The Johnson Viking Ranger
by Greg Latta, AA8V
How The Differential/Timed Sequence Keying Circuit Works:

Click here for a higher resolution (larger) schematic.

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Introduction:
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 low B+ 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 VFO from being transmitted. This is accomplished by doing two things:

1. The VFO is grid-block keyed using the minimum voltage necessary to reliably turn off the VFO. When the blocking voltage is removed, the VFO 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 VFO 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 keyed in proper sequence. They must be turned on a short time after the VFO has had a chance to stabilize, and they must be turned off as quickly as possible, before the VFO has a chance to turn off.

If the previous two rules are followed, any chirp created by the VFO 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 VFO. 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 VFO 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 VFO must turn on fully before at least one other circuit down the signal chain also turns on.

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

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 the E.F. Johnson Company in several of their transmitters, including the Viking Ranger, and it was also used by Jim Trutko, W8EXI, when he built the Wingfoot VFO Exciter.

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

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 low or positive bias on the left triode. The current flowing in the left triode passes through the 22kohm cutoff bias resistor producing a large voltage drop across the resistor. This passes through to the VFO and cuts it off.

2. The bias produced across the cathode bias resistor also passes through the 100kohm 2nd grid resistor onto the grid of the right triode, cutting off the right triode.

3. Negative bias from the bias supply passes through the 100kohm 2nd grid resistor to the key and the delay network. The capacitor in the delay network is charged up, cutting off the crystal oscillator/buffer and the multiplier.

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 removes the bias from 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 VFO. The VFO 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 47kohm resistor in the delay network is removed. This causes the 0.047uf capacitor to begin discharging. After about 2ms, when the capacitor is sufficiently discharged, the blocking bias is removed from the crystal oscillator/buffer and multiplier. This satisfies the requirement that the VFO 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 VFO, and the VFO is on.

3. The crystal oscillator/buffer and multiplier grid leak resistors are grounded through the key and 47kohm resistor in the delay network, turning all of them on.

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

1. The negative voltage on the grid of the right triode is restored. This places large negative 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 large voltage drop across the resistor. This restores cutoff bias to the VFO, turning it off. All of this happens instantaneously, but the VFO 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 VFO must remain on long enough for at least one other circuit down the signal chain to turn off before the VFO.

2. The negative voltage on the 47kohm resistor in the delay network is restored. This causes the 0.047uf capacitor to begin charging. After about 2ms, when the capacitor is sufficiently charged, the blocking bias is restored to the crystal oscillator/buffer and multiplier, 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