Jan Jozef Oksiuta
Brisbane QG62lk
Australian Amateur Radio Station

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Home Projects - Dip Oscillator

Dip Oscillator

This is a piece of test equipment that is so simple and so useful that I consider it one of my best projects ever. For those unfamiliar with with the concept - it is simply a tunable, calibrated LC oscillator used for determining the approximate resonant frequency of an external LC circuit. The function can be extended to the measurement of unknown inductances by resonating with a known capacitor and then calculating the value. It can also work as a simple signal generator, RF energy detector and a handy test receiver for checking radio transmitters.

Completed oscillator with plug-in colis and other accessories
Completed dip meter with coils, dials and accessories

Principle of operation

The oscillator circuitry is constructed in a way that allows for inductive coupling of the oscillator's coil with the coil of the resonant circuit under test. The oscillator is then manually tuned. When the oscillator's frequency approximates that of the circuit under test it causes a reduction in the amplitude of oscillation. Oscillation amplitude is monitored through a rectifier and analogue meter movement, and the match of resonant frequencies manifests itself as a "dip" in an otherwise steady amplitude reading - hence the popular name for this device. The wanted frequency is then read from the oscillator's calibrated dial.

Historically, these devices were known as GDO - grid-dip-oscillator - in reference to the grid electrode of a vacuum tube. Of course these days they are made with semiconductor devices.

In "receiving" mode, the oscillator is switched off and the device acts in a manner similar to a crystal radio - with a resonant circuit and detector diode. The meter reading will be proportional to the amount of RF energy received and will reach a peak when the resonant frequency of the LC circuit matches that of of the received signal. Alternatively, the meter can be substituted by an audio amplifier or just high-impedance headphones for reception of amplitude-modulated signals. For acoustic reception of CW, SSB or similar modes, the oscillator is turned on, and the inductively-coupled monitored RF signal mixed with the local oscillator produces a beat note/demodulated audio signal at the output of detector diode.

Circuit description and construction

There are no revolutionary concepts in the project and I based my device on the popular design published by Drew VK3XU. I only added a buffer/amplifier stage to enable the device to be also used as a signal generator, and made some other minor changes.

The oscillator is in a Hartley configuration using a commonly available FET. The tuned circuit employs a two-section variable capacitor mounted with its dial in the aluminium project box, and a set of plug-in coil/capacitor modules for different frequency ranges, each seated on a 9-pin D-type connector. Most of the other components are soldered directly to the pins in the matching socket. Each of the two sections of the variable capacitor is connected to a separate pin, enabling the user to construct a variety of coil modules with or without jumpers, using either the smaller 60 pF section, the other section of 160 pF, or both in parallel depending on desired frequency span. Signal from the oscillator is fed to the a source-follower buffer amplifier for use as a signal generator, and also rectified and supplied to the meter movement for monitoring the amplitude of oscillation as described above.

The schematic below is my original quick hand drawing and does not show the details of individual coils - these were constructed by trial-and-error windings on a variety of insulated tubes until acceptable frequency ranges were obtained. In the end I satisfied myself with a set of seven overlapping frequency bands with seven coil modules, covering in total a range of frequencies from 1 to 32 MHz. Different type of variable capacitor available, or different requirements for individual band spread would necessitate its own set of coils, and their final choice is best left to the individual constructor. The schematic also omits some after-thought additions such as LEDs, headphone/meter switch, and battery voltage check switch.

Dip oscillator schematic
Dip oscillator schematic

Tuning, testing and usage

The variable capacitor dial is best calibrated with the device in oscillator mode and listenig to the oscillator's carrier tone on a general coverage receiver. Rather than having a densely populated single dial for all bands I used multiple removable dials, colour-matched to individual coil modules. Calibration marks were initially hand-drawn on a piece of cardboard, then scanned into a bitmap computer file. The resuting image was then used to creaate a neat printed dial for each band.

To estimate the resonant frequency of an unknown resonant circuit, position the device (operating in oscillator mode) to ensure proximity of its coil and the coil under test; adjust potentiometer to achieve near maximum deflection of the movement meter, then slowly rotate the dial until a sharp dip is observed in the meter reading. Change coils (and dials) as necessary to find the appropriate frequency band. The frequency displayed on the dial at which the dip is observed will be the approximate resonant frequency of the LC circuit under test.

Whilst the oscillator is powered on, the device can be used as a crude direct conversion receiver or CW/SSB monitor
by bringing it into proximity with an RF energy source and connecting high impedance headphones in place of the meter movement. A close match of the oscillator's frequency with that of the monitored RF source will produce an audible tone, or SSB demodulation. This mode can also be very useful for detecting harmonics or any other unwanted components in the transmitted signal.

In passive monitoring mode the oscillator is simply switched off and the device behaves in a manner similar to a crystal radio - with the meter raching maximum deflection at resonance. If the monitoring source is amplitude modulated then headphones can be used to monitor the modulation.

Among the dials, one can see what looks like tinfoil spiral-shaped coil. It is a common anti-theft device included with goods such as CD,s books, and other flat-packed items- simply a resonant circuit that can be checked with our device. The shoplifting detector gates in supermarkets are simply a monster-sized version of a dip oscillator.

Dip oscillator

Added 2014-08-25

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Created with CompoZer and Blue Griffon. Last updated 2014-08-25