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  • #883
    Gerry O’Hara

    These simple, versatile and very useful instruments are usually found in the radio amateur’s shack rather than the radio repair workshop, the grid dip oscillator (GDO) is a multi-function device that can be very useful in circumstances when other test equipment simply will not do.

    A GDO essentially consists of a tuned circuit, the coil of which is made to be interchangeable with other coils of differing inductance to allow a wide range of frequencies to be tuned (photo of my Millen GDO attached). The coil and tuning capacitor form part of a simple oscillator circuit, using either a valve (usually a triode), or an FET or bipolar transistor in later designs, coupled to a meter. With a few additional switches and sockets, including maybe DC and/or audio amplifiers, the tuned circuit can be made to act as an ‘absorption wavemeter’ (AW), a simple signal generator, or beat frequency oscillator (BFO). Their simplicity and versatility in use) made GDO’s very popular ‘homebrew’ pieces of test equipment for the radio amateur.

    In the GDO mode, the coil of the GDO is placed so that it loosely couples to the coil of an external (un-powered) tuned circuit who’s resonant frequency is to be determined (see the post on the Zenith ‘silver mica disease’). The GDO is tuned across its calibrated frequency range and the meter reading monitored until a dip occurs, indicating that power is being absorbed by the external tuned circuit, indicating that it is at resonance. In AW mode, the GDO is un-powered, and is used to check the oscillating frequency of a powered tuned circuit, as in a transmitter or local oscillator, here resonance being indicated by a peak in meter reading. The GDO can be used as a simple tuneable oscillator in place of a signal generator, or in beat frequency oscillator (BFO) mode, a pair of ‘phones is plugged into the GDO and when the internal circuit is oscillating close to an external oscillating circuit, an audible tone (heterodyne) may be heard in the ‘phones.

    There are many models available from such companies as Heathkit and Millen, though I am not sure if any are still being made. A GDO was one of my first ‘home brew’ projects in my youth whan I first became a ham – they are very simple to make and not that difficult to calibrate, though the commercial ones, such as my Millen, as the ‘bees knees’.

    Download Alignment and Winter [1024×768].jpg. (Caution: This file may not be virus scanned.)

    Robin in Kansas
    Forum Participant

    I was wondering if the GDO is also called a frequency counter. Also, I’ve usually heard them being used with ham equipment. Are they useful for us regular BC radio guys, too?

    Gerry O’Hara

    A GDO is not a ‘frequency meter’ as such – that term (or the term ‘frequency counter’) is normally used for devices that provide a digital readout of frequency (‘Digital Frequency Meter’, or ‘DFM’). The Grid Dip Oscillator (GDO) is a simple analogue device that pre-dates a digital frequency meter by several decades. I find a GDO to be useful for certain things in radio receiver maintenance. For example, I have used one to determine the resonant frequency of a tuned circuit (actually an IF transformer/capacitor combo) in a circuit having the tuned circuit un-powered, or use the same instrucment ‘Absorption Wavemeter’ mode when the circuit containing the tuned circuit who’s resonant fequency needs to be determined is powered. A GDO can even be used as a simple signal generator or tracer. They have the advantages of verstaility, low cost, ruggedness and ease of operation. They have limited accuracy however, though if you also have an accurate signal generator/DFM, this can be overcome.

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