A Simple VFO for 80 and 160 Meters

This VFO is simple to build and is quite stable. The VFO produces a two phase output (2 outputs), making it well suited to supplying a signal to either single ended RF amplifiers, where only one of the outputs is used, or to balanced (push-pull or single ended push-pull) RF amplifiers where both outputs would be used. By using a frequency counter, the need for a mechanically calibrated dial is elminated, significantly recucing construction complexity.

The frequency counter is an inexpensive "Citizen's Band" counter. These counters may be found on Ebay and other electronic equipment vendors. Most any frequency counter may be used.

Schematics:

VFO Schematic (.PDF)

To the right:Inside of the VFO showing all of the oscillator and buffer components. The large capacitor to the left is the coarse frequency adjustment, and the smaller capacitor is the fine frequency adjustment.

Circuit Details

Referring to the VFO Schematic, the circuit uses a Hartley oscillator operating between approximately 7 to 8 mHz. The buffered oscillator output is then divided by 2 for 75 meter operation or divided by 4 for 160 meter operation. The oscillator runs constantly (at 7 to 8 mhz) when the VFO is turned on. The frequency dividers are turned off during receive, eliminating any signal "bleed" at the operating frequency during receive. By leaving the 7 mHz master oscillator running all the time, there is no transmit-receive oscillator warmup drift.

The analog output from the buffer stage (Q2) is fed to a J-K flip flop. The flip flop will change state on each rising edge of the oscillator output signal. This will guarantee a symmetrical output waveform regardless of the symmetry of the output from the master oscillator itself. On 75 meters, the oscillator output is divided one time (by 2), and on 160 meters the oscillator output is divided by 4. The output from the VFO is a standard TTL (3.5V square wave) output.

The VFO includes a relay that can be used to enable the operation of other circuits or relays. When the VFO is producing output, the relay closes. When the VFO is in standby, the relay will open. There is a small delay between the time when the VFO is put into standay, and when it actually goes to standby. This delay allows the builder to configure a transmit/receive system that allows output and other circuits to become inactive before the VFO output stops. The VFO relay opens only when the VFO output actually stops. Ideally, the VFO relay should be used to prevent the high voltage power supply from supplying power, should the VFO not be enabled for whatever reason.

To the right: Closeup of the inside of the VFO. Note the construction of the "gimmick" capacitor.

The VFO relay should not be the only source of control for the high voltage supply, rather the relay should be in series with the coil of the high voltage power supply control relay(s) as ONE of the conditions for high voltage supply operation.

Frequency Counter

The frequency counter shown is a simple and inexpensive counter, originally designed for use in conjunction with a Citizen's Band Transceiver. This counter includes an external input, allowing the counter to be used for general purpose applications. This makes it quite suitable for use with this VFO. Power for the counter (+12V) is taken from the VFO's +12V power supply, since the counter uses very little power.

The frequency counter takes a 1V peak to peak signal, and the input impedance of this counter appear to be relatively low. To the end, a seperate filtered buffer stage (Q5) is included solely for the purpose of driving the frequency counter. The frequency counter driver output is filtered to remove high frequency signals that may cause false triggering of the counter.

To the right: Back of the VFO showing the J-K flip flop (divider), the power supply regulators, the failsafe relay and the frequency counter output stage. The yellow wire in the foreground is connected to the frequency counter input.

Construction

The VFO should be constructed in a mechanically stable enclosure. Thick aluminum or copper enclosure parts should be used, as even very small movements of the sides, top, bottom or back will cause the VFO to "wobble". Although the leads should be as short and as as stiff as possible, the layout is not particularly critical. The buffer stage (Q2) should be physically close to the point where the VFO signal exits the shielded compartment. All power leads entering the VFO shielded compartment should be bypassed at the point of entry to prevent stray RF from interfering with the master oscillator. The master oscillator circuitry including the coil and capacitors should be kept away from other circuits, and particularly from any wires entering or leaving the shielded compartment.

All mechanical components should be securely mounted to the VFO cabinetry. The coil should not be placed too close to any of the sides, top or bottom of the VFO cabinet, and the ends of the coil should be at least a coil diameter from the cabinet.

Important: Because all components in the VFO are operating at RF, including the divider IC and the signal sensing and most of the fail-safe relay control circuits, all of the circuitry should be built over a solid copper ground plane. Ground and bypass capacitor leads should be kept relatively short. All outputs should be carried on properly terminated, shielded cable.

A Simple VFO for 80 and 160 Meters

Schematics:

Circuit Details

Frequency Counter

Construction

Adjustment and Testing

[Under Construction - more text to come!]