ex SP2EBP, VK2EBP
Jan Jozef Oksiuta
|Australian Amateur Radio
DC to light, homebrewing,
minimalist antennas and projects, QRSS, QRPp and less
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.
with plug-in colis and other 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
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
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
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
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
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.
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|Created with CompoZer
and Blue Griffon. Last updated 2014-08-25