The W8EXI Wingfoot VFO Exciter
How The Differential/Timed Sequence Keying Circuit Works:

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This page discusses how the differential/timed sequence keying circuit works from key up, to key down, and back to key up. For a detailed discussion of the individual components in the circuit and what they do see the following link:
Differential/Timed Sequence Keying Schematic Diagram and Circuit Description

General Comment:
When I first encountered the circuit below, I was, quite frankly, thoroughly confused. It is an uncommon circuit and the operation is confused by the fact that it uses two power supplies, one positive ground (the bias supply), and one negative ground (the plate supply). However, after making voltage measurements and spending a good deal of time studying the circuit, I now understand exactly how it works. This page is my way of passing that information on to you.

Main Purpose of Differential (Timed Sequence) Keying:
The main purpose of differential keying is to prevent chirp caused by keying the oscillator from being transmitted. This is accomplished by doing two things:

1. The oscillator is grid-block keyed using the minimum voltage necessary to reliably turn off the oscillator. When the blocking voltage is removed, the oscillator quickly turns on and quickly stabilizes. This minimizes the time it takes for any frequency changes (i.e. "chirp)" to occur. When the blocking voltage is restored, the oscillator turns off slowly, since it is barely cut off. This delays the time it takes for the chirp to occur.

2. Other circuits in the transmitter must also be keyed in proper sequence. They must be turned on a short time after the oscillator has had a chance to stabilize, and they must be turned off as quickly as possible, before the oscillator has a chance to turn off.

If the previous two rules are followed, any chirp created by the oscillator will not be transmitted, and the result is a clean, chirp free signal.

What the Differential Keying Circuit Must Do:
The differential keying circuit must do four things:

1. The differential keying circuit must provide an adjustable cutoff voltage for the oscillator. The exact cutoff voltage needed for the best keying depends on many things, such as the type of oscillator tube used, the brand of oscillator tube used, the age of the oscillator tube, etc. By making the cutoff voltage adjustable, it can then be precisely adjusted by listening to the keying for best performance. When set to the proper value, the oscillator will turn on quickly, yet turn off slowly.

2. The differential keying circuit must allow the adjustable cutoff voltage to be turned off and on instantaneously by the key, while also allowing the key to control the other circuits in the transmitter.

3. When the key is closed, the oscillator must turn on fully before at least one other circuit down the signal chain also turns on.

4. When the key is opened, the oscillator must remain on long enough for at least one other circuit down the signal chain to turn off before the oscillator.

The above requirements might seem difficult to implement, especially using 1950s technology, but the circuit below does the job. The basic circuit was first published in an article in the September Issue of QST Magazine titled "De Luxe Keying Without Relays" by T.H. Puckett, W2JXM. It was used by Jim Trutko, W8EXI, when he built the Wingfoot VFO Exciter and it was also used by the E.F. Johnson Company in several of their transmitters, such as their Viking Ranger.

How The Differential/Timed Sequence Keying Circuit Works:
Scroll down for a complete description of how the differential keying circuit works.

Circuit voltages during key up and key down.
Green indicates key up. Red indicates key down.
All voltages are with respect to ground. Bias voltages are with respect to the tube cathodes.

To understand how the circuit works, it is best to
consider the following four situations:

It is assumed that the keying adjust potentiometer has been set
for the best keying by listening to the transmitter in a receiver.

Key Up:
When the key is up, three things are in place:

1. The left triode is turned on. The negative bias produced by the cathode bias resistor and the positive bias from the keying adjust potentiometer combine to place +1.4V of bias on the left triode. The current flowing in the left triode passes through the 22kohm cutoff bias resistor producing a voltage drop of 62.5V across the resistor. This passes through the 100kohm grid leak resistor and cuts off the oscillator.

2. The bias produced across the cathode bias resistor also passes through the 100kohm 2nd grid resistor and combines with the positive bias from the bias rail to place -35.1V of bias on the right triode grid, cutting off the right triode.

3. Negative bias from the bias supply passes through the 100kohm 2nd grid resistor to the key, the bias rail, and the 2E26 grid. The capacitors in the bias rail are charged up and the final amplifier and 1st and 2nd buffers are all cut off.

Key Closes:
When the key closes, three things happen:

1. The negative voltage on the grid of the right triode is shorted to ground. This places +4.1V of bias on the right triode grid, turning on the right triode. The current in the right triode flows through the cathode bias resistor greatly increasing the voltage across the resistor. This extra cathode bias voltage passes through the keying adjust potentiometer and 1st grid resistor to the grid of the first triode, cutting it off. This removes the cutoff bias from the oscillator. The oscillator turns on quickly because the minimum blocking bias was used to turn it off. All of this happens instantaneously, without delay. This satisfies the requirement that differential keying circuit must allow the adjustable cutoff voltage to be turned off and on instantaneously by the key, while also allowing the key to control the other circuits in the transmitter.

2. The negative voltage on the final amplifier is immediately removed, turning it on.

3. The negative voltage on the 47kohm resistor feeding the bias rail is removed. This causes the 0.05uf capacitor to begin discharging. After about 2ms, when the capacitor is sufficiently discharged, the blocking bias is removed from the 1st and 2nd buffers, and they turn on, after the oscillator has turned on. This satisfies the requirement that the oscillator must turn on fully before at least one other circuit down the signal chain also turns on.

Key down:
When the key is down, three things are in place:

1. The right triode is turned on because the grid is grounded through the key. The current through the right triode produces a large voltage across the cathode bias resistor.

2. The left triode is turned off because of the large negative bias from the cathode bias resistor. There is no cutoff bias on the oscillator, and the oscillator is on.

3. The final amplifier, 1st buffer, and 2nd buffer grid leak resistors are all grounded through the key and 47kohm resistor in the bias rail, turning all of them on.

Key Opens:
When the key opens, three things happen:

1. The negative voltage on the final amplifier is immediately restored, turning it off.

2. The negative voltage on the grid of the right triode is restored. This places -35.1V of bias on the right triode grid, turning off the right triode. The drop in current flowing through the cathode bias resistor greatly reduces the voltage across the resistor. This reduction in cathode bias voltage passes through the keying adjust potentiometer and 1st grid resistor to the grid of the first triode, turning it on. This current in the triode passes through the 22kohm cutoff bias resistor producing a voltage drop of 62.5V across the resistor. This restores cutoff bias to the oscillator, turning it off. All of this happens instantaneously, but the oscillator turns off slowly because the minimum cutoff bias (as set by the keying adjust potentiometer) is used. This satisfies the requirement that when the key is opened, the oscillator must remain on long enough for at least one other circuit down the signal chain to turn off before the oscillator.

3. The negative voltage on the 47kohm resistor feeding the bias rail is restored. This causes the 0.05uf capacitor to begin charging. After about 2ms, when the capacitor is sufficiently charged, the blocking bias is restored to the 1st and 2nd buffers, and they turn off.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu