444 BELL SYSTEM TECHNICAL JOURNAL 



where B is the width of IF band in cycles per second (for this purpose con- 

 veniently measured between 6 db points) and y is the speed of frequency 

 sweep in cycles per second per second. For best results this nominal width 

 should be similar to the width of transmitter pulse for which the IF and 

 video circuits are designed. This means that the scanning speed should be 

 in the neighborhood of B^/2. 



A CW input of the same frequency as the transmitter produces in the 

 output of the IF detector merely a direct current to which the video amplifier 

 and radar indicator do not respond. However, CW test signals may be 

 utilized by observing on a d.c. meter (built into the radar or separate) the 

 change produced in detector current or converter current. 



Receiver Sensitivity 



Just as in radio, the sensitivity of a radar receiver is defined as the mini- 

 mum received signal that is perceptible in the presence of set noise.* At 

 microwave frequencies atmospheric disturbances are usually negligible, so 

 that unless accidental or deliberate interfering signals are present, the 

 operating sensitivity is the same as the intrinsic sensitivity of the receiver. 



Receiver sensitivity is commonly stated as the minimum perceptible signal 

 power in db referred to a milliwatt, (abbreviated as dbm). . In practice, 

 the receiver sensitivity depends upon the noise figure of the converter, the 

 conversion loss, and the noise figure of the IF amplifier. If an RF amplifier 

 is used, as is the practice at lower microwave frequencies, its noise figure is 

 likely to be controlling. By noise figure in each case is meant the noise 

 power in comparison with the thermal noise. The thermal- noise in watts 

 delivered to a load is kTB, where k is Boltzmann's constant, T is absolute 

 temperature in degrees K, and B is the frequency bandwidth. Thus for a 

 4 mc band at 25° C the thermal noise is — 108 dbm. With good design the 

 over-all receiver noise is of the order of 10 to 15 db higher than thermal noise. 

 The minimum detectable signal is usually not equal to the receiver noise but 

 depends on the type of indicator, particularly on whether the presence of an 

 echo is indicated by spot deflection or spot modulation. 



With a CW signal generator, receiver sensitivity is measured by deter- 

 mining the minimum input power necessary to produce a perceptible change 

 in meter reading. This affords a satisfactory^ relative measure of receiver 

 performance, but since the radar indicator usually permits better visual 

 discrimination against noise, the minimum input as read with the meter 

 ordinarily differs from the minimum pulse input for barely discernible indica- 

 tor response. 



* The term noise is commonly used even though the disturbances are observed on a 

 cathode-rav screen. 



