Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Grid-dip meter calibration
The grid-dip meter isn't intended for extremely accurate measurements, but it is frustrating to find that the oscillator frequency does not always conform to what the calibration scale indicates. There is a simple solution which is not usually mentioned in the g.d.o. instruction books.
The outside rotor plates of the oscillator tuning capacitor are slotted, and by bending these leaves in certain combinations it is possible to add or subtract capacitance anywhere along the tuning curve. If the capacitance is increased in the area that engages at the high-frequency end, then the entire scale will be moved to a lower value unless there is a corresponding reduction of capacitance (outward bending of the plate) in the area that comes into action as the low-frequency end is approached. The g.d.o. coils are wound and checked to a standard so if one coil is made to track properly the others will fall into line also.
Some g.d.o.'s use the Colpitts oscillator circuit so only one section of the split-stator capacitor needs to be worked on since the capacitors are in series across the coil. The calibration problem isn't serious at low frequencies where the station receiver can be used to check the g.d.o. frequency. But on the higher frequencies for which there are usually no frequency monitoring devices, it is nice to know that you can depend on the g.d.o. dial.
Edson B. Snow, W2BZN.