Dip Meter: Grid Dip Oscillator GDO

The dip meter is a versatile item of test equipment that can be used for making RF measurements from testing resonance to checking for RF emissions.

Dip Meter Tutorial Includes:
Dip meter / GDO basics How to use a dip meter / GDO Using GDO to measure inductance Using GDO to measure capacitance Measuring antenna resonant frequency Measuring feeder electrical length

The dip meter often called a grid dip oscillator or GDO is a simple but effective test instrument that can give an indication of many RF parameters of circuits and components.

The name grid dip oscillator came from the when these test instruments used vacuum tubes or thermionic valves, but today, with semiconductors being the most widely used active elements the names include gate-dip-meter, dip meter or simply, dipper.

The dip meter or grid dip oscillator is able to measure the test circuit without any direct probing -the dip meter coil simply needs to be placed close to the target unit. As a result of its convenience, was widely used to build, repair and adjust the radio, television and any other items of RF equipment.

A typical analogue grid dip oscillator GDO / dip meter showng the main controls including the large tuning dial

What is a dip meter

A dip meter or dip oscillator is an instrument which contains an oscillator which can be tuned over a wide range of frequencies. Generally there are several ranges which can be chosen by the use of external plug-in coils.

The operation of a dip meter or grid dip oscillator depends upon the fact that when a tank or tuned circuit of an oscillator is placed close to another resonant circuit the oscillator current will drop when tuned to the resonant frequency of the external circuit.

The centre of the dip indicates the actual resonant frequency of the circuit to which the dip meter or grid dip oscillator is coupled.

By placing the dip oscillator close to a tuned circuit in this way it is possible to check the resonant frequency of almost any tuned circuit regardless of whether it is on a circuit board or whether it forms part of an aerial. Thus a dip meter or grid dip oscillator is essentially a form of calibrated variable frequency oscillator in which it is possible to monitor the oscillator current.

Apart from acting as an oscillator, most dip meters have the facility to turn the oscillator off so that they can be used as an absorption wavemeter. In this mode the dip meter can be used to pick up strong signals like the RF field near a transmitter or feeder carrying RF power. In this form they are very useful for checking the frequency band of a transmission.

It can be seen that a dip meter can be used in a number of different ways. By using a little ingenuity they can be used to perform a great variety of measurements which are very useful when setting up and experimenting with aerials.

Grid dip oscillator / dip meter circuit operation

When the coil of the grid dip oscillator or dip meter is placed close to a resonant circuit under test, then some of the energy from the dip meter oscillator coil is transferred to that of the resonant circuit.

The coupling between the two circuits reaches a maximum when the frequency of the dip meter oscillator is the same as that of the resonant circuit under test.

The coupled energy is supplied from the grid dip oscillator / dip meter and this causes the amplitude of the oscillation to fall. As the meter placed in the GDO circuit monitors the current drawn, and this is proportional to the oscillation level, a fall in the current is seen under these circumstances.

In this way, the resonant frequency of the circuit under test corresponds to the frequency of the minimum point of the dip. This frequency can be read directly from the scale on the grid dip oscillator or dip meter.

The advantage of this form of measurement is that the components do not have to be powered up, nor do they need to be removed from the circuit to test them.

Advantages and disadvantages of the dip meter

The dip meter or grid dip oscillator is a relatively simple item of test equipment. This provides advantages and disadvantages.

Essentially the dip meter or GDO is a very simple instrument, but one that can give a considerable degree of insight into various aspects of the performance of radio frequency circuits and elements.

The dip meter is not particularly accurate, but can be used to give broad indications of parameters and performance.

Grid Dip Oscillator / Dip Meter Advantages

Simplicity: therefore the GDO circuit is simple and costs are relatively low.
Versatility: The dip meter is a versatile instrument that can be used in a variety of ways to provide several measurements.
Low cost: The dip meter / grid dip oscillator has a very much lower cost than other RF instruments and therefore is within the financial reach of many radio amateurs and enthusiasts.
Does not require a direct connection: The dip meter only requires its coil to be placed close to the col or tuned circuit that is being tested. No direct connection is needed. This makes it very convenient to use.

Grid Dip Oscillator / Dip Meter Disadvantages

Inaccuracy: Simplicity - therefore the dip meter / GDO is very inaccurate.
Free running oscillator: The grid dip oscillator or dip meter uses a free running oscillator and therefore this is subject to all the tolerances of this type of circuit. It is not able to offer the digital accuracy of many of today's items of radio equipment.
Coupling: Coupling the tuned circuit of the oscillator within the GDO to an external circuit means that the oscillator circuit is 'pulled' by the external circuit and this leads to even more inaccuracy.

When used with care and a good understanding of its limitations, the dip meter or grid dip oscillator, GDO is a very handy instrument. It quickly gives a good indication of many parameters, often to an accuracy that is sufficient for many applications.