Tesla Oscillator

Buying dapoxetine online || VFO || Eritromicina oftalmica nombre generico || Disulfiram price || Bandpass Filters || First Mixer & Amplifier || Crystal Filter

Receiver Range/SSB Mods/Modifying NBPF/VFO Frequencies
VFO Stabilization || VFO Tuning with a CD ROM

Tesla (Vackar) VFO

Circuit History || Circuit Used in Receiver || RF Choke Information
Main Tuning Capacitor || Modifying Bandwidths || Relay/IR Operation || References

In the April 1999 issue of QRP Quarterly, Chris Trask, N7ZWY, states in his article, "LC Oscillators - A Brief History", (speaking of the Vackar Oscillator) "The last of the significant improvements in LC oscillators was developed at the same time as the Gouriet circuit was being devised, ...Created by Jiri Vackar in Czechoslovakia, the stability is improved by placing a variable capacitor... The same circuit, also known as a Tesla Oscillator, was developed independently in Italy by O. Landini."

So I decided to use the name Tesla Oscillator, in honor of my favorite inventor.

The tuning capacitor tabs (top of image) can be seen through the holes in the PCB. A 39pf NPO is soldered to the first tab on the left. A 10pf NPO capacitor is shown soldered to the last tab on the right.

The relay on the right (10.545) is turned on by a Photo Transistor located between the two VFO amplifiers, triggered by an IRED at the 3.547 MHz crystal filter. The voltage to this relay provides voltage to the 10.455 relay circuit. The 10.455 relay can turn on only when the 10.545 relay is on.

An IR (infrared) diode is at the lower left hand corner, covered with a black tube. The IRFU220 switches on the 10.455 relay when an IRED on the 20 meter bandpass filter is turned on, increasing the gate voltage, through the IR diode, of the IRFU220 above 4 volts.

A regular kit LED must be used at the 10.455 relay. The voltage drop to that part of the circuit will not throw the relay with a Super-Bright LED. A Super-Bright Blue LED is shown installed next to the 10.545 relay.

The stabilizer parts for the VFO are now integrated in the VFO section with the new Rev2 boards. The new circuit provides outstanding stability and is very easy to connect. The cable from the stabilizer output connects to the "Stabilizer In" box in the VFO section.

If the stabilizer is not used, the 47pf capacitor and the 2/8 trimmer are put in the empty footprints shown above outside of the "Stabilizer Parts" box.

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The VFO was taken from the July 1997 issue of QEX, "Meet the Vackar: The Simple, Stable VFO You've Been Looking For", by Mark L. Meyer, WU0L.

Meyer's circuit follows the capacitor rules for a 'real' Vackar VFO, and his tutorial on achieving stability is excellent.

The following diagram is a modification of his circuit using a single varicap and a 2N3904 temperature compensator. This circuit is optimized for 4.570MHz.

A variation of the original circuit that appeared in QEX.

Circuit Used in the Receiver

The three VFO frequencies are 14.068 MHz, 10.545 MHz, and 10.455 MHz. The relays are switched by IR devices, using the circuit at the bottom.

I designed the basic oscillator for 12 MHz. This determined the inductance for the coil and the capacitor values off the Drain and Gate of the 2N5486. The capacitor values were 220pf (split between two values - 100pf & 120pf poly caps).

The coil is 18 turns on a T68-6. The feedback capacitor value (120pf) is split between 39pf (C5) and 82pf (C6) NPOs.

The oscillator was set at 14 MHz by removing capacitance between the coil to ground. The other places, off the drain and gate, had some, but not enough effect on the frequency to use.

Relays are used to add additional capacitance between the coil and ground to reach 10.545 MHz and 10.455 MHz. IR switching is used to switch the relays.

The range of the VFO is about 9 MHz to above 15 MHz with the 120pf feedback capacitor value and 220pf on the drain and gate.

Extending the range is done by changing the value of the feedback capacitor. Lower the feedback capacitor value for higher frequencies and raise the value for lower VFO frequencies.

A "Huff and Puff" frequency stabilizer is available as an option for increasing the stability of the VFO.

For information on the "Huff and Puff" frequency stabilizer, check out "'Huff and Puff' Frequency Stabilizer".

Important RF Choke Information

Caution: When building and using this circuit you can blow the 2N5486 in a split second by shorting any of the capacitors off the coil to ground.

Shorting the coil to ground on the opposite side of 12 Volts generates a spike of voltage that instantly blows the gate of the FET. This is very similar to what is used in automobile ignition circuits with a point type distributor.

Do not use a molded choke in a Vacker VFO circuit. A hand wound FT37-43 or FT50-43 (20 turns) or the ones in the kit, FT50A-75 (7 turns) with #24 wire, will hold its choking ability if you momentarily short the coil. The molded chokes use very fine wire. Even the shortest full voltage spike will blow their choking ability, but not necessarily their ability to pass DC.

The RF choke is a drift contributing part. Ferrite material absorbs heat quickly, way ahead of the toroid or capacitors in the VFO. It is important to cover the choke with melted candle wax (or some other suitable material) to slow the temperature change to approximately the same rate of the poly caps to enhance frequency and mechanical stability.

Two values were tried on a FT50A-75, a 16 turn (1 mHy), and a 7 turn, (220uH) for the RF choke. The 7 turn choke moved less in value with temperature changes.

Main Tuning Capacitor

The photo above shows the tuning capacitor soldered to the bottom of the printed circuit board. Holes are in the PCB above the tuning capacitor tabs, so they can be easily soldered.

The tuning capacitor is a five section variable, 30-200-30-86-25, front to back.

Soldering the tuning capacitor on the board allows a quick setup and stable VFO. Soldering the main tuning capacitor on the PCB board resolves the hardest part of building a receiver: mounting the main tuning capacitor.

The two end sections of the tuning capacitor are used: the front one for 40 and 20 meters and the rear one for 30 and 17 meters. The front section tunes the 10.545 MHz (40 meters) and 10.455 MHz (20 meters) frequencies. Relays on the top side determine which frequency is running, switched by IR devices located at the Crystal Filter and the 20/17 Bandpass Filter.

The rear section of the main tuning capacitor tunes the 14.068 MHz frequency used for 30 and 17 meters. The coupling capacitor sets the bandwidth. A 6.8pf NPO capacitor gives 100 kHz bandwidth, 14.068 MHz to ~14.168 MHz. With a 10pf NPO cap, the bandwidth will include WWV as well as the amateur bands, 14.100 MHz to ~14.200 MHz.


Modifying Bandwidths

The coupling capacitors are in series with the tuning capacitor, so raising the coupling cap value raises the effective capacitance of the tuning capacitor, giving a wider bandspread. For absolute maximum bandspread, the coupling cap could be shorted and all the capacitance of the main tuning cap would be in the circuit.

Conversely, lowering the coupling capacitor lowers the effective capacitance of the main tuning cap, which shortens the bandwidth of the VFO.

Coupling capacitors are noted with an arrow - 39 NPO and 10 NPO

The 39pf NPO capacitor is used for the 40 and 20 meter bands (10.545 MHz and 10.455 MHz). The 10pf NPO (or 6.8pf NPO) capacitor is used for the 30 and 17 meter bands (14.000 MHz).

For setting different bandwidth values, experiment with both the coupling capacitor and adding/changing sections of the main tuning capacitor.

Do not change the 100pf and 120pf poly capacitors unless major frequency changes are being made to the VFO (way outside the 9 MHz to 15 MHz range of the VFO).