FRANKLIN OSCILLATOR

•     The local oscillator for, a balanced modulator generating single or double sideband amplitude modulated wave is based on the Franklin oscillator, that was so popular in the first half of this century.

•      The Franklin circuit used two vacuum tubes, this circuit uses two direct coupled FET’s.

•     The Inductor Li is wound on a ceramic former, about 1/2 inch (12.5mm) Diameter.

•      Ceramic coil former can be made from an old 1KW electric fire (heater) element by nicking the element with a hacksaw, and breaking it on a hard surface.

•      Drilling holes in it will not help. The main tuning capacitor is a double bearing type, value about 2Opf.

•     The trimmer cap is a good quality air spaced type. The 5Opf fixed cap (47pf actually) is polystyrene.

•     The trimmer marked, 5p should be adjusted to the minimum value that gives reliable operation, a 5mm ceramic trimmer, 3/20pf would be appropriate.

•     The VFO should be built in a well screened box.

•     The VFO output is about 700m Vpp, further amplification will be needed to drive a high level diode, or mosfet mixer.

•      Here a digital VFO stabilizer is used, so good long term frequency stability will not be too difficult to achieve.

•     The LO. AMP. Increases the VFO signal level to about +17 dBm (50 mW).

•     This uses a 2N2219A transistor.

•     The transformer  is wound on a small ferrite ring (the same type as used for the balanced modulator), 6 turns bifilar winding.

•      Twist two lengths of pvc insulated wire together, use two different colors T low pass filter is designed to work at 11.5 MHz.

•     The inductor is 12 Turns on a T50-2  core.

•      If your rig has a different L.O. frequency, you will need to re-calculate the LPF component values.

•     The 5R resistor is two 10 R resistors in parallel.

APPLICATION OF FRANKLIN OSCILLATOR:

•     The simple, two diode balanced modulator is still one of the best ways of generating a DSB/SSB signal.

•     The local oscillator used here is Franklin oscillator.

•     You can use Silicon switching diodes, 1N914, 1N4148, Germanium point contact diodes, 1N34, or Schottky (Hot Carrier) diodes.

•      Both transistors are BC548’s, for the reasons mentioned above.

•     The transformer is wound on a high permeability (ui = 850) ferrite core.

•     2K2 resistor at the output, supplies a few mA to a diode switch in my receiver.

•     This allows the same crystal filter to be used for transmit and receive.

•     If only transmitter is built, this resistor should be omitted.

•     The modules constructed so far will work on any band.

•     Decide what band you want to use, work out the local oscillator frequency, allowing for the IF offset, then build a suitable oscillator / PLL / DDS.

•      An SSB transceiver requires the 80 Metre band.

•     The IF frequency here is 7.8MHz, the local oscillator frequency range required is, 1 1.3 MHz to 11.6 MHz to cover from 3.5 to 3.8 Mhz.

•      This local Franklin oscillator can easily be modified to work on other frequencies, if you are using a different IF.