MICROWAVE RADAK TESTING 445 



RF Osc ilia lor s 



Beat oscillator tubes for radars deliver (with sufficient decoupling or isola- 

 tion to prevent undue frequency pulling) a power of the order of milliwatts. 

 This power being adequate for most test purposes, such tubes are well 

 adapted for use as signal generators. 



Throughout the 'greater part of the microwave range, reflex velocity- 

 modulated tubes,2 |3Q(.jj |-he type ^[^^ built-in cavity (Pierce-Shepherd) tuned 

 mechanically or thermally, and that with external cavity (McNally) tuned 

 by plugs, vanes or adjustment of dimensions, have been used. The former 

 is more convenient for general use but the latter usually permits wider fre- 

 quency coverage. Oscillation occurs when the repeller voltage is adjusted 

 so that the round trip transit time corresponds to an odd number of quarter 

 wavelengths. Ordinarily there are several different ranges of repeller 

 voltage, corresponding to different numbers of quarter waves, each of which 

 supports oscillation over a range of frequencies, called a mode of oscillation. 

 Pulsing or frequency modulation is accomplished by applying a pulse or 

 sawtooth wave to the repeller. 



At the longer microwaves, a triode with closely spaced electrodes, or 

 so-called "lighthouse" tube^ has been employed in a tuned-plate tuned- 

 grid oscillator of the positive grid type. Two coaxial lines, conveniently 

 placed one inside the other, provide the tuning, with the feedback through 

 interelectrode capacitances supplemented by loop coupling. The inner 

 cavity (between plate and grid) controls the frequency of oscillation, while 

 the outer cavity (between grid and cathode) provides a suitably high grid 

 impedance. Mechanical arrangements are provided for tracking the tuning 

 of the two cavities over a wide frequency range. 



Some Design Principles 



Standard signal generators which have been employed in the past for 

 measuring the sensitivity of radio receivers usually deliver a known voltage 

 across a low impedance. This voltage is applied in series with a dummy 

 antenna to the receiver under test. In the microwave range this technique 

 is inconvenient, and signal generators are designed to deliver test power on 

 a matched impedance basis. Receiver sensitivity is stated in terms of 

 power (dbm) instead of volts. 



The components of a signal generator or other test unit are commonly 

 arranged along a microwave transmission line. The wave guide type of 

 line possesses certain advantages over a coaxial line in affording a lower 

 loss, facilitating attenuator design as discussed in a subsequent section, etc. 

 Hence the wave guide type of line is used in test equipment for those wave- 

 lengths where its size is not excessive, i.e. from about 4,000 mc upwards, and 

 coaxial line for lower frequencies. 



