Keyed Oscillator Transmitter

The KEYED OSCILLATOR TRANSMITTER most often uses a MAGNETRON as the power oscillator. The following discussion is a description of a magnetron used as a keyed oscillator radar transmitter.

The figure below, view A, shows the typical transmitter system that uses a magnetron oscillator, waveguide transmission line, and microwave antenna. The magnetron at the bottom of the figure is connected to the waveguide by a coaxial connector.

High-power magnetrons, however, are usually coupled directly to the waveguide. A cutaway view of a typical waveguide-coupled magnetron is shown in view B.

View A: Keyed oscillator transmitter physical layout.

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View B: Typical-magnetron.

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The magnetron is an electron tube in which a magnetic (H) field between the cathode and plate is perpendicular to an electric (E) field. Tuned circuits, in the form of cylindrical cavities in the plate, produce rf electric fields. Electrons interact with these fields in the space between the cathode and plate to produce an ac power output.

Magnetrons function as self-excited microwave oscillators. These multicavity devices may be used in radar transmitters as either pulsed or cw oscillators at frequencies ranging from approximately 600 to 30,000 megahertz. If you wish to review magnetron operation in more detail, refer to NEETS, Module 11, Microwave Principles) or go to my tutorials in the "Microwaves" section.

Let’s examine the following characteristics of a magnetron used as a pulse radar transmitter oscillator stage:

· Stability

· Pulse characteristics

· The magnet

· Output coupling

Stability

In speaking of a magnetron oscillator, STABILITY usually refers to the stability of the mode of operation of the magnetron. The two main types of mode instability are MODE SKIPPING and MODE SHIFTING.

Mode skipping (or misfiring) is a condition in which the magnetron fires randomly in an undesired, interfering mode during some pulse times, but not in others. Pulse characteristics and tube noises are factors in mode skipping.

Mode shifting is a condition in which the magnetron changes from one mode to another during pulse time. This is highly undesirable and does not occur if the modulator pulse is of the proper shape, unless the cathode of the magnetron is in very poor condition.

Pulse Characteristics

PULSE CHARACTERISTICS are the make up of the high-voltage modulator pulse that is applied to the magnetron. The pulse should have a steep leading edge, a flat top, and a steep trailing edge. If the leading edge is not steep, the magnetron may begin to oscillate before the pulse reaches its maximum level. Since these low-power oscillations will occur in a different mode, the mode of the magnetron will be shifted as the pulse reaches maximum power. This mode shifting will result in an undesirable magnetron output.

For the same reason (to prevent mode shifting), the top of the modulator pulse should be as flat as possible. Variations in the applied operating power will cause variations in the mode of operation. The trailing edge of the pulse should also be steep for the same reason--to prevent mode shifting.

Magnet

The purpose of the MAGNET is to produce a fairly uniform magnetic field of the desired value over the interaction space between the cathode and plate of the magnetron. The strength of the magnet is critical for proper operation. If the magnetic field strength is too high, the magnetron will not oscillate. If the magnetic field strength is too low, the plate current will be excessive and power output will be low. Frequency of operation will also be affected.

Since the strength of the magnet is critical, you should be careful when handling the magnet. Striking the magnet, especially with a ferromagnetic object, will misalign the molecular structure of the magnet and decrease the field strength.

Output Coupling

The OUTPUT COUPLING transfers the rf energy from the magnetron to the output transmission line (coaxial line or waveguide). A number of considerations impose restrictions upon the output circuit. The wavelength (frequency) and the power level of the magnetron output energy determine whether the transmission line to the antenna will be waveguide or coaxial line.

The coaxial output circuit consists of a length of coaxial line in which the center conductor is shaped into a loop and inserted into one of the magnetron cavities for magnetic coupling. The load side of the coupling line may feed either an external coaxial line or a waveguide. If the external line is coaxial, the connection may be direct or by means of choke joints. If the external line is a waveguide, the output circuit must include a satisfactory junction from the coaxial line to the waveguide. One type of junction used quite often is the PROBE COUPLER. The probe coupler acts as an antenna radiating into the waveguide.

The waveguide output may be fed directly by an opening (slot) into one of the magnetron cavities, as shown in view B of the figure above. This opening must be covered by an iris window to maintain the vacuum seal.

The peak power ratings of magnetrons range from a few thousand watts (kilowatts) to several million watts (megawatts). The average power ratings are much lower, however, and vary from a few watts to several kilowatts. Additionally, many of the magnetrons used in modern radar systems are tunable in frequency. Typically, a tunable magnetron can vary the output frequency ±5 percent about the center of its frequency band. Thus the carrier frequency of radar can be changed to obtain the best operation or avoid electronic jamming on a particular frequency.

Modulator signals of many thousands of volts are applied to the magnetron cathode during operation. These high voltage levels require large glass posts to insulate the cathode and filaments from the anode block. In some high-power magnetrons, the cathode is completely enclosed in a container filled with insulating oil.

WARNING: All radar transmitters contain lethal voltages. Extreme care and strict observance of all posted safety precautions are essential when working on a radar transmitter.

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