Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

IMPULSE NOISE 243center of the pass band, the deviation being caused by grid currentwhich detunes one circuit during high amplitude impulses.From further tests, not shown here, the grid-leak type of limiterseems to be particularly good in avoiding the detuning due to gridcurrent.in spite of itsIn fact, it seems to be the most desirable type of limiter tested,apparent inability to remove the amplitude modulation.If Figure 4 is re-examined keeping Figure 8 in mind, it can be seen thatthe grid-leak limiter does remove the low-frequency components fromthe envelope of the wave.This type limiter may also be used as a sourceof automatic volume control voltage. It may be used in combinationwith the plate voltage limiter to obtain still better limiter action.3. In the output circuits of the discriminator the two sides of thecircuit must have the same fidelity characteristics. A good fidelity balanceis seldom obtained in practice because the output circuit is usuallygrounded on one side instead of the center. As a result, there is morecapacity across the grounded side than across the high side. The extracapacity can be compensated for, as shown in Figure 1.4. From the standpoint of noise reduction the limiter has littlevalue, providing the circuit constants are properly balanced. Thelimiter helps to reduce the noise only when the instantaneous frequencyof the carrier is off center. If the signal is accurately tuned in, thismeans that the improvement takes place only on noise that occursduring a modulation peak.Noise which occurs only during modulationpeaks is very likely to be masked by the modulation. However, thedesirability of a limiter isaffected by other considerations than noisereduction. For example, the limiter does reduce the distortion if theselectivity curve is not flat over the 200-kc frequency band. This maymake it desirable to retain the limiter. The grid-leak type of limiterseems to be almost as effective for noise reduction as the plate-voltagelimiter in spite of itsmodulation.demonstrated inability to remove the amplitudeA more accurate idea of the degree of usefulness of a limiter canbe obtained by referring to Table I. The upper table indicates thedegree of noise reduction obtainable for relatively weak noise pulses.It indicates that the use of a limiter,and the symmetry of the selectivitycurve are unimportant, providing the instantaneous frequencyof the carrier wave corresponds to the frequency of zero output at thecenter of the discriminator curve. It also indicates that if the frequencyof the carrier wave has a slightly different value, the limiteris of importance, but a symmetrical selectivity curve is still notnecessary.