Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

236 FREQUENCY MODULATION, Volume Ivoltage swing is limited to something less than the d-c plate voltageemployed.The first oscillogram of the left-hand column is fordegrees phaseangle with a moderate amount of gain so that only partial limiting isobtained. For the second oscillogram the gain was increased so thatlimiter action was almost perfect. The third and fourth repeat the conditionsexcept that the phase angle between signal and noise is 180degrees. This shows that the plate voltage limiter is a rather goodlimiter.The second column of oscillograms is for a grid-leak limiter. Thistype of limiter obtains the limiting action by allowing the tube to biass/n?2s/n - '/2PlateGridPlateGridVoltageLeakVoltageLeakLimiter Limiter * LimiterLimitero°ModerateGain0*HighGain180°HighGainFig. 4—Oscillograms demonstrating the relative effectiveness of platevoltage and grid leak limiters.itself off as the signal increases in amplitude.to keep the circuit response as fast as possible.It is obviously importantFor these oscillograms100,000 ohms and 20 micromicrofarads were used. The other conditionsfor the second column are the same as for the first.These testsseem to indicate that the grid-leak limiter is not as satisfactory as theplate voltage type. The grid condenser apparently cannot charge uprapidly enough to follow the pulsation. However, it does build up someextra bias during the pulsation, and after it is over, the carrier is attenuatedfor a short time until the extra bias leaks off.The third and fourth columns in Figure 4 repeat the conditions of