Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

390 FREQUENCY MODULATION, Volume IMeasurement of A-M RejectionThe measurement of a-m rejection can be carried out by means ofa signal generator which can be simultaneously frequency and amplitudemodulated. The type of pattern obtained when there are residualbalanced and unbalanced components of amplitude modulation inoutput has been described in connection with Figure 23. Typicaloscillograms are shown in Figure 12.The measurement of the a-m rejection of a ratio detector can bedescribed in terms of the pattern obtained for a given frequencydeviation, i.e., for a given per cent of frequency modulation, and for agiven per cent of amplitude modulation.theIdeally the pattern obtainedshould remain a diagonal line regardless of the presence of the amplitudemodulation. The extent to which this pattern departs from astraight line indicates to what extent the detector fails to rejectamplitude modulation.While it would be desirable to be able to quotea single figure of merit to indicate the a-m rejection, it is essentialto check the performance at various levels and to interpret the wedgeshapedpatterns which are obtained so as to determine the extent ofthe residual balanced and unbalanced components of amplitude modulationin the output.In addition to the method of using a signal which is simultaneouslyfrequency and amplitude modulated, it is possible to make a-m rejectionmeasurements by using a generator which is only frequency modulated;To measure a-m rejection using this method, an f-m signal isapplied at full deviation corresponding to 100 per cent modulation,and a battery equal in value to the rectified output isshunted acrossthe stabilizing condenser. This battery will have negligible effect onthe f-m output. Now the input signal is reduced in value until thef-m output becomes distorted as a result of the diodes being biasedby the stabilizing voltage, and the f-m output drops to zero. The ratiobetween the initial input signal and the minimum input signal forwhich the f-m output becomes distorted is then a measure of theamount of downward modulation that the detector can reject. Forexample, if this ratio is r, then the per cent downward modulationwhich can be handled is 100 (r — l)/r. The performance of a typicalratio detector as measured by this method has been shown in Figure 11.To determine the rejection of the detector for upward amplitude modulation,the same setup is used, and the change in the output is notedas the input signal is increased.Since the amplitude of the input signal is not varied dynamically,this method will not indicate any unbalanced component which may