analyzer which can perform analysis and frequency performance of an antenna with very accurate data. However, the dip meter is used for rapid resonance checks, primarily for circuits of transmitters, receivers, or other tank circuit elements. One additional feature that is not used much is the ability to determine relative Q of coils, whether toriods or air gap coils.
When cost is an issue, a dip meter can provide some similar functions and in a few cases can perform some specialized test functions that are not practical with an antenna analyzer. Example: Relative effects of loose and tight coupling. The ability to move the dip meter's coil around a coil under test, allows for fast analysis and visual display, in real time, of the coupling effects.
In this site, the acronym GDO (Grid Dip Oscillator) is used synonomously with dip meter, understanding that solid state circuits are the modern replacement.
For those who wish to construct there own dip meters, you will find the links helpful from the variety of circuits shown and principles of design.