GB2425415A - Phase locking magnetrons - Google Patents

Abstract

The invention relates to a magnetron using the frequency pushing characteristics of a phase locked loop to control the magnetron frequency, where the locking signal from the phase locked loop is introduced to the magnetron via a directional coupler. This enables the magnetron to have a high output power while only using a low input power as the directional coupler isolates the low- and high-power parts of the circuit from each other. For example, a 20dB - 25dB directional coupler may enable the input signal injected into the magnetron to be -47dB - 50dB lower than the output power of the magnetron.

Description

Phase Locking Magnetrons Magnetrons in mass production are produced at a

relatively low cost per kW of output power with output power of the order of I kW or less for use in domestic microwave ovens. High power magnetrons having power outputs in the range 5 to 20 kW, as required for specialised industrial processing applications, are produced in low numbers and have a relatively high cost per kW of output power.

It is desirable therefore to combine the output of mass produced magnetrons to give low cost, high power microwave sources.

Magnetrons are normally operated as free running oscillators. In order to efficiently combine the output of several magnetrons it is necessary to lock their phases of oscillation. It is well known that magnetrons can be phase locked by injecting a signal of the correct phase into their output port ("Injection-locked magnetron with low-input power" Oar Research Review, vol.7, no.4, Apr11 1968, pp.11. USA.). Where this has been done, it has until now been necessary to inject a relatively large locking signal whose power is typically 10 dB below the output power of the magnetron. This has made the technique very expensive to apply as the injection amplifiers, and the circulators needed to isolate them, are very much more expensive than the high power magnetrons themselves.

Techniques for combining the power from phase locked sources are well established. Phase locked microwave sources are normally achieved by amplifying the signal from a master oscillator using lOTs, Klystrons, TWTs or solid state amplifiers. Many designs for combining cavities exist.

The advance here is a technique for phase locking a magnetron with an injection signal -47dB - 50dB below the output power of the magnetron combined with the use a 20dB - 25dB directional coupler that isolates the low cost low power injection amplifier and its low power circulator from the high power circuit.

The scheme for isolating the injection amplifier is shown in figure 1. The importance of the 20dB -25dB coupler is the fact that once the power level of the locking signal has been reduced by a factor of 100 or more the cost of the circulator needed to protect and isolate the low power amplifier is dramatically reduced. The use of this magnitude of coupling is only possible with locking techniques requiring very small signals.

Specifically if locking can be achieved with a signal to power level of 10 log L then it is required that

L

where S is the power output of the injection amplifier, P is the power output of the magnetron and the degree of coupling is lOlogY dB. The power handling requirement of the circulator would then be PY+S=-+S

SL

Table 1 gives, as an example, the power handling requirement of the circulator as a function of the magnetron power and the injection signal power when locking is achieved with a -50dB signal ( L 10).

_________________ Injected Signal Power Magnetron Power 1W 2W 5W lOW 1kW 11 7 7 11 2kW 41 22 13 14 5kW 251 127 55 35 10kW 1001 502 205 110 Table I Power rating for injection amplifier isolation circulator The advance claimed which enables the magnetron output to be locked with a signal -47dB - 50dB below the power of the magnetron is to use the pushing properties of the magnetron as the current controlled oscillator in a phase lock loop at the same time as applying an injection signal.

A layout using low cost components that might be used to achieve this is shown in figure 2. This invention makes it possible to combine the output power from an array of inexpensive low power magnetrons without requiring a high power source for the locking signal. It therefore provides a means by which the cost of -.3- generating microwave power for industrial applications can be significantly reduced.

In this circuit a reference signal is taken from the magnetron via a directional coupler, a circulator and probably an attenuator to an RF PLL Frequency Synthesizer i.c. such as Analog Devices ADF 4113.

Such an i.c. gives a current output in proportion to the phase difference between the divided signal from the magnetron and the divided signal from a reference oscillator. This current then becomes the input to a controller that varies the output current of the high voltage d.c. power supply that drives the magnetron.

The controller would typically be a loop filter. The power supply would typically be an industrial specification switched mode power supply.

It has been established experimentally that such an arrangement can easily control the frequency of 1 kW 2.45GHz CW magnetron to a bandwidth of 300kHz.

With the Phase Lock Loop (PLL) but no injection signal the phase jitter between the reference signal multiplied up to the centre magnetron frequency and the magnetron frequency itself is much greater then a few degrees hence on its own the PLL does not provide the opportunity for power combining.

We have found that if we take the reference signal, multiply it up to the centre magnetron frequency and inject it at a level of-5OdB into the output port of the magnetron, the phase of the magnetron is locked to better than a few degrees.

Although injecting locking is well known, injection locking with such a small signal has never been demonstrated before. Key issues with respect to achieving this are the level of RF isolation between the magnetron signal and the reference signal at the input to the voltage controller oscillator (VCO) giving the drive to the injection amplifier.

Claims (3)

1. Claims 1. An injection-locked magnetron in which the frequency of the

   magnetron is additionally controlled by a phase-locked loop through its frequency pushing characteristics.
2. 2. An injection locked magnetron as described in claim I in which the locking signal is injected through a directional coupler.
3. 3. Injection-locked magnetrons as described in claims I and 2 whose power is combined through the use of a power combining device or network.