The W8EXI Wingfoot VFO Exciter
Oscillator Schematic Diagram and Circuit Description:

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General Information:
The heart of the Wingfoot VFO Exciter is, of course, the VFO itself. It is here that the ultimate stability and sound of the transmitter are determined. The series tuned Colpitts or Clapp oscillator is by and far the circuit of choice for this application. The circuit features a large inductance to capacitance (L/C) ratio which limits the current flowing in the main oscillator coil. Limiting the current in the coil minimizes temperature changes in the coil which lead to drift and slow chirp. The circuit also uses very small coupling between the tuned circuit and the oscillator tube, which minimizes the effects that changes in the tube (such as those that occur when the oscillator is keyed) have on the frequency of the oscillator. This helps to minimize the chirp (frequency change) that occurs when the oscillator is keyed, resulting in a much better sounding signal.

Because of the large capacitances (1780pf) that are across the tube elements and the large inductance of the oscillator coil, it is possible to mount the LC circuit in a separate cabinet called the "Remote Grid Box" and connect it to the rest of the oscillator via shielded coaxial cables, marked "Grid Cable" and "Cathode Cable" in the schematic.

Mounting the resonant circuit away from the heat producing main chassis minimizes drift caused by temperature changes. Changes in room temperature still affect the frequency of the oscillator, but these changes are much less than those that would have been caused by the tubes and other heat producing components on the main chassis. When the room temperature is reasonably constant, this oscillator/VFO has a stability rivaling that of modern, synthesized, transceivers.

Ten bandspread/bandset combinations (!) can be selected by a front panel switch to adjust the frequencies covered by the oscillator. In practice I have found that only need two ranges are really needed. One is set to cover between 3500kHz. and 3557kHz. This gives the most bandspread on 80m, 40m, 20m, and 15m and is perfect for a CW operator such as myself who likes to operate at the bottom end of the bands. Another is set for operation between 3366.7kHz to 3383.3kHz. When tripled, this yields an output between 10100kHz and 10150kHz for operation in the 30m amateur band.

Series Tuned Colpitts (Clapp) Oscillator
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Series Tuned Colpitts (Clapp) Oscillator:

Main Coil: The oscillator is the heart of the transmitter, but the oscillator coil is the heart of the heart. The coil must be constructed to minimize any changes in inductance due to temperature changes. Even the normal oscillator current in the coil can cause sufficient heating to alter the inductance of the coil. It is best to use an air core coil, since changes in the permeability of ferromagnetic materials (powdered iron, etc.) with temperature will cause changes in inductance. The wire should be as heavy and stiff as possible. In a Clapp oscillator, the tuning capacitance should be as small as possible, which requires that the inductance to be relatively large. The Wingfoot VFO uses a coil wound on a 2" diameter fiberglass form. 19 turns of #15 AWG enameled copper wire spaced over 1.5" are wound on the form. This yields an inductance of approximately 15uH. The turns are then glued in place.

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Tuning, Bandspread, and Bandset Capacitors: The Wingfoot VFO transmitter uses a tuning arrangement similar to others in use at the time (1940s to 1960s) that consisted of a series/parallel arrangement of capacitors. This arrangement allows the range of frequencies covered by the oscillator to be easily adjusted. The values shown in the schematic are the maximum values for the variable capacitors. The actual value depends on the range of frequencies the operator chooses to cover. The 100pf bandspread capacitor and 75pf main tuning capacitor in series form an adjustable variable capacitor. For instance, assuming that the minimum capacitance of each capacitor is 10pf, if the bandspread capacitor is set to its minimum capacitance of 10pf, the combination will produce an effective capacitance ranging from 5pf to 8.8pf as the main tuning capacitor is varied from its minimum to maximum. If the bandspread capacitor is set to its maximum capacitance of 100pf, the combination will produce an effective capacitance of 9.1pf to 42.9pf when the main tuning capacitor is varied from its minimum to maximum. Thus, by adjusting the bandspread capacitor the change in capacitance can be varied from 3.8pf to 33.8pf. The capacitance of the bandset capacitor is added to the effective capacitance of the bandspread and main tuning capacitors and allows the maximum capacitance of the entire combination to be adjusted. For example, if the bandspread capacitor is set to 100pf (fully meshed) and the bandset capacitor is set to say, 50pf (half meshed), then the capacitance of the entire combination will vary from 59.1pf (50pf + 9.1pf) to 92.9pf (50pf + 42.9pf) as the main tuning capacitor is varied from its minimum to its maximum.. Typically, the main tuning capacitor is fully opened and the bandset capacitor adjusted to the highest frequency desired. The main tuning capacitor is then fully meshed and the bandspread capacitor is adjusted to the lowest frequency desired. This process is then repeated over and over until the desired results are obtained.

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Grid Coupling Capacitor: The effective tuning capacitor (the series/parallel combination of the bandspread, main tuning, and bandset capacitors) is connected in series with the grid coupling capacitor and cathode feedback capacitor. The effective capacitance of these three series connected capacitors is then connected in parallel across the main tuning coil, forming a parallel resonant circuit. The fact that the junction between the effective tuning capacitor and the cathode feedback capacitor is grounded does not affect this result. The voltage developed across the main tuning coil is unequally divided across these three capacitors in inverse proportion to their capacitance. The HIGHER the capacitance, the SMALLER the voltage developed across it. Most of the voltage, about 85%, appears across the effective tuning capacitor. The remaining 15% is split evenly between the grid coupling capacitor and the cathode feedback capacitor. Thus, about 7.5% of the total voltage across the main coil appears across each of these capacitors. The 7.5% that appears across the grid coupling capacitor is applied, via the connecting coaxial cables, across the grid and cathode of the 6AG7 tube, where it is amplified. Because this capacitor is quite large, the capacitance of the cables and the input capacitance of the tube have little effect on the operation of the oscillator. Since only 7.5% of the total voltage appears across this capacitor, changes in the tube caused by, for example, keying the oscillator or changing the tube, have little effect on the oscillator. It is this weak coupling between the resonant circuit and the input of the tube that is an important characteristic of the Clapp oscillator.

Cathode Feedback Capacitor: The signal applied to the grid of the tube is amplified and part of the amplified signal is taken from the screen grid (not the plate) of the tube and is shunted to ground through the screen bypass capacitor. This signal must get back to the cathode of the tube, but it can't pass through the cathode RF choke, which blocks any RF. Instead, it travels from ground back through the cathode feedback capacitor to the cathode of the tube. As it travels through the feedback capacitor, it causes a small voltage to appear across the capacitor. This voltage is in phase with the oscillation in the resonant circuit and tends to assist the oscillation, restoring any energy that was lost. The voltage across a capacitor is in inverse proportion to the charge on the capacitor. Because of the large size of the feedback capacitor, only a small voltage is developed across the capacitor. This minimizes the effect of the feedback on the oscillator, keeping the feedback to a minimum. This keeps the size of the oscillation in the resonant circuit as small as possible, minimizing any heating of the main tuning coil caused by the oscillating current. The large capacitance also minimizes the effects of the connecting coaxial cable. It is this weak coupling between the resonant circuit and the output of the tube (feedback) that is another important characteristic of the Clapp oscillator.

Coaxial Cables: In the Clapp oscillator, large capacitances are shunted across the tube elements, and a relatively large inductance is used in the resonant circuit. This allows the resonant circuit to be remotely connected to the oscillator tube via coaxial cables. This removes the resonant circuit from the heat produced by the oscillator tube and surrounding components and greatly improves the stability of the oscillator. RG-58 coaxial cable is typically used, and the cables can be up to several feet long if desired, though they should be kept as short as possible.

Grid Leak Resistor: When the circuit is oscillating, some of the RF is rectified by the diode action of the grid and cathode. This causes a voltage to develope across the grid leak resistor, providing operating bias for the tube. The resistor also allows the grid-block keying voltage to reach the grid of the tube while keeping the RF on the grid from flowing back to the keying circuit.

Grid Leak Capacitor: The 1780pf capacitors and the 470pf grid leak capacitor are ins series and connected across the grid leak resistor. Their combined series capacitance (about 300pf) smooths out any voltage variations across the resistor keeping the tube bias steady. The 470pf capacitor by itself also tends to bypass any RF leaking through the resistor to ground, preventing it from getting to the keying circuit.

Cathode RF Choke: Direct current must be allowed to flow to the tube cathode, but RF must be blocked so that it flows through the cathode feedback capacitor instead. This is done by connecting the cathode to ground through an RF choke, which blocks RF but passes DC. The value here is not critical. 2.5mH is a commonly available value.

Meter Bypass Capacitor: In a transmitter, RF can get into places it shouldn't by accident. If a meter is connected across the oscillator metering resistor to monitor the oscillator current, stray RF can get into the meter and upset the reading. The meter bypass resistor effectively short circuits to ground any RF that might try to get into the meter.

Oscillator Metering Resistor: One of the unusual features of the Wingfoot VFO Exciter is the use of current metering resistors throughout the transmitter. One of these is included in the cathode lead of the oscillator. By connecting a voltmeter across the resistor and using Ohm's law, the total oscillator current (cathode current) can be determined.

Screen Bypass Capacitor: The screen grid of the tube serves as the plate of the oscillator in an electron coupled oscillator. Some of the RF amplified by the tube must get back to the resonant circuit to support oscillation. The screen bypass capacitor allows the RF on the screen grid to pass through to ground while preventing the screen DC supply from being short circuited. The RF eventually flows through the cathode feedback capacitor back to the cathode of the tube. When it flows through the cathode feedback capacitor, it provides the necessary feedback to keep the oscillator going.

Plate Coupling Capacitor: RF appearing at the plate must be fed to the next stage (cathode follower) for amplification. However, the DC plate voltage must not be allowed to pass through to the next stage. The plate coupling capacitor allows the RF to pass through while blocking the DC. The value is not critical.

Plate RF Choke: Direct current must reach the plate of the tube for proper operation, but the RF appearing on the plate must not be permitted to reach the plate power supply. The plate RF choke allows the DC to pass through, while preventing any RF from getting through. The value here is not critical. 2.5mH is a commonly available value.

Plate Bypass Capacitor: As extra insurance that any RF from the plate of the oscillator does not get into the plate power supply, a capacitor is used to short circuit any RF that might have leaked through the plate RF choke to ground.

6AG7 Tube: In 1950, an important article in QST magazine, "Crystal-Controlled Oscillators, A Review of Modern Crystals, Circuits and Tubes" (QST, March 1950, C. Vernon Chambers, W1JEQ) addressed several points concerning crystal oscillators, including which tube to use. In that article, various electron-coupled circuits were tried along with a variety of tubes: the 6AG7, 6F6, 6V6GT, and 6L6. Among the many conclusions in the article, one came through loud and clear, which I quote here: "Of the four tubes tested the 6AG7 is by far the best from every standpoint." As a result of that article, virtually all crystal oscillator circuits in the ARRL handbook for the next 15 years featured or recommended the use of the 6AG7. Though this oscillator isn't crystal controlled, the arguments in the article still apply, so a 6AG7 was used in this circuit. You can click here for a 6AG7 data sheet.

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