GDO circuits bonanza !

The Gate Dip Oscillator (GDO) is one of the most useful tools for the radio amateur. You can use it to tune LC circuits, as test RF source, check antennas, etc. This GDO circuits bonanza features many previously unpublished GDO circuits from (primarily) Dutch magazines and books.

Bouwen en luisteren – IAN R. SINCLAIR

This book in the “GDO circuits bonanza” contains many simple – yet very useful circuits. To help the constructor a simple layout sketch is given based on vero strip-board. The circuit is build around a single NPN transistor. The transistor (TR1) is set-up in common-base amplifier, the basis is held at a fixed voltage and is connected to AC ground via capacitor C2. This configuration is probably choosen because the input capacitance does not sugger from the Miller effect, which degrades the bandwidth of the common-emittor configuration. This implies that the collector and emitter from the transistor are in phase. Thus a simple small capacitor from collector (output) to emitter (input) is sufficient to provide positive feedback and hence start oscillation. The LC circuit comprised of L1 and VC1 only provides a high resistance and thus (high) amplification at it’s resonance value, which is given as ƒ = 1 / (2 * π * √ L * √ C).

For every component the position in the vero board is given, with the description from which to which copper strip the component should be mounted.

 

Theorie en praktijk voor zendamateurs – J.L. Molema

Two other ones in the GDO circuits bonanza. This time from the book “Theorie en praktijk voor zendamateurs” (Theory and practice for radioamateurs), a collection of circuits and little background information on their workings. It features two GDO both based on junction transistors.

The first circuit is set-up around a PNP transistor, again in common-base configuration. The circuit itself is a little difficult to follow because, unlike current conventions were the positive rail is drawn a top and the ground below, it use the old drawing style were both the supply rail and ground rail are drawn next to each other. Both 3k3 resistors are used to set the base at half the supply rail and 6.8nF cap is used to connect base to ground for AC. The 820Ω resistor and 10nF cap are used for power decoupling the transistor’s collector. The meter read-out is different in a couple of ways. First the signal is taken from the low-impedance emitter. Thus the read-out circuitry will load the LC circuit far less than the design above. Potentially offering greater stability. The second-change is that a transistor is used to amplify the signal, thus a less sensitive meter couple theoretically be used. Finally the meter is connected to a bridge circuit, one leg being the 4k7 resistor and the transistor and the second being the 10k trimmer pot. Although, the circuit recommend using a trimmer here, a normal pot can be used to your advantage as well. With the 50K pot one can then set the sensitivity, with the 10k pot one can the shift the zero-point and thus zoom-in on a small dip.

One thing that should be pointed out, however, is that there is no protection for the meter M. If you are not careful you can easily destroy it when fiddling with the controls.

Coil details are given in the page scans below.

 

The second circuit is a more traditional design. If you are familiar with the classic Heathkit GD-1B and the Eico 710 models, you’ll probably recognize the circuit immediately. The emitter is held at ground via the 470Ω and 1nF. The LC circuit is connected between basis and collector. Basis and collector are in this configuration 180º out-of-phase. The LC circuit provides another 180º, because the mid-point of the capacitor is connected to ground. Sensitivity is set via the 5k log  potentiometer that changes both the base voltage and the collector. If the DC voltage is increased, the amplitude of the oscillation will also increase.  Th output is sampled from the collector and rectified. Because there is no amplification and more sensitive meter is required.

Coil details are given in the page scans below.

 

Ontvangers – F.A.S. Sterrenburg

The next book in the “GDO circuits bonanza ” is probably one of the best books on radio receivers available in the Dutch language. It describes almost every aspect of radio reception: radio propagation, receiver principles, receiver specifications, planning and construction, antennas, receiver inputs, oscillators, IF amplifiers and the list just goes on and on…! It is all written down in an easy to read concise format, with many many examples. I bought his book at a library selling off some surplus books some 20+ years ago and it is still my go-to book for many aspects. It really is a pity it is only available in Dutch!

Of course being a book of 1978 many new techniques in receiver design, such as H-mode mixers, DDS VFOs, SDR and new modulation techniques as WSPR, are not covered. But then you cannot run, if you cannot walk, can you? So, I would definitely recommended this book to any youngster planning on building a receiver.

In chapter 15 “Meetinstrumenten” (Measuring instruments) many principles and some simple to build tools are described including two dippers. The first one is build around a EM87 tuning indicator tube, very cleverly using the triode portion of the tube as the oscillator. Oscillator and indicator are all in one tube. Although the author, very politely, doesn’t mention it, the circuit is from it’s own hand. It was originally published in the Dutch magazine Radio Electronica 1974, page 49. The author goes to great length to describe how to build the circuit, enclosure, which coils to use and how to calibrate the circuit.

After the previous circuits, the oscillator configuration itself should be becoming familiar: an LC circuit connected between a tube’s grid (gate) and plate (drain), the central part of the capacitor grounded to provide the necessary 180º phase shift. The second half of the tube (the actual indicator) is then used to display the amplitude of the oscillation. In the original article you can find a great deal of information about the construction and the coils used.

The second dipper described in the book is totally different. It is a transistor dipper, again in common-base configuration. However, now a couple of diodes are used to rectify the output and stabilize the amplitude of the oscillator signal. This agc signal is then also used to indicate the dip.

The use of a negative power supply looks maybe a bit strange but it is actually quit clever. In this way one half of the LC circuit can be connected to the ground, minimizing hand effects. I am not sure if the actual rectification circuit around D1 and D2 is drawn entirely correct. It seems strange to put D1 in anti-parallel with T2…

You can find more information in the page scans below..

 

Kleine zenders en oscillatoren – Herbert Brosch

This booklet is written by German amateur Herbert Brosch (now SK), who has compiled a collection of many simple circuits about oscillators and small transmitters. The GDO feaured in this book is classic Collpits based GDO with a J-FET and uses a sensitive meter to display the rectified gate current. Not even a basic 100nF buffer capacitor is included in the schematic….

You can find more information in the scans below:

Although I am not a great fan of this book, it features some nice and unorthodox circuits, like this Push-pull oscillator: