Typical magnetron impedance characteristics
Some of the following parameters are often found in magnetron specifications.
The frequency of a magnetron is broadly proportional to the size of the resonant magnetron cavity. When the amount of power into the magnetron is changed, either by switch-on or by a change in operating conditions, the amounts of power dissipated in the anode (and cathode) change with consequent changes in temperature. Since this changes the physical size of the cavity, the frequency of the magnetron is altered. Most of this drift happens within a few seconds of the change; after 10 to 30 minutes (depending on type) the frequency stabilises.
Any change in the ambient conditions that affect the anode temperature also causes a frequency change. This could be changing air temperature or pressure, a change in mounting plate temperature, or in coolant flow rate or temperature. This change is usually specified for each magnetron in kHz/°C. This value is almost invariably negative for magnetrons, that is frequency falls with increasing temperature.
The oscillating frequency is affected by the electron density in the interaction space of the magnetron – this is a function of the anode current. If the top of the current pulse is not flat, this will result in modulation of the frequency as well as of the power level.
Typical frequency pushing curve for 10 kW 3rd generation marine magnetrons (MG 5241).
The datasheets for some types include maximum limits on frequency pushing, expressed in MHz/A (megahertz per ampere) over a specified current range. Unless otherwise specified, the frequency pushing is measured with the magnetron feeding a matched load, and can be greater under mismatched conditions.
This is a measure of change of frequency with change of phase of load mismatch, and it is clearly desirable to minimise this characteristic in most magnetrons. The pulling figure is usually defined as the maximum frequency change when a fixed external mismatch (usually 1.5:1 VSWR but sometimes 1.3:1 VSWR) is moved one half wavelength in the output waveguide.
The pulling figure is a characteristic determined by the degree of coupling between the anode and output systems. Although a high degree of coupling gives good power output and efficiency, it gives poorer jitter and pulling characteristics. Consequently, the magnetron designer must choose the best compromise.
As the V/I curve is nearly horizontal, any change in the operating conditions will have little effect on the anode voltage, but a large effect on the current. Abnormal magnetron operation is often indicated by incorrect anode current, even though the anode voltage has not changed noticeably. The effects of changing load and magnetic field are also indicated.
Time jitter (or starting jitter) is the random variation in time delay between the leading edge of the applied voltage pulse and the leading edge of the detected RF output pulse. To a large extent, time jitter occurs as a function of the interface between the particular modulator and magnetron. Magnetron specifications refer to a desired range for the ”rate of rise of voltage” (RRV) for stable operation. RRV is defined as the steepest slope of the leading edge of the applied high voltage pulse, measurable above 80% amplitude, and is usually expressed in kV/µs (kilovolts per microsecond). If the value of RRV is too high, there is insufficient dwell time at the normal firing potential of the magnetron to permit a smooth transition into oscillation, consequently random delays occur in establishing stable oscillation. This is usually expressed as pulse-to-pulse variation in nanoseconds rms.
In extreme cases, where the delay in starting produces an RF output pulse having less than 70% of the energy content of a normal pulse, the pulse is considered to be ”missing”. In addition to specifying a maximum permissible time jitter, magnetron specifications contain the parameter of missing pulses, expressed as a maximum percentage of the total number of high voltage pulses applied over a three minute test period.
If it were possible to remove frequency variations due to pushing, pulling, thermal drift, temperature coefficient, shock, vibration and all other external effects, there would still be a small amount of frequency modulation (FM) on each magnetron transmitted pulse, and from pulse to pulse. This residual FM is random in nature and results from a number of minor uncontrollable factors.
In most system applications, random FM is small enough to be unimportant. However, in MTI radars it is a parameter that must be considered in calculating the maximum attainable MTI improvement factor for the system.