This page is a detailed discussion of the individual components in the differential/timed sequence keying circuit and what they do. For a detailed discussion of how the differential/timed sequence keying circuit works from key up, to key down, and back to key up see the following link:
How The Differential/Timed Sequence Keying Circuit Works
General Information On Differential/Timed Sequence Keying:
One of the most consistent things that I hear on the air is that the Ranger is a great sounding CW rig. This is due to the use of differential or timed sequence keying. This type of keying was introduced in the 1950s and made "break in" keying practical. 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 in the 1950s, and it was also used by the E.F. Johnson Company (who called it "timed sequence keying") in several of their transmitters, including the Viking Ranger.
In transmitters such as the Viking Ranger, the transmitter frequency is the same as the oscillator frequency or an integer multiple of the oscillator frequency. This means that whenever the oscillator is on it can be heard in the station receiver, and must be turned off during receive periods. In the early years of amateur radio the transmitter had to be manually switched from receive to transmit to turn the oscillator on and off, or the oscillator could be wired to the antenna transmit/receive switch, so that the switch that controlled the antenna also turned the oscillator on and off.
One might wonder: why not just turn the oscillator on and off with the key? In other words, why not just key the oscillator? This is definitely an option, but turning an oscillator on and off causes the frequency to change during the turn-on/turn-off periods, which introduces what is commonly called "chirp" in the transmitted signal. Early oscillators were especially susceptible to chirp, and sounded awful when keyed. However, with the introduction of the Clapp/Series Tuned Colpitts Oscillator the chirp was greatly decreased (but not eliminated), and keying the oscillator became an option. The chirp from a keyed Clapp oscillator was acceptably low to some, but too much for others. Complete elimination of the chirp in a keyed oscillator required a technique known as differential or timed sequence keying.
In differential keying, several stages of the transmitter are keyed in sequence. The oscillator is keyed first, followed a short time later by another stage (or stages) in the transmitter. If the delay in keying the other stages is long enough, and the oscillator stabilizes fast enough, the oscillator chirp is over before the rest of the transmitter is keyed, and the chirp isn't transmitted. Likewise, the later stages are unkeyed first, before the oscillator is unkeyed, eliminating the chirp on turn-off.
For differential keying to work, the oscillator must turn on very fast, yet turn off slowly. This is accomplished by using grid block keying and making the cutoff bias on the oscillator adjustable so that it can be set to a value that just barely turns off the oscillator on key-up. When the key goes down, the bias is removed and the oscillator quickly begins to operate. When the key goes up, the circuit takes a while to turn off, since it is barely cut off. In the Ranger keying circuit below, the left half of the12AU7 tube is used to provide adjustable cutoff bias for the oscillator. This cutoff bias is controlled by the key through the right half of the 12AU7. However, the key also controls several other stages in the transmitter, but the circuit is arranged so that the other stages are keyed a short time after the key closes. This circuit is nearly identical to the circuit used in the Wingfoot VFO Exciter.
Click On A Section of the Schematic
Below for Information on That Part of the Circuit:
|Cutoff Bias Resistor|
|1st Grid Resistor|
|VFO Keying Adjust Control|
|2nd Grid Resistor|
|Bias Voltage Divider|
The VFO in the Ranger transmitter is keyed by applying a negative voltage to the grid of the VFO tube. This is known as grid block keying.
The keying circuit allows an adjustable voltage to be applied to the VFO grid so that the minimum voltage necessary for reliable cutoff is used. This causes the oscillator to turn on as quickly as possible, and also causes the oscillator to turn off more slowly.
Cutoff Bias Resistor:
Current flowing through the left half of the 12AU7 tube flows through the cutoff bias resistor R38. The voltage drop developed across R38 is connected to the VFO grid through the VFO grid leak resistor and cuts off the VFO.
On key up, the current flowing through the left half of the 12AU7 is controlled by the keying adjust potentiometer.
1st Grid Resistor:
R40, the 1st grid resistor, allows the negative cathode bias developed across the cathode resistor and the positive bias from the keying adjust potentiometerto flow through to the grid of the left section of the 12AU7. The bias on the left grid is the sum of these two voltages.
VFO Keying Adjust Control:
The VFO keying adjust control R39 feeds an adjustable positive voltage to the grid of the left triode. (Remember the bias supply is positive ground.) The grid bias on the left triode is the sum of the negative voltage from the cathode bias resistor and the positive voltage from this control. By varying this control, the net grid voltage, and thus the current through the left triode (and the voltage across the cutoff bias resistor), can be controlled.
In practice, this control is adjusted so that left triode is just conducting enough to reliably shut off the VFO off when the key is up.
Current flowing through both triodes flows through the cathode resistor. This produces a negative bias that is then applied to both the left and right triodes through the left and right grid resistors.
When the key is closed, the current through the right triode greatly increases and the voltage across this resistor greatly increases. This increased negative voltage travels to the left triode grid, cutting off the left triode, and turning on the VFO.
The 12AU7 is a rugged, low gain (u), dual triode. The plate dissipation for each section is 2.75 watts, which is very good given the small size of the 12AU7. The 12AU7 has an amplification factor (u) of about 17 and a transconductance of about 2200 umhos. The 12AU7A and 5814A are electrically equivalent to the 12AU7 and can be used instead of the 12AU7.
High gain is undesirable in this circuit, since the tube is being used only as a voltage regulator and switch. Excessive gain could lead to undesirable self oscillation.
You can click here for a 12AU7A data sheet.
2nd Grid Resistor:
The 2nd grid resistor allows the negative cathode bias developed across the cathode resistor to flow through to the grid of the right section of the 12AU7.
+300V from the low B+ supply is connected to the plate of the right triode. The right triode is thus effectively connected to both the low B+ and bias power supplies in series, since the cathode/plate current of the right triode flows through the cathode resistor to the negative side of the bias power supply.
When the right triode is turned on (when the key is closed), it raises the voltage across the cathode resistor by a substantial amount.
The Johnson Ranger uses grid block keying. This means that during key up a negative voltage is applied to various parts of the transmitter to turn them off. When the key is pressed, this voltage is shorted to ground, turning on the various stages.
The voltage on the key when it is open is about -50V. When the key is closed, the current is only a few mA. Any mechanical key or bug can easily handle the negative voltage and small current, but this may cause problems for some electronic keyers. An electronic keyer must be able to switch a voltage that is negative with respect to ground and it must be able to handle an open key voltage of about -50V.
The key line also runs to the operate switch. When the operate switch is in "Tune" mode the key is grounded. When the operate switch is in the "Phone" mode the multiplier cathode is also connected to the key line so that the multiplier cathode is also keyed in "Phone" mode.
The left end of resistor R43 is connected directly to the key. When the key is up, the negative voltage across the key flows through R43 and charges up capacitor C89. This voltage is then applied to the crystal oscillator/buffer grid through the bias voltage divider, and also to the multiplier grid, shutting these circuits off. When the key is closed, capacitor C89 discharges through R43 to ground, turning on the crystal oscillator/buffer and the multiplier. The time it takes C89 to discharge introduces a delay of about 2ms between when the key closes and when the crystal oscillator/buffer and the multiplier turn on.
When the key is opened, C89 begins to charge up, but, as before, this takes time. The keying of the crystal oscillator/buffer and the multiplier stages is therefore delayed a couple of milliseconds after the opening or closing of the key. The keying of the VFO however, is not delayed. As a result, the crystal oscillator/buffer and multiplier stages are always keyed a couple of milliseconds after the VFO.
On key down, the delay in the keying of the crystal oscillator/buffer and multiplier stages allows the oscillator to stabilize after it is keyed so that any chirp that occurs when the key goes down is not transmitted.
When the key opens, the oscillator takes longer than 2ms to shut off because the blocking voltage is kept to a minimum by the setting of the VFO Keying Adjust Control. This allows the crystal oscillator/buffer and multiplier stages to shut off completely before the oscillator, so that any chirp that occurs when the oscillator is unkeyed is not transmitted.
Bias Voltage Divider:
The voltage needed to cut off the crystal oscillator/buffer stage is less than the voltage needed to cut off the multiplier stage. Excess cutoff voltage is undesirable, so the keying voltage for the crystal oscillator/buffer stage is applied to a voltage divider consisting of resistors R44 and R45. These drop the blocking voltage on the crystal oscillator/buffer stage to about 3/4 of that applied to the multiplier stage.
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